Báo cáo Y học: The cytoplasmic C-terminus of the sulfonylurea receptor is important for KATP channel function but is not key for complex assembly or trafficking pdf

Eur. J. Biochem. 269, 5303–5313 (2002) Ó FEBS 2002

doi:10.1046/j.1432-1033.2002.03249.x

The cytoplasmic C-terminus of the sulfonylurea receptor is important
for KATP channel function but is not key for complex assembly
or trafficking
Jonathan P. Giblin*, Kathryn Quinn* and Andrew Tinker
Centre for Clinical Pharmacology, Department of Medicine, University College London, The Rayne Institute, UK

ATP-sensitive K+ channels are an octameric assembly of
two proteins, a sulfonylurea receptor (SUR1) and an ion
conducting subunit (Kir 6.0). We have examined the role of
the C-terminus of SUR1 by expressing a series of truncation
mutants together with Kir6.2 stably in HEK293 cells. Biochemical analyses using coimmunoprecipitation indicate
that SUR1 deletion mutants and Kir6.2 assemble and that a
SUR1 deletion mutant binds glibenclamide with high affinity. Electrophysiological recordings indicate that ATP sensitivity is normal but the response of the mutant channel
complexes to tolbutamide, MgADP and diazoxide is dis-

turbed. Quantitative immunofluorescence and cell surface
biotinylation supports the idea that there is little disturbance
in the efficiency of trafficking. Our data show that deletions
of the C-terminal most cytoplasmic domain of SUR1, can
result in functional channels at the plasma membrane in
mammalian cells that have an abnormal response to physiological and pharmacological agents.

ATP-sensitive potassium channels (KATP) are present in the
plasma membrane of a number of tissues and are also
present in endomembranes such as mitochondria. They
have been proposed to be involved in a number of
physiological and pathophysiological processes and form

a link between cellular metabolism and membrane excitability. For example, in the pancreas, KATP regulates insulin
release and in vascular smooth muscle, it is regulated by
vasodilators and influences blood flow in certain vascular
beds. KATP is an octameric protein complex composed of
two subunit types namely a pore forming subunit (Kir6.1,
Kir6.2), a member of the inwardly rectifying family of K+
channel, and the sulfonylurea receptor subunit, a member of
the ATP binding cassette family of proteins (SUR1,
SUR2A, SUR2B). The assembly of a particular pore
forming subunit with a particular SUR generates currents
with a characteristic single-channel conductance, nucleotide
regulation and pharmacology [1–4]. Kir subunits have a
cytoplasmic N and C terminus with two transmembrane
domains and a pore forming H5 loop [5,6]. SUR has
multiple transmembrane domains with two large intracytoplasmic loops, the first and second nucleotide binding
domains (NBD1 and NBD2), which contain consensus
sequences for the hydrolysis of nucleotides (Walker A and B
motifs) [7,8].

The trafficking of the KATP channel complex has been
the subject of some investigation. It was initially observed
that coexpression of the two proteins was necessary to
generate significant plasmalemmal currents [9]. A series of
studies have supported the idea that the Kir6.2 and
SUR1 subunits have small peptide motifs (RKR) that
either prevent the export of the protein from the ER and/
or retrieve it from the Golgi [10]. The simultaneous
masking of these two signals by interaction of the two
proteins allows the channel complex to proceed through
the biosynthetic pathway to the membrane. It has also

been suggested that the most distal part of the C-terminus
of SUR1 contains an anterograde signal that allows
export from the ER [11]. Potentially, this has clinical
ramifications as a number of mutations in persistent
hyperinsulinaemic hypoglycaemia of infancy [12] cluster in
this domain of the protein. Persistent hyperinsulinaemic
hypoglycaemia of infancy is an hereditary disease characterized by inappropriately high levels of insulin release
and hypoglycaemia in children at birth. It is hypothesized
that deletion of the most distal part of the C-terminus
(only seven amino acids) leads to the removal of a
forward trafficking signal and retention in the ER. A
phenylalanine (1574) and a leucine (1566) were established
as being particularly important. However a splice variant
of SUR1 has been described and cloned from a
hypothalamic cDNA library that removes exon 33 and
results in a frameshift and premature termination of the
C-terminus preceding the Walker A and B motifs [13].
Two other deletions were constructed at the beginning
and end of the C-terminus in exon 33. All these were
functional upon expression and the truncation of 253
amino acids did not affect the magnitude of macroscopic
currents. In a related vein, a recent report has shown a
SUR1-MRP1 C-terminal chimaera is able to traffic to the
plasma membrane when coexpressed with Kir6.2 [14].

Correspondence to A. Tinker, Room F2, 4th Floor, Centre for Clinical
Pharmacology, Department of Medicine, University College London,
The Rayne Institute, 5 University Street, London WC1E 6JJ, UK,
Fax: + 44 20 76912838, Tel.: + 44 20 76796192,
E-mail:

*Note: These authors contributed equally to this work
(Received 2 July 2002, revised 19 August 2002,
accepted 11 September 2002)

Keywords: K channel; inward rectifier; ATP-sensitive; trafficking and assembly; sulfonylurea receptor.


Ó FEBS 2002

5304 J. P. Giblin et al. (Eur. J. Biochem. 269)

There are several potential explanations for these different
results. Firstly, there are differences in the expression
system used (Cos cells in [11] vs. Xenopus laevis oocytes in
[13] and [14]) and secondly the details of the deletions
vary. To try to resolve these differences we have
constructed a series of deletions in the second nucleotide
binding domain, expressed and studied their biochemical
and functional behaviour in a human kidney cell line
(HEK293 cells).

MATERIALS AND METHODS
Molecular biology
Standard subcloning techniques were used throughout. A
SUR1 mutant with a myc epitope was used as previously
described [15]. In our initial studies we constructed a 146
amino acid deletion of SUR1 with the myc epitope. The
pcDNA3 vector was digested with NotI/ApaI. Sense and
antisense oligonucleotides corresponding to an artificial
gene fragment encoding the myc epitope and stop codon

together with compatible overhangs were annealed and
ligated into pcDNA3. A NotI fragment of SUR1 (from
SUR1 in pcDNA3) was then subcloned into this construct. Subsequently a range of SUR1 deletions, tagged
with the 10 amino acids constituting the myc epitope, were
constructed from SUR1 in pBluescript (SK–). A two stage
PCR strategy was used to generate a deletion cassette with
a 5¢-SfiI site in the clone and the myc epitope followed by
a stop codon and an AvrII site. The native fragment was
replaced by this cassette and resulting product subcloned
into the XhoI/XbaI sites of pcDNA3 using a SalI/SpeI
digest. Kir6.2 and Kir6.2mycHis6 was expressed in
pcDNA3.1/Zeo (Invitrogen). Kir6.2mycHis6 was generated from a previous study [15]. The sequence of all
mutants was confirmed by DNA sequencing using the
dRhodamine Terminator cycle sequencing kit (Applied
Biosciences) and an automatic sequencer (ABI 377,
Perkin-Elmer).
Cell culture and transfection
HEK293 cells were cultured, transfected and stable cell lines
expressing Kir6.2, Kir6.2mycHis6 and SUR1-myc and
deletion mutants generated as previously described [15]. A
number of new monoclonal lines have been generated in this
study and they are detailed as appropriate in the text. For
selection with G418 we used 727 lgỈmL)1, for Zeocin
364 lgỈmL)1 and in combination 727 lgỈmL)1 G418 and
364 lgỈmL)1 Zeocin.
Antiserum production in rabbits
A peptide corresponding to amino acid residues 942–955
of hamster SUR1 (ETVMERKASEPSQGC, final cysteine
added for coupling purposes) was synthesized and linked
to keyhole limpet haemocyanin before injection into

rabbits using standard protocols (Regal Group Ltd,
Great Bookham, Surrey, UK). Bleeds were assayed for
activity using an antibody capture assay [16] and bleeds
showing reactivity were affinity purified. Kir6.2 antisera to
the C-terminus of the channel and myc hybridoma cells
were used as previously described [15,17].

Gel electrophoresis, radioligand binding,
immunoprecipitation and immunofluorescence
SDS/PAGE, Western blotting, radioligand binding,
immunoprecipitation and immunofluorescence staining
were carried out as previously described [15,17] with some
modifications for colocalization experiments. In colocalization experiments slides were incubated with the first
primary antibody [a 1 : 500 dilution of anti-(Kir6.2
C-terminus) Ig] and the appropriate secondary antibody
(a 1 : 300 dilution of a rhodamine-linked secondary goat
anti-rabbit Ig) for 1 h each as previously described [17].
After washing, the second primary antibody was applied
overnight (a 1 : 500 dilution of anti-myc Ig) and the
appropriate secondary antibody (a 1 : 300 dilution of a
fluorescein-linked secondary goat anti-mouse Ig) was then
applied the following day for 1 h. The antibody reactive to
the Kir6.2 C-terminus was raised to the peptide sequence
DALTLASSGPLRKRSC and has been characterized
previously [17]. The anti-myc Ig used was purified on
Protein A Sepharose CL-4B (Amersham-Pharmacia biotech) from mouse 9E10 hybridoma cell line culture
supernatant under high salt conditions using a standard
method (p311; [16]). All fluorophore conjugated secondary
antibodies were purchased from Molecular Probes Inc.
Gel densitometry was performed using SCION IMAGE as

detailed in the help section in the program.
For the imaging work, slides were viewed and analysed
using a computer based image analysis system (OPENLAB 3.1,
Improvision) coupled to a Zeiss Axiovert 100M microscope
set up for epifluorescence equipped with an appropriate
filter set that allowed imaging of both fluorescein and
rhodamine fluorophores (XF66-1 multiband filter, Omega
optical). Images used for quantitative analysis and colocalization experiments represented a focal plane taken through
the middle of the cell. Scattered and out-of-focus fluorescent
signal was removed from the image by deconvolution. The
image was deconvolved from a z-stack of 31 images with
0.2 lm spacing using the OPENLAB software (volume
deconvolution module). The quantitative assay was performed on the deconvolved images using the OPENLAB
advanced measurements module. The theory behind the
quantitative assay is given in Fig. 6A. In colocalization
experiments, deconvolved images of the cell at each
excitatory wavelength were obtained and merged using the
OPENLAB software.
Procedure for surface biotinylation
Stable cell lines were cultured in 100 mm2 tissue culture
dishes as previously described [15] until 75–95% confluent.
Each dish was washed three times with ice-cold phosphate
buffered saline (NaCl/Pi – 10 mM phosphate buffer,
2.7 mM KCl, 137 mM NaCl, pH 7.4, prepared from
tablets supplied from Sigma, Poole, UK) before incubation with 3 mL 0.5 mgỈmL)1 EZ-Link Sulfo-NHS-LCBiotin (Pierce, Rockford, Illinois, USA) in NaCl/Pi at
4 °C for 30 min. Dishes were washed twice with ice-cold
NaCl/Pi before incubation with NaCl/Pi + 100 mM glycine at 4 °C for 20 min to quench any remaining
unreacted biotin reagent. Cells were then lysed by scraping
into 250 lL 1% (w/v) SDS in Tris buffered saline (Tris/
NaCl – 50 mM TrisHCl, 150 mM NaCl, 5 mM KCl,

Ó FEBS 2002

Role of the cytoplasmic C-terminus of SUR1 (Eur. J. Biochem. 269) 5305

pH 7.4) containing 1 mM phenylmethylsulfonyl fluoride
and a protease inhibitor cocktail (Roche Complete
EDTA-free, Roche Diagnostics Limited, Lewes, Sussex,
UK). The lysates were subsequently incubated at 65 °C
for 10 min before dilution with 1 mL 1% (v/v) Triton in
Tris/NaCl containing 1 mM phenylmethanesulfonyl fluoride and the protease inhibitor cocktail. Lysates were then
incubated on ice for 1 h before being briefly sonicated (2 s
using an MSE Soniprep 150 probe sonicator at half
power). Sonicated lysates were centrifuged at 20 000 g for
30 min to pellet any insoluble material and biotinylated
proteins were isolated from the supernatant as described
below. A 50-lL sample was removed at this point and
used for subsequent SDS/PAGE and Western blotting
analysis.
Isolation of surface biotinylated proteins
The supernatant obtained from the surface biotinylation
procedure was incubated with 150 lL of a pre-equilibrated
1 : 1 slurry of Ultra-link Immobilized NeutrAvidin biotin
binding protein (Pierce) overnight at 4 °C with gentle
rotation. The slurry was pre-equilibrated with binding
buffer [1% (v/v) Triton, 0.20% (w/v) SDS in Tris/NaCl].
After the incubation period, the binding resin was pelleted
by centrifugation (20 000 g for 2 min at 4 °C) followed by
five washes with 1 mL binding buffer. Bound protein was

recovered by incubation with 80 lL 6 · Laemmli gel
loading buffer (350 mM TrisHCl pH 6.8, 10.28 (w/v) SDS,
36% (v/v) glycerol, 0.012% (w/v) bromophenol blue,
200 mM dithiothreitol) at 100 °C for 3 min. Eluted proteins
were subsequently analysed by SDS/PAGE followed by
Western blotting.
Electrophysiology
Whole-cell and inside-out patch clamp recordings were
performed using an Axopatch 200B amplifier (Axon
Instruments) and were digitized using a Digidata 1200
interface before capture to a computer hard disk. Whole-cell
current signals were filtered at 1 kHz and sampled at 2 kHz,
and analysed using PCLAMP6 software (Axon Instruments).
Unitary single-channel currents were filtered at 2 kHz and
sampled at 5 kHz, and analysed to determine either mean
NPo (Number of channels · Open probability) during a 30
second sweep (using pClamp6), or mean current (using
Satori, Intracel Ltd). Patch pipettes were pulled using a PP830 pipette puller, and fire-polished using a MF-830
microforge (both Narishige). Pipettes had resistances of
1.5–3 MW for whole-cell recording and 6–9 MW for
single-channel recordings. The capacitance of pipettes was
reduced by coating pipettes with a parafilm/mineral oil
suspension and compensated for by using the amplifier.
Series resistance during whole cell recording was compensated to at least 70% using amplifier circuitry. The pipette/
bath solution contained in mM; 107 KCl, 1.2 MgCl2,
1 CaCl2, 10 EGTA, 5 Hepes (with 33 mM KOH to pH 7.2)
and the bath/pipette solution; 140 KCl, 2.6 CaCl2, 1.2
MgCl2, 5 Hepes (pH 7.4) for whole cell/single-channel
work. Whole-cell pipette solutions were supplemented with
ATP and ADP, and pH was re-adjusted to 7.2 (see figure

legends for nucleotide concentrations used in each stable
line).

Data analysis
Radioligand binding experiments were fitted by the
binding isotherm y ¼ BmaxỈx/(x + Kd) where y is the
bound specific radioligand (pmolỈmg protein)1) and x is
the radioligand concentration. For dose–response curves
with inhibition of current by tolbutamide the data from
individual experiments were expressed and fitted to:
% inhibition of maximal current ¼ a + (b ) a)/(1 +
(x/Ki)h) where b was fixed at zero and a represents the
limiting inhibition (expressed as percentage inhibition of
maximal current), h the hill coefficient and Ki the EC50
for inhibition. Mean Ki was calculated by fitting
individual experiments with the curves and calculating
mean parameters as indicated. Statistical analysis was
carried out using one-way ANOVA with an appropriate
posthoc test or Students t-test as appropriate (ORIGIN
v6.0 and PRISM v3.0). Statistical significance is as
indicated in the legend and text. Data are presented as
mean ± SEM.

RESULTS
To facilitate biochemical studies, we generated a polyclonal
rabbit antisera raised to a short peptide sequence in the first
nucleotide binding domain of SUR1 (see Materials and
methods). Figure 1A and B show the characterization of the
specificity of the antisera for immunoblotting and immunofluorescence, respectively. It is apparent that the antisera
recognizes bands of the correct molecular mass in a

SUR1 + Kir6.2 stable line (a band at  150 kDa and
another at  170 kDa) but not in stable lines expressing
SUR2A + Kir6.2, SUR2B + Kir6.1 and wildtype nontransfected HEK293 cells (Fig. 1A). The antisera also
recognizes a number of other proteins of lower molecular
mass endogenous to HEK293 cells however, this does not
influence the nature of our conclusions below. When
assayed using immunofluorescence, the antisera reacted
with a stable line expressing SUR1 + Kir6.2 but not one
expressing SUR2B + Kir6.1 (Fig. 1B). The signal was
competed with by the immunogenic peptide (Fig. 1B) and
incubation with the secondary alone led to no signal (not
shown).
Generation of SUR1 deletions and stable cell lines
Using standard molecular cloning methods (see Materials
and methods) we generated a series of SUR1 deletions
tagged with the 10 amino acid myc epitope (Fig. 2A). In our
initial studies we first examined a 146 amino acid deletion
(SUR1del146myc) and subsequently engineered a series of
these. We generated a polyclonal stable cell line in HEK293
cells stably expressing SUR1del146myc by selection with
G418 and stable monoclonal cell lines expressing
Kir6.2 + SUR1myc (line A), Kir6.2 + SUR1del101myc
(line B), Kir6.2 + SUR1del145myc (line C1) and
Kir6.2mycHis6 + SUR1del146myc (line C2), Kir6.2 +
SUR1del196myc (line D) and Kir6.2 + SUR1del249myc
(line E) with G418 and Zeocin as previously described
[15,18]. Lines were screened using biochemical and subsequently electrophysiological methods. Line C1 was labile
with multiple passages often losing current. Line C2 was
more stable and could be passaged for long periods without

5306 J. P. Giblin et al. (Eur. J. Biochem. 269)

Ó FEBS 2002

Fig. 1. Characterization of SUR1 antibody by
Western blotting and immunofluorescence. The
Western blot in (A), probed with a 1 : 2000
dilution of anti-SUR1 Ig, is derived from an
8% polyacrylamide gel. Lanes were loaded
with 8 lg of cellular homogenates of the stable
lines indicated. WT denotes untransfected
HEK293 cells. The positions of molecular
mass markers (in kDa) are indicated to the left
of the blot and the position of SUR1 is indicated by the arrow. Note that SUR1 migrates
as a doublet with bands of approximately 150
and 170 kDa in size. The blot represents an
exposure to film of 30 s. The images in (B)
show images of cells from the stable lines
indicated stained with a 1 : 500 dilution of
anti-SUR1 Ig. Reactivity is observed against
cells expressing SUR1 but not against cells
expressing SUR2B. SUR1 reactivity is
abolished by preincubation of the antibody
with 1 mgỈmL)1 antigenic peptide for 1 h. The
rhodamine-linked secondary antibody was
used at a 1 : 300 dilution. All images were
captured at the same exposure (1 s) and
magnification. The scale bar represents 5 lm.

loss of current. As a result most electrophysiological studies
were performed on this line.
Kir6.2 interacts with SUR1 and the SUR1 deletions
We used a coimmunoprecipitation strategy to examine the
interaction of Kir6.2 with SUR1-myc and SUR1-myc
deletions. Immunoprecipitation of SUR1-myc and deletion constructs was performed using a mouse monoclonal
antibody to myc to avoid problems with discrimination
between the rabbit immunoglobulin heavy chain and
Kir6.2 that might occur if the precipitating antibody were
rabbit in origin. Immunoprecipitation of SUR1 was
confirmed by probing with the rabbit polyclonal antisera
to SUR1. Figure 2B shows the coimmunoprecipitation of
Kir6.2 with SUR1-myc and each of the deletions. The
upper blots show the level of protein expression in the
selected lines prior to immunoprecipitation. Kir6.2 is
detected using a rabbit polyclonal antisera raised to the
C-terminus of the protein (see Materials and methods and
[17]). Controls for the immunoprecipitation protocol have
previously been shown [15]. We further showed the close
association and localization of Kir6.2 and SUR1-myc by
costaining cells with the myc mouse monoclonal antibody
and the rabbit anti-(Kir6.2 C-terminus) Ig. In addition, we
also examined the colocalization of Kir6.2 with the most
profound deletion SUR1del249myc. In Fig. 2C it is
apparent that the two signals colocalize in both line A
and line E. Thus the ability of Kir6.2 to assemble with
SUR1 is not grossly affected by deletion of the second
nucleotide binding domain.

Functional properties of the deletion mutants

coexpressed with Kir6.2
Electrophysiological techniques revealed that all the deletion mutants were able to express significant currents at the
plasma membrane of HEK293 cells. In Fig. 3A and B
representative traces are shown from whole cell patch clamp
recordings from line A (Ai and Aii) as compared to the most
profound deletion in line E (Bi and Bii). After rupture of the
membrane to gain access to the intracellular contents,
whole-cell currents increased to a steady-state value. The
magnitude of this was dependent on the pipette ATP
concentration. Currents in line A were substantially inhibited by 100 lM tolbutamide, whereas those in line E were
only partially inhibited (see below). Both currents were
completely inhibited at hyperpolarized potentials by 10 mM
Ba2+ (Fig. 3Aii and Bii). All the SUR1 deletions in the
monoclonal stable lines gave rise to currents significantly in
excess of those from wildtype cells [15]. However the
magnitude of these varied with clonal isolates and some
lines possessed smaller currents than others. With 1.2 mM
ATP in the patch pipette, the current density at )50 mV
was: in line A ¼ 586 ± 117 pA/pF (n ¼ 14); in line
B ¼ 9.5 ± 2.1 pA/pF (n ¼ 11); in line D ¼ 51.2 ± 14.1
pA/pF (n ¼ 7) and in line E ¼ 492 ± 136 pA/pF (n ¼ 11).
With 0.6 mM ATP in the patch pipette, the current density
at )50 mV in line C2 was 313 ± 44 (n ¼ 8) pA/pF. This
electrophysiological behaviour was reflected in variations
in the expression of the relevant components as determined by immunoblotting. Figure 3C shows representative
recordings of single-channels in the inside-out configuration.


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Role of the cytoplasmic C-terminus of SUR1 (Eur. J. Biochem. 269) 5307

Fig. 2. Assembly of SUR1 C-terminal deletion mutants with Kir6.2. The scheme in (A) shows the boundaries of the C-terminal deletion mutants
of SUR1. The putative first nucleotide binding domain (NBD1) lies between amino acid residues 697–895 and the putative second nucleotide
binding domain (NBD2) between residues 1359–1582, as indicated by the shading. The Walker A and B consensus sequences in NBD2 are
located between residues 1379 and 1385 and residues 1503–1507, respectively. The upper panels of Western blots (B) show the expression of the
SUR1 deletion mutants and Kir6.2 in each of the monoclonal lines indicated. Two species of SUR were observed corresponding to putatively
immature and maturely glycosylated forms. Two bands were also sometimes observed for Kir6.2, the smaller band possibly representing a
proteolytic fragment or a partially processed form. Cell line homogenate (8 lg) was loaded into each lane. WT represents a lane loaded with
8 lg of nontransfected cell homogenate. The lower set of blots (B) show the results of coimmunoprecipitation experiments performed on
0.8 mg of solubilized cell line homogenate. The myc monoclonal antibody was used to immunoprecipitate myc-tagged SUR mutants.
Immunoprecipitation of SUR1 mutants with concomitant immunoprecipitation of Kir6.2 was observed for all lines tested, indicating that the
C-terminus of SUR1 is not required for biochemical interaction with Kir6.2. Lanes were loaded with 50% of the total eluate and blots were
probed with the anti-SUR1 and anti-Kir6.2 Ig. All blots shown represent a 30 s exposure to photographic film. Blots were probed with the
antibodies as indicated. The SUR1 and Kir6.2 antibodies were both used at a 1 : 2000 dilution. SUR1 and Kir6.2 were resolved on 8 and 12%
polyacrylamide gels, respectively. The images in (C) show colocalization of Kir6.2 and SUR1myc (line A) and Kir6.2 and SUR1del249myc
(line E). SUR1 was detected using the myc mouse monoclonal antibody purified with Protein A Sepharose (1 : 500 dilution – see Materials and
methods) in conjunction with a fluorescein anti-mouse secondary Ig. Kir6.2 was detected using the polyclonal rabbit anti-(Kir6.2 C-terminus)
Ig (1 : 500 dilution) in conjunction with a rhodamine-linked anti-rabbit secondary Ig. Cells were imaged at each excitatory wavelength and the
resulting images overlaid using the imaging software. The green scale bar represents 5 lm. The images shown represent deconvolved images
(see Materials and methods).


5308 J. P. Giblin et al. (Eur. J. Biochem. 269)

Ó FEBS 2002

Fig. 3. Examples of Kir6.2/SUR1 and NBD2 deletion currents in stably transfected HEK293 cells. (Ai) Voltage-clamp recording of line A
currents, and (Bi) voltage-clamp recording of line E currents, evoked during 250 ms voltage steps between )100 mV and +100 mV in 10 mV
increments from a holding potential of 0 mV. Currents were blocked by 100 lM tolbutamide, and the block was reversed on subsequent

washout. (Aii) Voltage clamp recording of line A current, and (Bii) voltage-clamp recording of line E current, evoked during a 2-s ramp
between )100 mV and +100 mV, showing block by 10 mM BaCl2 (dashed line), which was reversible on washout. (C) Inside-out patch
recordings of single-channel currents through stably expressed channels at a holding potential of )60 mV, with inhibition of channel activity by
100 lM ATP shown in both line A (Ci) and line E (Cii). All currents were recorded in symmetrical 140 mM K+, and whole cell recordings
carried out with 1.2 mM ATP in the patch pipette.

Fig. 4. Effects of deletions in the NBD2 domain of SUR1 on channel sensitivity to glibenclamide, diazoxide and tolbutamide. (A) Representative
radioligand binding curve from a single experiment. Points are specific binding activity (bars indicate SD of triplicate results). In this particular
example, the best fit binding isotherm has a Kd ¼ 1.47 nM and Bmax ẳ 30.1 pmolặmg protein)1. (B) Eects of 100 lM tolbutamide on currents in
lines A, B, C2, D and E, expressed as percentage inhibition of maximal current, with 1.2 mM ATP in the patch pipette for lines A and E, 600 lM
ATP for line C2 and 100 lM ATP for lines B and D. (C) Concentration–response curves for the effect of tolbutamide (1–500 lM) on A (d), E (j)
and C2 (n) currents, expressed as percentage inhibition of maximal current. Figures for Ki, h and a in text. Pipette ATP concentrations as in (B). All
currents recorded at )60 mV in symmetrical 140 mM K+. (D) Effect of 10 min perfusion with 300 lM diazoxide on currents in lines A, B, C2, D
and E. Currents expressed as I/Ic, with 3 mM ATP in the patch pipette in lines A, E and D and 1.2 mM ATP in the pipette in lines B and C2. Oneway ANOVA using Dunnett’s multiple comparison test was used to test for a significant difference in the effects on lines B-E, compared with A, where
* shows P < 0.05 and ** shows P < 0.001. Numbers in brackets denote number of samples.


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Role of the cytoplasmic C-terminus of SUR1 (Eur. J. Biochem. 269) 5309

It shows the inhibition of channels in line A and E by ATP.
The deletion of the C-terminus of SUR1 does not substantially affect the response to this nucleotide (Fig. 3Ci and ii)
and quantitation is shown in Fig. 5B.
However other electrophysiological and pharmacological
parameters were disturbed. The interaction of the channel
complex with the sulfonylurea class of drug was examined
next. We first performed radioligand binding with tritiated
glibenclamide to the polyclonal SUR1del146myc cell line.
The Kd for drug binding is not significantly changed: for

SUR1del146myc Kd ¼ 2.02 ± 0.35 nM (n ¼ 6) (a representative example is shown in Fig. 4A) and for SUR1-myc as
previously published [15] Kd ¼ 1.25 ± 0.17 nM (n ¼ 4)
(not significant, P ¼ 0.12 unpaired t-test after logarithmic
transformation). Next, we examined the electrophysiological
effect of tolbutamide using the whole cell configuration of the
patch clamp (Fig. 4B.C). Concentrations of tolbutamide
(100 lM) known to act selectively by binding to SUR1 and
not to have significant pore blocking effects were used. The
lines containing the SUR1 deletions had currents that were
reduced but not to the same extent compared to control.
Figure 4B summarizes the percentage inhibition of current in
these lines. Thus progressive deletion of the C-terminus of
SUR1 affects the efficacy of drug response. To further
characterize this, dose–response curves were constructed for
two of these and it was found that the Ki was not significantly
affected once the differences in limiting value were taken
into account. The Ki for tolbutamide inhibition of the current
in line A was 8.1 ± 3.3 lM and h ¼ 1.45 ± 0.1 and
a ¼ 87.3 ± 3.9 (n ¼ 10), compared with 18.7 ± 7.5 lM,
h ¼ 1.63 ± 0.3 and a ¼ 51.3 ± 4.0 for line C2 (n ¼ 6) and
6.2 ± 2.8 lM, h ¼ 1.43 ± 0.33 and a ¼ 33.0 ± 4.2 for
line E (n ¼ 8) (for Ki P ¼ 0.09 after log transformation for
all comparisons, one way ANOVA with Bonferronni correction). Figure 4C shows the mean pooled data at each point
fitted with the best-fit curve (to the means) using nonlinear
regression. The resultant parameters are very similar but not
identical to those determined after analysis of each individual
experiment. The latter however, allows statistical analysis of
the relevant data.
The response to bath applied diazoxide was determined in
the whole-cell configuration. The application of 300 lM

diazoxide induced large whole-cell currents in line A but led
to little increase in the other lines (Fig. 4D). Recent studies
have shown an interrelationship between diazoxide stimulation and MgADP stimulation [19,20]. Thus we examined
bath applied MgADP in inside-out patches on line A and
line E. In the former line there was pronounced stimulation
whilst in the latter there was a much more modest
stimulation (Fig. 5A and B). We also performed analogous
experiments with line C2 and observed very little stimulation
with MgADP (not shown).
In summary, the deletion of the C-terminus of SUR1
results in functional channels that cannot be stimulated by
diazoxide and MgADP. The affinity of sulfonylurea interaction is not affected, however, the net resultant inhibitory
effect is decreased as the C-terminus is progressively deleted.
Membrane trafficking studied using
immunofluorescence
The presence of significant membrane currents is an
indication that trafficking is not significantly impaired.

Fig. 5. Effects of deletions on channel sensitivity to ADP. (A) examples
of inside-out patch recordings of single-channel currents through stably expressed channels, with unitary single-channel A currents shown
in (Ai) and unitary single-channel E currents shown in (Aii). Both
records show the effects on channel activity of application of 100 lM
(Mg)ATP and 500 lM (Mg)ADP. (B) Comparison of normalized (I/Ic)
single-channel currents between A (n ¼ 9) and E (n ¼ 5) in the presence of 100 lM ATP and 100 lM ATP with 500 lM ADP. Student’s
paired t-test used to test for a significant change in normalized NPo on
addition of ADP, where * ¼ P < 0.05. All currents recorded at
)60 mV in symmetrical 140 mM K+.

However the possibility of differences in the relative level
of expression of the different mutants may confound such

a picture. For example, it is possible for there to be
significant membrane currents but for a significant
fraction of protein to be retained intracellularly. A
number of elegant and imaginative methods have been
developed to examine these problems [10,21]; however,
they suffer from the complication that calibration for
relative expression must be obtained independently. We
took a different approach. The fraction of immunofluorescent signal at or close to the plasma membrane was
calculated in a number of cells. First a mask was defined
round the edge of the cell and then a further mask was
defined (performed by shrinking the original mask by
0.4 lm in each direction), which excludes the membrane
and membrane associated area but includes the intracellular contents. The total fluoresence in each situation is
calculated from mean pixel intensity multiplied by the
number of pixels. The ÔmembraneÕ associated fluoresence
is calculated by subtracting the two and the ÔmembraneÕ


5310 J. P. Giblin et al. (Eur. J. Biochem. 269)

signal is normalized to the total fluorescence. Potentially
this controls internally within each cell for differences in
expression. The principle of the assay is shown in Fig. 6A.
The sensitivity of such an assay was tested in detecting
the membrane translocation of SUR1 and Kir6.2 when
expressed independently and in the situation where they are
coexpressed. To facilitate these studies we generated stable
lines expressing Kir6.2 with Zeocin selection and SUR1
with G418 (see Materials and methods). The fraction of
immunofluorescence associated with the membrane was

compared between SUR1 and SUR1 + Kir6.2 stable lines
probed with the anti-SUR1 Ig and between Kir6.2 and
Kir6.2 + SUR1 stable lines probed with the anti-(Kir6.2
C-terminus) Ig. Representative images and the quantitative
data are shown in Fig. 6B–D. In both cases there was a
measurable and significant increase in membrane associated
immunofluorescence in keeping with the visual appearance
of the images (see Discussion). The fraction retained upon
single expression of a subunit (SUR1 or Kir6.2) and the
increase upon coexpression was approximately the same
when detected by either antibody.
We then used this approach to examine the trafficking of
Kir6.2 in lines A and line E. Representative images and the

Ó FEBS 2002

quantitative data are shown in Fig. 7A and B. We were
unable to detect a significant difference in trafficking of
Kir6.2 when coexpressed with full length SUR1-myc and
the most profound deletion, SUR1del249myc. In other
words even with the most profound deletion there is no
gross alteration in the efficiency of trafficking as assessed
using immunofluorescence.
As our assay will not distinguish between protein
located at the membrane or just below it, we used a
further experimental approach to examine this question.
We labelled membrane proteins with biotin using a cell
impermeable reagent (see Materials and methods) and
purified the resulting products using an avidin derivative
complexed to a solid support. A sample of the cellular

lysate prior to purification and samples after purification
are then subjected to Western blotting with antibodies to
SUR1 and Kir6.2. Figure 8A shows that only small
amounts of SUR1 are labelled when expressed in a stable
cell line alone. However, coexpression of SUR1-myc with
Kir6.2 in line A results in significant labelling. Furthermore, coexpression of Kir6.2 with the most profound
deletion, SUR1del249myc, in line E results in comparable
signal. We also noted that Kir6.2 was purified under such

Fig. 6. Study of KATP channel subunit trafficking using immunofluorescence. The scheme
in (A) shows the principle of the quantitative
trafficking assay. The images in (B) show
representative deconvolved images of cells
expressing either Kir6.2 or Kir6.2 + SUR1
stained with a 1 : 500 dilution of anti-(Kir6.2
C-terminus) Ig. The images in (C) show representative deconvolved images of cells expressing either SUR1 or Kir6.2 + SUR1
stained with a 1 : 500 dilution of anti-SUR1
Ig. An anti-rabbit rhodamine-linked secondary Ig was used to detect bound primary
antibody. Note the increase in membrane associated staining when both subunits are coexpressed. The graphs in (D) show the data
from the quantitative trafficking assay. It can
be observed that coexpressing of channel
subunits significantly changes the percentage
of membrane associated fluorescence corresponding to both Kir6.2 and SUR1. The scale
bar on the images represents 5 lm.


Ó FEBS 2002

Role of the cytoplasmic C-terminus of SUR1 (Eur. J. Biochem. 269) 5311

Fig. 7. Analysis of the effect of coexpression of SUR1del249myc on
Kir6.2 trafficking. The images in (A) show representative deconvolved images of cells coexpressing either Kir6.2 + SUR1myc (line
A) or Kir6.2 + SUR1del249myc (line E) stained with a 1 : 500
dilution of the anti-(Kir6.2 C-terminus) Ig. Note the presence of
membrane associated staining in both images. The scale bar on
the images represents 5 lm. The data obtained from the quantitative trafficking assay is shown in (B). There is no significant
difference in the percentage of fluorescence associated with the
membrane when Kir6.2 is coexpressed with either SUR1myc or
SUR1del249myc.

conditions (Fig. 8B) and due to the presence generally of
a single distinct band was better suited to quantification.
Thus assuming a high efficiency for purification it is
possible to estimate the fraction of cell surface protein by
performing gel densitometry (see Materials and methods
and figure legend). In line A 2.55 ± 1.2% and in line E
1.93 ± 0.38% of Kir6.2 is surface biotinylated (n ¼ 4,
not a significant difference). Finally, our conclusion is
further supported by the presence of substantial and
comparable currents in line A and line E.

DISCUSSION
In this study we report a comprehensive study in a
mammalian cell line of the effects of deletion of the SUR1
C-terminus on assembly, trafficking and function of the
KATP channel complex. The data reported here show that
the cytoplasmic C-terminus of SUR1 has a key role in
channel function but is not absolutely required for complex
assembly or trafficking. Our data support aspects of
previous studies but also extend those observations

[11,13,14].
The cytoplasmic C-terminus of SUR1 containing NBD2
is mutated in persistent hyperinsulinaemic hypoglycaemia
of infancy. The disease is autosomal recessive and is
characterized by profound neonatal hypoglycaemia and

Fig. 8. Analysis of channel subunit trafficking using surface biotinylation. The Western blots shown in (A) and (B) were probed with
1 : 2000 dilutions of anti-SUR1 and anti-Kir6.2 Ig, respectively. Lanes
labelled lysate were loaded with approximately 1.5% of the total lysate
obtained from the surface biotinylated cells. The lanes labelled AP
were loaded with approximately a third of eluate obtained from the
neutravidin binding column and corresponds to surface-biotinylated
protein. The blots shown represent exposures to film of 20–25 s. The
positions of molecular mass markers and their sizes in kDa are shown
on the left of each blot. The experiments were repeated on three
further occasions with similar results. In (A) it can be observed
that coexpression of Kir6.2 + SUR1myc (line A) and Kir6.2 +
SUR1del249myc (line E) results in increased surface biotinylated
SUR1 protein compared with SUR1 expressed alone. In (B), surface
biotinylated Kir6.2 can be observed when coexpressed with either
SUR1myc or SUR1del249myc. Quantitative analysis was also performed to show that there was no difference in the proportion of
surface biotinylated Kir6.2 when coexpressed with either SUR1myc or
SUR1del249myc (refer to main text for more details).

inappropriate insulin secretion from the pancreas. A
number of point mutations cluster in this region of
SUR1; the resulting frameshift mutations lead to premature truncation of the protein [12]. A number of studies
have examined various aspects of the role of the
C-terminus, including NBD2, in KATP channel function.
Sharma et al. [11] performed limited deletions of the

C-terminus and identified a forward trafficking signal that
increased plasma membrane currents. The removal of as
few as seven amino acids led to loss of channels at the
plasma membrane based on the reduction of sulfonylurea
inhibited Rb+ flux, a chemiluminescent trafficking assay
and the absence of an apparent higher molecular mass
glycosylated species of SUR1. In contrast, Sakura et al.
[13] observed pronounced expression of plasma membrane
currents that was not quantitatively different from wildtype. In addition, Schwappach et al. [14] showed that a


Ó FEBS 2002

5312 J. P. Giblin et al. (Eur. J. Biochem. 269)

SUR1-MRP1 C-terminal chimaera (in which the C-terminus of SUR1 was replaced by MRP1) when coexpressed
with Kir6.2 could reach the plasma membrane. One
potential explanation for this is that the trafficking in
these two systems is quite different. Indeed it is now well
established that the commonest of the mutations (DF508)
in the CFTR protein (cystic fibrosis transmembrane
conductance regulator) causing cystic fibrosis forms a
functional chloride conductance in Xenopus laevis oocytes
but not in mammalian cells. The explanation is that the
trafficking is temperature dependent: it occurs at 23 but not
37 °C [22,23]. Furthermore, there are a number of rarer
mutations in cystic fibrosis that affect mainly the
C-terminus of CFTR and lead to premature truncation
of the protein in an analogous fashion to those occurring in
SUR1 in persistent hyperinsulinaemic hypoglycaemia of

infancy. In a series of elegant studies, Lukacs and
colleagues have shown that trafficking through the secretory pathway is not disturbed and maturation occurs
normally [24,25]. However C-terminally deleted CFTR is
degraded more quickly than wild-type protein via a novel
transport mechanism that occurs from the plasma membrane to the proteosome [24,25]. Our data in a human cell
line support the idea that the deletion of the C-terminus of
SUR1 does not give channel complexes predisposed to
temperature dependent trafficking. We also examined
whether there was a subtle relative trafficking defect by
using a quantitative immunofluorescence assay and cell
surface biotinylation and were unable to demonstrate
major disturbances in the trafficking. However the assays
may not be sensitive enough to pick up a decrease in
forward trafficking from ER to Golgi or increase in
degradation rate with C-terminal deletion as occurs with
CFTR. It is also conceivable, however, that these mutants
may traffic differently in different cell lines and that there
may be trafficking determinants in the region defined by
the largest and smallest deletions. It is worth pointing out
that the immunofluoresence assay will not distinguish
between protein at the membrane or in submembranous
vesicles. As might be expected given the ability of the
SUR1 deletion mutants to form functional channels at the
membrane, there was also no gross disturbance of interaction between the subunits as assessed by coimmunoprecipitation.
However, our data do reveal quite profound disturbances
in the function of the mutant channel complex. The ATP
sensitivity is not significantly disturbed and this is supported
by more detailed studies in Xenopus laevis oocytes [13].
More interestingly however, as observed by Sakura et al.,
profound deletion of the C-terminus of SUR1 generates

channels that are insensitive to MgADP and diazoxide. In
addition we demonstrate here that only relative minor
deletions also impair the ability of diazoxide to activate the
channels. In particular intact Walker A and B motifs are
necessary for diazoxide activity. The current models of the
interrelationship of channel opener binding and action,
ATP binding and hydrolysis and MgADP binding and
stimulation are complex [19,20,26,27,27–34]. Much of this
data is compatible with the idea that the hydrolysis of
MgATP at NBD2 results in an MgADP bound conformation that leads to channel activation. Openers stabilize the
latter state and this effect is modulated by nucleotide
binding at NBD1. Our data emphasize the central role that

a fully intact NBD2 of SUR1 plays in stimulation by
MgADP and diazoxide and support the above hypothesis.
Sulfonylurea action is more subtly altered. The Kd for
tritiated glibenclamide binding is not significantly changed.
However tolbutamide, at doses known to interact predominantly with the sulfonylurea receptor [35], causes only a
partial reduction in current but with an identical Ki.
Tolbutamide has the same affinity for interaction but the
efficacy is decreased. Our data differ from those of Sakura
et al. [13] who found no difference for high affinity
tolbutamide block when measuring whole-cell currents in
two-electrode voltage clamp recordings of macroscopic
currents in Xenopus laevis oocytes (they did see some
reduction in effect in inside-out patches). However, they
used metabolic poisoning to elicit these currents and this
may account for the difference. In addition, we demonstrate
that this phenomenon occurs with even relatively modest
deletions.

What do the observations say about the mechanism of
sulfonylurea block? Recent studies [36,37] indicate that the
binding pocket is located within the final group of
transmembrane domains between NBD1 and NBD2. It is
interesting that the coexpression of two SUR hemi molecules is necessary for the full reconstitution of the
sulfonylurea binding site [38]. Our data are compatible with
the idea that the binding is unaltered but occupation of its
site results in less pronounced effects and a change in
efficacy dependent on NBD2. It emphasizes that the
occupation of the sulfonylurea binding site need not
necessarily translate itself into channel closure [39]. Secondly
it suggests that a significant proportion of the sulfonylurea
effect may be directly related to the antagonism of MgADP
induced openings dependent on an intact Walker A and B
motif in NBD2. Speculatively, it supports an emerging
picture in which NBD2 and the final group of transmembrane domains interact functionally to modulate the pore
forming subunit. Finally, these studies demonstrate as in
our previous work [15] that assembly and trafficking may be
well preserved but functional coupling between SUR1 and
Kir6.2 is critically determined by a number of interdependent factors.

ACKNOWLEDGEMENTS
This work was supported by BBSRC, British Heart Foundation,
Diabetes UK and the Wellcome Trust. We are grateful to Professor
S. Seino for providing Kir6.2 cDNAs and Professor J. Bryan for
providing SUR1 cDNA.

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