Báo cáo khoa học: Mapping of the 45M1 epitope to the C-terminal cysteine-rich part of the human MUC5AC mucin potx

Mapping of the 45M1 epitope to the C-terminal
cysteine-rich part of the human MUC5AC mucin
Martin E. Lidell
1
, Jacques Bara
2
and Gunnar C. Hansson
1
1 Department of Medical Biochemistry, Go
¨
teborg University, Sweden
2 U-673 INSERM, Ho
ˆ
pital Saint-Antoine, Paris, France
Mucins are large glycoproteins found on mucosal sur-
faces throughout the body. They are divided into mem-
brane-bound and secreted mucins, and the latter group
can be further subdivided into gel-forming and non-
gel-forming mucins [1]. The gel-forming mucins consti-
tute the main structural component of the mucous layer
protecting the underlying epithelial surfaces against the
often harsh environment present in the lumen. So far,
five gel-forming mucins (MUC2, MUC5B, MUC5AC,
MUC6, and MUC19) have been described, and their
expression appears to be tissue-specific [2–7]. In the
normal situation, MUC5AC is primarily expressed in
the airways and stomach, and it constitutes a major
component of the respiratory and gastric mucus.
The gastric M1 antigen was originally defined by
several polyclonal antibodies, and it was reported that
this antigen is expressed early during human colonic

carcinogenesis [8,9]. In the normal gastrointestinal
tract, M1 is present in the columnar mucous cells of
the surface epithelium of gastric mucosa, but not in
the colon [10]. However, M1 is found in the goblet
cells of fetal colon [11] and in colorectal adenomas
[8,9] and adenocarcinomas [10,12]. M1 has also been
shown to be expressed in human pancreatic ductal
adenocarcinomas [13]. Twelve hybridoma cell lines
secreting mAbs against M1 have been isolated after
screening of their supernatants for strong immunoreac-
tivity on colon adenomas and their lack of reactivity
in normal colon, as observed with the polyclonal anti-
bodies against M1. Two of the mAbs, 2-11M1 and
9-13M1, have been shown to recognize epitopes present
in the N-terminal cysteine-rich part of MUC5AC [14].
One mAb, 1-13M1, recognizes an epitope found in the
second and fourth CysD domains of MUC5AC [14].
Another five mAbs, 19M1, 21M1, 463M, 589M, and
62M1, recognize epitopes mapped to the C-terminal
cysteine-rich part of MUC5AC [15,16]. The epitopes
for the last four mAbs, 2-12M1, 58M1, 166M1, and
Keywords
antibody; 45M1; monoclonal; MUC5AC;
mucin
Correspondence
G. C. Hansson, Department of Medical
Biochemistry, Institute of Biomedicine,
Go
¨
teborg University, Box 440,

405 30 Gothenburg, Sweden
Fax: +46 31 416108
Tel: +46 31 7863488
E-mail:
(Received 25 October 2007, accepted
28 November 2007)
doi:10.1111/j.1742-4658.2007.06215.x
Mucins are large glycoproteins protecting mucosal surfaces throughout the
body. Their expressions are tissue-specific, but in disease states such as cys-
tic fibrosis, inflammation and cancer, this specificity can be disturbed.
MUC5AC is normally expressed in the mucous cells of the epithelia lining
the stomach and the trachea, where it constitutes a major component of
the gastric and respiratory mucus. A number of mAbs have been raised
against the gastric M1 antigen, an early marker for colonic carcinogenesis.
Several of these mAbs recognize epitopes present on MUC5AC, suggesting
that MUC5AC is the antigen. However, some of the mAbs raised against
the gastric M1 antigen are widely used as antibodies against MUC5AC,
despite the fact that their specificity for MUC5AC has not been clearly
shown. In this study, we have tested the reactivity of the latter antibodies
against a recombinantly expressed C-terminal cysteine-rich part of human
MUC5AC. We demonstrate for the first time that the widely used mAb
45M1, as well as 2-12M1 and 166M1, are true antibodies against
MUC5AC, with epitopes located in the C-terminal cysteine-rich part of the
mucin.
Abbreviation
GDPH, Gly-Asp-Pro-His; goat-a-mouse-AP, goat anti-(mouse IgG) coupled to alkaline phosphatase.
FEBS Journal 275 (2008) 481–489 ª 2007 The Authors Journal compilation ª 2007 FEBS 481
45M1, have not been deciphered so far, although they
have been tested against recombinantly expressed por-
tions of MUC5AC. Despite this, 45M1 has been widely

used as a mAb against MUC5AC [17–19], and is even
sold as such. The 2-12M1 mAb is also sold as a
mAb against MUC5AC, although its specificity for
MUC5AC has not been fully established.
In this study, we have tested the reactivities of the
11 mAbs towards the complete C-terminal cysteine-rich
part of human MUC5AC. The results presented here
show that 2-12M1, 166M1 and 45M1 really are mAbs
against MUC5AC and that their epitopes are located
in the C-terminal cysteine-rich part of this mucin.
Results and Discussion
The recombinant C-terminal cysteine-rich part
of human MUC5AC
In this study, the reactivities of the mAbs against M1
were tested against the C-terminal cysteine-rich part of
human MUC5AC. This part was expressed in CHO-K1
cells as fusion proteins containing a myc-tag followed by
either the 1041 C-terminal amino acids (M-MUC5AC-
CH-long) [20] or 959 C-terminal amino acids
(M-MUC5AC-CH-short) of human MUC5AC and a
His-tag (Fig. 1). The murine Ig j-chain signal sequence
was used to direct the protein synthesis into the secre-
tory pathway. L31 [21], the cDNA clone used in previ-
ous studies aimed at mapping the epitopes of the mAbs
against M1 to the C-terminal part of MUC5AC [15,16],
does not encode the complete C-terminal cysteine-rich
part of human MUC5AC. It lacks a sequence in the
5¢-end that corresponds to the major part of the last
CysD domain found in MUC5AC. The missing sequence
is present in the NP3a clone reported by Meerzaman

et al. [22]. The L31 and NP3a clones were used as
templates when constructing the plasmids encoding
M-MUC5AC-CH-long and M-MUC5AC-CH-short;
the former contains the complete C-terminal cysteine-
rich part, and the latter the sequence corresponding to
the L31 clone with an extra nine amino acids added to
its N-terminus (Fig. 1). When expressed in CHO-K1
cells, M-MUC5AC-CH-long forms disulfide-linked
dimers in the endoplasmic reticulum and is partially
cleaved at a Gly-Asp-Pro-His (GDPH) sequence located
in the von Willebrand D4 domain during its transport
through the secretory pathway [20]. After cleavage, the
fragments are still held together by disulfide bonds.
Reduction of these releases a C-terminal fragment
(C2-H) that can be detected with a mAb against His
5
,
and an N-terminal fragment (M-C1) that can be
detected with a mAb against myc. Both the dimer and
the monomer can be detected with either the mAb
against myc or the mAb against His
5
. The recombinant
MUC5AC C-terminus and its cleavage products were
used to map the epitopes recognized by the mAbs
against M1.
Immunoreactivity of mAbs against M1 towards a
recombinant MUC5AC C-terminal cysteine-rich
part
To test the reactivity of the mAbs against M1

towards the human MUC5AC C-terminal cysteine-rich
Fig. 1. The recombinant C-terminal cysteine-rich part of human
MUC5AC. A schematic picture of the recombinant C-terminal cyste-
ine-rich part of human MUC5AC is given in the upper part of the
figure. The GDPH-cleavage site as well as the epitopes for the
mAbs against myc (amyc) and His
5
(aHis
5
) are indicated. M-C1,
N-terminal cleavage fragment; C2-H, C-terminal cleavage fragment.
The lower part of the figure shows an alignment of the N-terminal
MUC5AC sequences of M-MUC5AC-CH-long and M-MUC5AC-CH-
short and the corresponding sequences of human, rat and mouse
MUC5AC. Long, M-MUC5AC-CH long; Short, M-MUC5AC-CH
short; Mouse, mouse MUC5AC (AJ511871) [29]; Rat, rat MUC5AC
(U83139) [30]. ELO9 (AJ001402) [25], L31 (Z48314) [21], NP3a
(U06711) [22]; GeneBank ⁄ EMBL Databank accession numbers are
given in parentheses after each clone.
45M1, a human mAb against MUC5AC M. E. Lidell et al.
482 FEBS Journal 275 (2008) 481–489 ª 2007 The Authors Journal compilation ª 2007 FEBS
part, cell lysate from CHO-K1 cells expressing
M-MUC5AC-CH-long was separated by SDS ⁄ PAGE
under both nonreducing and reducing conditions, and
the proteins were blotted to membranes that were
probed with the different antibodies (Fig. 2). The
results after SDS ⁄ PAGE under nonreducing condi-
tions (Fig. 2A) confirm previous results showing that
19M1, 21M1, 463M, 589M and 62M1 react with the
C-terminal cysteine-rich part of MUC5AC [15,16].

The 1-13M1 and 2-11M1 mAbs do not react with the
recombinant MUC5AC C-terminus. This is in agree-
ment with previous work showing that these anti-
bodies recognize epitopes in the N-terminal part of
MUC5AC [14]. A triplet of reduction-sensitive bands
centered around 150 kDa is detected by 1-13M1.
These bands are also seen in nontransfected CHO-K1
cells, indicating that the bands are nonspecific (data
not shown). More interestingly, 2-12M1, 45M1 and
166M1 react with the recombinant MUC5AC C-termi-
nus. The epitopes of these antibodies have not been
deciphered previously, although they have been ana-
lyzed against the expression product of the L31 clone
[16]. One possibility would therefore be that the
epitopes of these antibodies are located within the
91 amino acids missing in the L31-encoded sequence.
The 58M1 epitope does not seem to be located in the
C-terminal cysteine-rich part of MUC5AC, as no
staining of the recombinant protein is seen. The
dimeric protein is stained by all the reacting anti-
bodies. The monomeric protein is also stained,
although more weakly, with all these antibodies
(longer exposure times, not shown).
The results after SDS ⁄ PAGE under reducing condi-
tions (Fig. 2B) show that, whereas the epitopes of
2-12M1, 45M1, 463M and 589M are reduction-sensi-
tive, those of 19M1 and 21M1 are not. This is in
agreement with previous results [23]. In addition, the
present results show that the 166M1 and 62M1 epi-
topes appear to be insensitive to reduction. The fact

that the recombinant MUC5AC is cleaved during its
biosynthesis allows us to delimit the region where the
reduction-insensitive epitopes are located. As the
M-C1 fragment is detected by 19M1 and 21M1, their
epitopes must be N-terminal to the GDPH-cleavage
site in the von Willebrand D4 domain of MUC5AC.
This is in agreement with the previously identified
region between the last CysD domain and the
von Willebrand D4 domain of MUC5AC [16]. The
62M1 mAb reacts with the C2-H fragment, indicating
that its epitope is located C-terminally to the GDPH-
cleavage site. This is also in agreement with previous
studies, in which the epitope has been located to the
C-terminal part of MUC5AC harboring the von Wille-
brand C domain and cysteine-knot domain [16]. In the
case of 166M1, a very faint band corresponding to the
C2-H fragment was detected, indicating that this mAb
also detects a C-terminal epitope (the band is more
clearly visualized in Fig. 3D, where the blot was devel-
oped for a longer time). Table 1 summarizes the reac-
tivities of the antibodies towards both reduced and
nonreduced M-MUC5AC-CH-long. In conclusion, our
results confirm that 19M1, 21M1, 463M, 589M and
62M1 are antibodies against MUC5AC, and that their
epitopes are found in the C-terminal cysteine-rich part.
More importantly, we find that the previously
unmapped antibodies 2-12M1, 45M1 and 166M1 also
A
B
Fig. 2. Western blot analysis of the recombinant MUC5AC C-termi-

nus using mAbs against M1. Cell lysates of CHO-K1 cells stably
expressing a recombinant MUC5AC C-terminal cysteine-rich part
(M-MUC5AC-CH-long) were separated by SDS ⁄ PAGE [3–
10% (w ⁄ v) gradient gel] under nonreducing (A) and reducing (B)
conditions. After western blotting, the membranes were probed
with mAbs against myc, His
5
or M1. The lane numbers and the
mAb used for detection are indicated in the lower part of the
figure. Dimer, dimeric M-MUC5AC-CH; Monomer, monomeric
M-MUC5AC-CH; M-5AC-CH, reduced M-MUC5AC-CH; C2-H, C-ter-
minal cleavage fragment; M-C1, N-terminal cleavage fragment. The
position of the very faint band that is detected by 166M1 and that
corresponds to the C2-H cleavage fragment is indicated by an
asterisk in (B). Positions of molecular mass standards are indicated
on the right-hand side.
M. E. Lidell et al. 45M1, a human mAb against MUC5AC
FEBS Journal 275 (2008) 481–489 ª 2007 The Authors Journal compilation ª 2007 FEBS 483
are antibodies against MUC5AC, with epitopes located
in the MUC5AC C-terminal cysteine-rich part.
Further localization of the epitopes of 45M1,
2-12 M1 and 166M1
In order to further locate the epitopes of the hitherto
unmapped 45M1, 2-12M1 and 166M1, these mAbs
were screened against M-MUC5AC-CH-long and
M-MUC5AC-CH-short. The 45M1 mAb is sold by
several companies as a specific mAb against
MUC5AC, and has been used as such in a number of
studies [17–19]. The specificity of 45M1 for MUC5AC
has never been clearly demonstrated. As our results

indicated that 45M1 reacted with a recombinant pro-
tein harboring the complete C-terminal cysteine-rich
part of MUC5AC, but not with a protein correspond-
ing to the L31 clone, we hypothesized that the
91 amino acids located N-terminally to the latter were
important for the antibody reactivity. This hypothesis
was tested by using 45M1 for detection after
SDS ⁄ PAGE and western blotting of cell lysates from
Table 1. Immunoreactivities of mAbs against M1 towards the
MUC5AC C-terminal cysteine-rich part. Staining intensity was esti-
mated from ) to +++.
mAbs
Reduced Nonreduced
M-5AC-CH M-C1 C2-H M-5AC-CH
1 Anti-myc +++ +++ ) +++
2 1-13M1 ))))
3 2-11M1 ))))
4 2-12M1 )))+++
5 58M1 ))))
6 19M1 ++ ++ ) +++
7 21M1 ++ ++ ) +++
8 45M1 )))+++
9 166M1 + ) ++
10 463M )))++
11 589M )))++
12 62M1 +++ ) +++ +++
13 Anti-His +++ ) +++ +++
A
B
C

D
Fig. 3. Western blot analysis of the recombinant MUC5AC C-termi-
nus using 45M1, 166M1 and 2-12M1. (A) Cell lysates of CHO-K1
cells and CHO-K1 cells transiently transfected with pSM-MUC5AC-
CH-short were subjected to affinity purification using Dynabeads
Talon beads. The purified proteins and cell lysates from CHO-K1
cells stably expressing M-MUC5AC-CH-long were analyzed by
SDS ⁄ PAGE (3–10% gradient gels) under nonreducing conditions.
After western blotting, the membranes were probed with either
45M1, 463M or mAb against myc (amyc). (B) Immunoprecipitations
using Dynabeads coated with either 45M1 or 463M were per-
formed from cell lysates of CHO-K1 cells, CHO-K1 cells transiently
transfected with pSM-MUC5AC-CH-short, and CHO-K1 cells stably
expressing M-MUC5AC-CH-long. The precipitates were analyzed by
SDS ⁄ PAGE (3–10% gradient gels) under reducing conditions, and
the proteins were blotted onto a membrane that was probed with
the mAb against myc. (C) Cell lysates of CHO-K1 cells and CHO-K1
cells transiently transfected with pSM-MUC5AC-CH-short were
subjected to affinity purification using Dynabeads Talon beads. The
purified proteins and cell lysates from CHO-K1 cells stably express-
ing M-MUC5AC-CH-long were analyzed by SDS ⁄ PAGE (3–10% gra-
dient gels) under nonreducing conditions. After western blotting,
the membranes were probed with 2-12M1. (D) Cell lysates from
CHO-K1 cells and CHO-K1 cells stably expressing M-MUC5AC-CH-
long were separated by SDS ⁄ PAGE (3–10% gel) under both reduc-
ing and nonreducing conditions, and the proteins were blotted
and subjected to detection by 166M1. AP, affinity purified;
R, reduced samples; NR, nonreduced samples; IB, mAb used for
detection; Dimer, dimeric M-MUC5AC-CH; Monomer, monomeric
M-MUC5AC-CH; M-5AC-CH, reduced M-MUC5AC-CH; C2-H, C-ter-

minal cleavage fragment. The weak band corresponding to the
C2-H cleavage fragment is indicated by an asterisk in (D). Positions
of molecular mass standards are indicated on the right-hand side.
45M1, a human mAb against MUC5AC M. E. Lidell et al.
484 FEBS Journal 275 (2008) 481–489 ª 2007 The Authors Journal compilation ª 2007 FEBS
CHO-K1 cells permanently expressing M-MUC5AC-
CH-long and of M-MUC5AC-CH-short affinity
purified from cell lysates of CHO-K1 cells transfected
with the pSM-MUC5AC-CH-short plasmid. The
SDS ⁄ PAGE separation was performed under nonre-
ducing conditions, as the previous results indicated
that the 45M1 epitope was reduction-sensitive. The
result clearly showed that the 91 amino acids N-termi-
nal to the L31 expression product are crucial for 45M1
reactivity, as only M-MUC5AC-CH-long was detected
(Fig. 3A). That similar amounts of M-MUC5AC-short
and M-MUC5AC-long were loaded is shown by the
reactivity of the mAb against myc. One reason for the
requirement of the N-terminal 91 amino acids could
be that these are necessary for the correct folding of
the protein, and that the lack of them leads to a
misfolded protein. The 463M mAb has a reduction-
sensitive epitope, indicating that a correctly folded pro-
tein is necessary for its reactivity. The 463M epitope
has previously been mapped to the von Willebrand
D4 domain of MUC5AC [16]. The result after using
the 463M mAb for detection in the western blot indi-
cates that M-MUC5AC-CH-short is correctly folded
at least as far N-terminally as the von Willebrand
D4 domain, as the reactivity of the mAb towards this

protein is as strong as that against M-MUC5AC-CH-
long. To rule out the possibility that the lack of reac-
tivity of 45M1 against M-MUC5AC-CH-short was
due to the denaturating conditions during SDS ⁄ PAGE,
this mAb as well as 463M was used for immunoprecip-
itations from cell lysates of cells expressing either
M-MUC5AC-CH-short or M-MUC5AC-CH-long
prior to SDS ⁄ PAGE and western blotting (Fig. 3B).
The results after probing the membrane with the mAb
against myc are consistent with those of the previ-
ous experiments, as M-MUC5AC-CH-long but not
M-MUC5AC-CH-short was precipitated by 45M1,
whereas both forms were precipitated by 463M. Hence,
the results imply that the 45M1 epitope is not
destroyed during SDS ⁄ PAGE, but rather that the epi-
tope is located in the part missing in M-MUC5AC-
CH-short. Although one cannot exclude the possibility
that the lack of the N-terminal 91 amino acids leads to
an incorrectly folded protein N-terminally to the
von Willebrand D4 domain, it is more likely that the
epitope for 45M1 is located within this sequence.
Moreover, this 91 amino acid sequence contains a part
of the last CysD domain preceded by a 13 amino acid
sequence that shows little homology with the corre-
sponding rat and mouse sequences. In contrast, the
fragment of the CysD domain, especially the sequence
between amino acids 18 and 52 (in the MUC5AC
sequence of M-MUC5AC-CH-long), shows strong
homology between the rat, mouse and human proteins
(Fig. 1). As 45M1 reacts strongly with the mucin from

both rat, mouse and human [24], our results suggest
that the 45M1 epitope is associated with such a
sequence alone or that it is discontinuous and built up
of parts found in both this sequence and more C-ter-
minal ones.
The 2-12M1 mAb is also sold as a mAb specific
against MUC5AC, although its specificity has not been
clearly demonstrated. The 2-12M1 epitope has been
reported to be reduction-sensitive [12], an observation
also supported by our results (Fig. 2B). The 2-12M1
mAb has previously been tested against COS-7 cells
transfected with an L31-containing expression plasmid,
but no reactivity against the expression product could
be detected [16]. However, our initial results showed
that 2-12M1 reacted with M-MUC5AC-CH-long
(Fig. 2A), suggesting that the 91 amino acids that are
absent in the L31 expression product could be neces-
sary to build the 2-12M1 epitope. The 2-12M1 mAb
was tested on western blots, like 45M1, after separa-
tion of the samples by SDS ⁄ PAGE under nonreduc-
ing conditions (Fig. 3C). The result shows that
2-12M1 reacted with both M-MUC5AC-long and
M-MUC5AC-short. In M-MUC5AC-CH-short, an
additional nine amino acids have been added N-termi-
nally to the L31 expression product. One possibility is,
therefore, that the epitope is located within these nine
amino acids. The results definitely show that 2-12M1
is a mAb against MUC5AC and that its epitope is
located in the C-terminal cysteine-rich part.
Our results indicated that 166M1 detected an epitope

located C-terminally to the GDPH-cleavage site, as a
very faint band corresponding to the C2-H fragment
was detected when probing western blots of cell lysates
from CHO-K1 cells expressing M-MUC5AC-CH-long
with this mAb (Fig. 2B). To rule out the possibility that
the band was nonspecific, cell lysates from both CHO-
K1 cells and CHO-K1 cells expressing M-MUC5AC-
CH-long were separated by SDS ⁄ PAGE under reducing
and nonreducing conditions, and the proteins were
blotted to a membrane, which was probed with 166M1
(Fig. 3D). The results clearly show that 166M1 detects
both nonreduced and reduced protein and that the
epitope is located within the C-terminal cleavage
fragment, as a specific band corresponding to the
C2-H fragment was detected in the reduced sample.
Testing 45M1, 2-12M1 and 166M1 for
cross-reactivity with MUC2
The gel-forming mucins share common domain struc-
tures and show a high degree of sequence homology,
M. E. Lidell et al. 45M1, a human mAb against MUC5AC
FEBS Journal 275 (2008) 481–489 ª 2007 The Authors Journal compilation ª 2007 FEBS 485
especially with regard to the positions of their cysteines
[1]. In many cases, these mucins also present an over-
lapping expression profile. Hence, it is important to
determine, when a particular antibody against mucin is
used, whether it shows cross-reactivity towards other
mucins. Among the gel-forming mucins, the MUC5AC
C-terminus shows the highest similarities with the
MUC2 C-terminus. To analyze potential cross-reacti-
vity, 45M1, 2-12M1 and 166M1 were tested against

the C-terminal cysteine-rich domain of human MUC2
(Fig. 4). Cell lysates from CHO-K1 cells expressing
either M-MUC5AC-CH-long or a recombinant MUC2
C-terminal cysteine-rich domain were separated by
SDS ⁄ PAGE and blotted onto a membrane. The 45M1,
2-12 M1 and 166M1 mAbs as well as the mAb against
myc were then used for detection. Only the mAb
against myc detected the recombinant MUC2 C-termi-
nal cysteine-rich domain, indicating that the mAbs
against MUC5AC do not cross-react with MUC2.
Hence, although human MUC5AC and MUC2 are
highly similar, 45M1, 2-12 M1 and 166M1 seem to be
specific for MUC5AC.
Conclusions and future aspects
This study shows that the widely used 45M1 as well as
2-12M1 and 166M1 are true mAbs against MUC5AC,
with their epitopes located in the C-terminal cysteine-
rich part of the protein. Fig. 5 shows a more detailed
map of the epitopes for the mAbs, where 45M1 recog-
nizes an epitope located in the N-terminal region of
the C-terminal cysteine-rich part of MUC5AC, pre-
sumably in the last CysD domain of the mucin. The
epitope of 2-12M1 is located in the sequence corre-
sponding to the expression product of the L31 clone,
whereas the epitope for 166M1 is located C-terminally
to the GDPH-cleavage site of MUC5AC. The mAbs
against M1 were selected, by screening the superna-
tants of hybridomas using immunohistology, as having
immunoreactivity against the mucus of goblet cells of
colon adenomas and a lack of reactivity against the

mucus of goblet cells of normal colon [12,16,23]. It is
noteworthy that among the 12 different mAbs isolated
by this technique, 11 recognized the product of one
unique gene: MUC5AC. The fact that the mAbs
against M1 are mapped to different parts of MUC5AC
Fig. 4. Testing the cross-reactivity of 45M1, 166M1 and 2-12M1
towards the C-terminal cysteine-rich domain of human MUC2. Cell
lysates from CHO-K1 cells and CHO-K1 cells stably expressing
either M-MUC5AC-CH-long or the corresponding C-terminal cyste-
ine-rich domain of human MUC2 were separated by SDS ⁄ PAGE
(3–10% gel) under nonreducing conditions. After western blotting,
the mAb against myc (amyc), 45M1, 166M1 or 2-12M1 was used
for detection. IB, mAb used for detection. The positions of the
dimeric forms of the recombinant MUC5AC and MUC2 proteins
are indicated on the left. Positions of molecular mass standards are
indicated on the right-hand side.
Fig. 5. Mapping of the 45M1, 166M1 and 2-12M1 epitopes on the C-terminal cysteine-rich part of human MUC5AC. The upper part of the
figure shows a schematic representation of full-length human MUC5AC with its domains. The C-terminal region of the protein is expanded,
and its domains and GDPH-cleavage site are indicated. In the lower part of the figure, the mapped regions for the 45M1, 166M1 and
2-12M1 epitopes are shown.
45M1, a human mAb against MUC5AC M. E. Lidell et al.
486 FEBS Journal 275 (2008) 481–489 ª 2007 The Authors Journal compilation ª 2007 FEBS
makes them potentially valuable tools for future stud-
ies of this protein. By use of 62M1 and the knowledge
that it reacts with an epitope mapped to the von Wille-
brand C domain and cysteine-knot domain of
MUC5AC, it could be shown in a previous study that
wild-type MUC5AC is partially cleaved at its GDPH
sequence [20]. This is an example showing that the use
of a well-defined mAb against M1 allows studies aimed

at deciphering cleavage patterns of the full-length
mucin. The present study maps the epitopes of three
previously unmapped mAbs against M1 to different
parts of the C-terminal cysteine-rich part of human
MUC5AC. The results showing that the widely used
45M1 is a true mAb against MUC5AC are of particu-
lar importance, as they verify the more than 80 previ-
ous publications in which 45M1 has been used as a
mAb against MUC5AC.
Experimental procedures
mAbs against human gastric mucin
Twelve mAbs against the peptide core of gastric mucins,
called mAbs against M1, were used in this study: 1-13M1,
2-11M1, 2-12M1, 9-13M1, 58M1 [12], 19M1, 21M1, 45M1
[23], 463M, 589M and 62M1 [16], and 166M1. The hybrid-
oma secreting 166M1 was raised from the cell fusion used
when isolating the hybridoma secreting 62M1 [16]. In short,
a mouse was immunized with gastric mucin isolated from
an OLe(a
)
b
+
) individual and purified using successive
chromatography steps on Sepharose 6B and 2B. The super-
natants of the hybridomas were screened, and the clones
were selected by their strong reaction against the gastric
surface epithelium and colon adenomas and their lack of
immununoreactivity on normal colon.
The mAb against myc was obtained from spent culture
media of the 1-9E10.2 hybridoma (ATCC CRL-1729).

Other antibodies used were: mAb against His
5
(Qiagen,
Valencia, CA, USA) and goat anti-(mouse IgG) coupled to
alkaline phosphatase (goat-a-mouse-AP) (Southern Biotech,
Birmingham, AL, USA).
Construction of MUC5AC vectors
The cDNA clone L31 [21], encoding almost the complete
C-terminal cysteine-rich part of human MUC5AC (lacks
the N-terminal 91 amino acids of this part), inserted into a
pBluescript vector, was used as template for the amplifica-
tion of the MUC5AC sequence. With use of the primer
pair 5¢-TATTCTAGAG AAGAGGGCCT GGTGTGCCGG
AACCAGGACC AGCAGGGACC CTTCAAG-3¢ (GH262)
and 5¢-ACGCGCTAGC TCAATGATGA TGATGATGGT
GCATGGGGGA CACTGGGACG CC-3¢ (GH263), the
MUC5AC-encoding sequence could be amplified by PCR.
The primer GH262 introduced an XbaI site in the 5¢-end of
the PCR product. It also added a nucleotide stretch coding
for the amino acid sequence SREEGLVCR. This sequence
is found N-terminally of the MUC5AC sequence encoded
by the L31 sequence [22,25]. The primer GH263 introduced
a His-tag and an NheI site in the 3¢-end of the PCR product.
The PCR product was ligated into the XbaI site of the previ-
ously described pSM vector [26]. This vector is based on the
pEGFP-C1 vector (Clontech, Palo Alto, CA, USA), where
the green fluorescent protein sequence has been replaced with
a murine Ig j-chain signal sequence from pSec Tag A
(Invitrogen, Carlsbad, CA, USA) followed by a myc-tag
(EQKLISEEDL). The resulting vector, pSM-MUC5AC-CH-

short, was used for transfection of CHO-K1 cells.
A vector encoding the complete C-terminal cysteine-rich
part, pSM-MUC5AC-CH-long, has been described before
[20]. In short, NP3a [22], a cDNA clone corresponding to
the 3¢-end of human MUC5AC inserted into a pBluescript
vector, was used as a template for PCR. With use of the
primer pair 5¢-GCTTCTAGAC ACGAGAAGAC AACCC
ACTCC C-3¢ (GH287) and 5¢-GCGAGGTCTC TGTGG
CGGTA TATGGTG-3¢ (GH288), the 5 ¢-part missing in the
L31 clone could be amplified. GH287 introduced an XbaI
site in the 5¢-end of the PCR product, and the amplified
product could then be ligated into the pSM-MUC5AC-
CH-short vector by its XbaI–BamHI sites.
Recombinant expression and tissue culture
The pSM-MUC5AC-CH-short plasmid, encoding an
Ig j-chain signal sequence followed by a myc-tag, the last
959 amino acids of the human MUC5AC C-terminal cyste-
ine-rich part and a His-tag, was transfected into CHO-K1
cells (ATCC CCL-61) using Lipofectamine 2000 (Invitro-
gen). Twenty-four hours after transfection, cell lysates were
prepared.
A CHO-K1 cell line stably expressing M-MUC5AC-CH-
long, the protein encoded by pSM-MUC5AC-CH-long, has
been described previously [20]. This protein encodes an
Ig j-chain signal sequence followed by a myc-tag, the com-
plete human MUC5AC C-terminal cysteine-rich part
(1041 amino acids), and a His-tag.
A CHO-K1 cell line expressing the last 981 amino acids
of human MUC2 (corresponding to the complete C-termi-
nal cysteine-rich part) fused to an Ig j-chain signal

sequence, a myc-tag and green fluorescent protein at its
N-terminus has been described previously [26].
The CHO-K1 cells were cultured as described previously
for LS174T [27], with the addition of 250 lgÆmL
)1
G418 to
the cells stably expressing recombinant proteins.
Preparation of cell lysates
The cell culture medium was removed, and the cells were
washed twice with NaCl ⁄ P
i
. The cells were lysed in lysis
M. E. Lidell et al. 45M1, a human mAb against MUC5AC
FEBS Journal 275 (2008) 481–489 ª 2007 The Authors Journal compilation ª 2007 FEBS 487
buffer [50 mm Tris ⁄ HCl, pH 7.9, 50 mm NaCl, 1% (v ⁄ v)
Triton X-100] containing protease inhibitors [2· Complete
(Roche, Indianapolis, IN, USA)] and 5 mm N-ethylmalei-
mide. After sonication (intensity 15) three times for 2 s
(MSE Soniprep 100 sonifier), the cell debris was removed
by centrifugation (16 000 g for 10 min at 4 °C).
Affinity purification of recombinant MUC5AC
C-terminus from cell lysates
Cell lysates from CHO-K1 cells and CHO-K1 cells tran-
siently transfected with the pSM-MUC5AC-CH-short plas-
mid were incubated with Dynabeads Talon beads (Dynal,
Oslo, Norway) for 2 h at 4 °C. The beads were washed
twice with lysis buffer before the proteins were eluted in
20 mm sodium phosphate (pH 7.4), 0.5 m NaCl, and 0.2 m
imidazole.
Immunoprecipitation

Fifty microliters of Dynabeads M-450 (Dynal) were washed
three times with NaCl ⁄ P
i
, 0.1% BSA, and 0.1% sodium
azide, and incubated on a shaker for 1 h at room tempera-
ture with 30 lL of hybridoma supernatant. The beads were
washed twice with lysis buffer before cell lysate was added
(1 mg of total protein). After incubation overnight at 4 ° C,
the beads were washed three times with lysis buffer before
the proteins were released into the sample buffer with
200 mm dithiothreitol at 95 °C for 5 min.
SDS
⁄
PAGE
The cell lysates were mixed with Laemmli sample buffer
with or without 200 mm dithiothreitol and incubated for
5 min at 95 ° C [28]. The samples were analyzed by discon-
tinuous SDS ⁄ PAGE, using 3–10% gradient gels with
3% stacking gels. The molecular marker used was the Pre-
cision Protein Standard (Bio-Rad, Hercules, CA, USA).
Western blotting and immunodetection
The proteins were transferred to poly(vinylidene difluoride)
membranes (Immobilon-PSQ, 0.20 lm; Millipore) in a
Transfer-Blot SD-Dry Transfer Cell (Bio-Rad) at
2.5 mAÆcm
)2
for 1 h. The transfer buffer used contained
48 mm Tris, 39 mm glycine, 1.3 mm SDS, and 10% (v ⁄ v)
methanol. After blotting, the membranes were placed in
blocking solution and incubated overnight at 4 °C. NaCl ⁄ P

i
containing 5% (w ⁄ v) milk powder and 0.1% (v ⁄ v) Tween-20
was used as blocking solution when using the mAb against
myc and the mAbs against M1 for detection.
BSA (2%, w ⁄ v) in 10 mm Tris ⁄ HCl, 100 mm NaCl and
0.1% (v ⁄ v) Tween-20 (pH 7.5) was used as blocking solution
when the mAb against His
5
was used for detection. After
blocking, the membranes were incubated with either mAb
against myc (1 lgÆmL
)1
), mAb against His
5
(100 ngÆmL
)1
)
or the mAbs against M1 (1 : 1000) in blocking solution for
2 h at room temperature. The membranes were washed
3 · 5 min with NaCl ⁄ P
i
containing 0.1% (v ⁄ v) Tween-20
and incubated with goat-a-mouse-AP (1 : 1000) in blocking
solution for 1.5 h at room temperature. After another
NaCl ⁄ Pi containing 0.1% (v ⁄ v) Tween-20 wash (3 · 5 min),
the membranes were developed with Nitro Blue tetrazolium ⁄
5-bromo-4-chloroindol-2-yl phosphate (Promega, Madison,
WI, USA).
Acknowledgements
We are indebted to Drs Francisco Real and Mary

Rose for partial cDNA clones. The work was sup-
ported by the Swedish Heart and Lung Foundation,
Swedish Research Council (No. 7461), IngaBritt and
Arne Lundberg’s Foundation, and the Swedish Foun-
dation for Strategic Research-Mucosa, Immunity and
Vaccine Center (MIVAC).
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