Alternative splicing of repetitive units is responsible for the polydispersities of integumentary mucin B.1 (FIM-B.1) from Xenopus laevis

The mouse Muc5b mucin gene: cDNA and genomic structures, chromosomal localization and expression

We report here the isolation and characterization of the mouse Muc5b mucin gene (mMuc5b). We determined its complete cDNA sequence, its genomic organization, and chromosomal localization. Moreover, we analyzed the expression of this gene by reverse-transcription PCR and in situ hybridization. The structure of the gene was determined from a genomic cosmid clone that encompasses the entire mMuc5b gene, including the 5'-flanking region. The mMuc5b gene spans approximately 36 kb and contains 49 exons. It is located on mouse distal chromosome 7. mMuc5b encodes at least two transcripts by alternative splicing of the second exon, the longest one being 14.9 kb in length. The deduced peptide contains 4782 amino acids. Its central region can be subdivided into 10 imperfect repeats, each composed of a cysteine-rich domain followed by a threonine, serine, and proline-rich mucin-type domain. It is flanked by cysteine-rich domains similar to cysteine-rich domains of pre-pro-von Willebrand factor. Comparison with its human homologue MUC5B revealed common features including high sequence similarities in the 5' and 3' regions, and the conservation of the genomic organization. In contrast, mMuc5b differs from its human homologue, since no highly tandemly repeated sequences could be identified within its central region. mMuc5b is expressed mainly in laryngeal mucous glands, and at a lesser extend in stomach and duodenum.

Structure of the Xenopus laevis TFF-gene xP4.1, differentially expressed to its duplicated homolog xP4.2

TFF-peptides (formerly P-domain peptides, trefoil factors) represent major secretory products of mucous epithelia in mammals and amphibia. The nucleotide sequence of a large portion of a gene encoding the TFF-peptide xP4.1 from Xenopus laevis and its genomic organization were determined in the present study. The peptide xP4.1 containing four TFF-domains is thought to represent the functional frog homolog of human TFF2 (formerly hSP). The xP4.1 gene analyzed spans a region of about 7 kb and consists of six exons. Each TFF-domain is encoded by a single exon flanked by type 1 introns typical of shuffled modules. The 5'-upstream region contains a TATA-box, and potential binding sites for hepatocyte nuclear factor 3 and AP-1. Furthermore, the cDNA sequence of a transcript named xP4.2 with 91% similarity to xP4.1 is presented. RT-PCR analysis revealed that xP4.1 and xP4.2 genes are differentially expressed. xP4.1 transcripts are detectable only in the stomach, but not in the esophagus, whereas xP4.2 transcripts are found both in the esophagus and in the stomach with a descending gradient from fundus to antrum.

Sequence of a second gene encoding bovine submaxillary mucin: implication for mucin heterogeneity and cloning

Secreted epithelial mucins are extremely large and heterogeneous glycoproteins. We report the 5 kilobase DNA sequence of a second gene, BSM2, which encodes bovine submaxillary mucin. The determined nucleotide and deduced amino acid sequences of BSM2 are 95.2% and 92. 2% identical, respectively, to those of the previously described BSM1 gene isolated from the same cow. Further, the five predicted protein domains of the two genes are 100%, 94%, 93%, 77%, and 88% identical. Based on the above results, we propose that expression of multiple homologous core proteins from a single animal is a factor in generating diversity of saccharides in mucins and in providing resistance of the molecules to proteolysis. In addition, this work raises several important issues in mucin cloning such as assembling sequences from seemingly overlapping clones and deducing consensus sequences for nearly identical tandem repeats.

Similarities of integumentary mucin B.1 from Xenopus laevis and prepro-von Willebrand factor at their amino-terminal regions

Frog integumentary mucin B.1 (FIM-B.1) contains various cysteine-rich modules. In the past, a COOH-terminal "cystine knot" motif has been found that is similar to von Willebrand factor; this region is generally known to be responsible for dimerization processes. Furthermore, a "complement control protein" motif is present as an internal cysteine-rich domain in FIM-B.1. We characterize here the missing 75% toward the NH2 terminus of the FIM-B.1 precursor by molecular cloning. Analogous to prepro-von Willebrand factor, four elements with considerable similarity to D-domains are present (i.e. D1-D2-D'-D3). These domains have been described as essential for the multimerization of von Willebrand factor. Thus, the general structure of FIM-B.1 resembles that of the human mucin MUC2 as well as prepro-von Willebrand factor; these three molecules at least seem to share common structural elements allowing similar multimerization mechanisms.