Genomic Organization of the 3′ Region of the Human Mucin GeneMUC5B

Abstract

MUC5B, mapped clustered with MUC6, MUC2, and MUC5AC to chromosome 11p15.5, is a human mucin gene of which the genomic organization is being elucidated. We have recently published the sequence and the peptide organization of its huge central exon, 10,713 base pairs (bp) in length. We present here the genomic organization of its 3 region, which encompasses 10,690 bp. The genomic sequence has been completely determined. The 3 region of MUC5B is composed of 18 exons ranging in size from 32 to 781 bp, contrasting thus with the very large central exon. The sizes of the 18 introns range from 114 to 1118 bp. Some repetitive sequences were identified in four introns. The peptide deduced from the sequence of the 18 exons consists of an 808-amino acid peptide. This carboxyl-terminal region exhibits extensive sequence similarity to MUC2, MUC5AC, and von Willebrand factor, particularly the number and the positions of the cysteine residues, suggesting that this domain may be derived from a common ancestral gene. The presence in these components of a cystine knot also found in growth factors such as transforming growth factor-␤ is of particular interest. Moreover, one part of this peptide is identical to the 196-amino acid sequence deduced from the cDNA clone pSM2-1, which codes for a part of the high molecular weight mucin MG1 isolated from human sublingual gland. Considering the expression pattern of MUC5B and the origin of MG1, we can thus conclude that MUC5B encodes MG1.Mucus is the layer that covers, protects, and lubricates the luminal surfaces of epithelial respiratory, gastrointestinal, and reproductive tracts. These basic properties are due to the viscous and viscoelastic properties of mucins, the major glycoprotein components of mucus. Mucins constitute a family of high molecular mass glycoproteins synthesized by the goblet cells of the epithelia and in some cases by submucosal glands (for more complete reviews, see Refs. 1-3).Alterations of the biosynthesis of mucins affecting the protein core and/or the carbohydrate content linked to the peptide have been observed in numerous pathological situations such as various adenomas and carcinomas, inflammatory diseases such as cystic fibrosis, asthma, chronic bronchitis, or inflammatory bowel diseases (4 -7). Moreover, the hypersecretion of mucins and the presence of alternating hydrophobic and hydrophilic domains in mucins have been shown to play a central role in the pathogenesis of cholesterol gallstones (8, 9).All apomucins contain tandemly repeated sequences rich in threonine and/or serine. Due to the high carbohydrate content, the peptide moiety of mucins has been difficult to characterize. cDNA cloning has enabled researchers to approach the study of the mucins over the past decade. Today, the membrane-associated mucin MUC1 and the secreted MUC7 are the only mucins for which the full-length cDNA and the genomic organization have been reported (10 -13). Both were revealed to be, in fact, small mucins. A complete cDNA of the large secreted mucin MUC2 (14 -17) has been...

The recombinant C-terminus of the human MUC2 mucin forms dimers in Chinese-hamster ovary cells and heterodimers with full-length MUC2 in LS 174T cells

“…The positions of the cysteine residues are very similar to the C-terminus of the vWF [3], but also to the human mucins MUC5AC [4,5] and MUC5B [6] as well as the PSM [7]. The cysteine residues in the far C-terminal end are part of the cystine-knot motif known to be involved in the dimerization of some human growth factors [2,23].…”

“…The vWF is then transported via the Golgi apparatus to the trans-Golgi network and secretory vesicles, where oligomerization of the protein starts by disulphide bonds involving the N-termini. Alignment of the human MUC2 mucin [2] with the human MUC5AC [4,5] and MUC5B mucins [6], as well as the porcine [7] and bovine [8] submaxillary mucins, shows sequence similarities in the positions of the cysteine residues of the C-termini. The sequence similarities between these mucins and the vWF suggest that these mucins might also assemble in a similar way.…”

Identification of Lys-Pro-Gln as a Novel Cleavage Site Specificity of Saliva-associated Proteases

“…Through traditional biochemical analyses, the structural characteristics of the proteins secreted by these glands have been established, generating the fundamental basis of the major salivary secretome (7). The most abundant salivary secretory proteins in these secretions combined are mucous glycoproteins 1 and 2 encoded by MUC5B and MUC7, respectively (8,9), amylase encoded by AMY1 (10), immunoglobulins, in particular sIgA, acidic proline-rich proteins (PRPs) encoded by PRH1 and PRH2 (11), basic PRPs encoded by PRB1 to PRB4 (11), and PBII (SMR3B) (12), agglutinin encoded by DMBT1 (13), cystatins encoded by CST1 to CST5 (14 -16) histatins encoded by HIS1 and HIS2 (17), and statherin encoded by STATH (18,19). Each of these proteins furthermore appears in families comprising numerous polymorphic isoforms displaying a high sequence homology (7,20).…”

Porcine Submaxillary Mucin Forms Disulfide-linked Multimers through Its Amino-terminal D-domains

“…Flanking the unique sequence domains are three half-cystine-rich domains at the amino-terminal side and one (240 residues) disulfide-rich domain at the carboxyl-terminal side (1). The half-cystine residues present at the carboxyl-terminal end of submaxillary mucin are conserved in the corresponding regions of human von Willebrand factor (3) and many secretory mucins, including frog integumentary mucin FIMB.1 (4); rat intestinal mucin (5); bovine submaxillary mucin (6); and human mucins MUC2 (7), MUC5B (8,9), MUC5AC (10), and MUC6 (11). Moreover, the 11 carboxyl-terminal half-cystine residues in human von Willebrand factor and those in the above secretory mucins are homologous to the 11 half-cystine residues in Norrie disease protein (norrin) (12).…”