MUC13, a novel human cell surface mucin expressed by epithelial and hemopoietic cells

Transmembrane mucins are glycoproteins involved in barrier function in epit helial tissues. To identify novel transmembrane mucin genes, we performed a tblastn search of the GenBank(TM) EST data bases with a serine/ threonine- rich search string, and a rodent gene expressed in bone marrow was identifi ed. We determined the cDNA sequence of the human orthologue of this gene, M UC13, which localizes to chromosome band 3q13.3 and generates 3.2-kilobase pair transcripts encoding a 512-amino acid protein comprised of an N-termin al mucin repeat domain, three epidermal growth factor-like sequences, a SEA module, a transmembrane domain, and a cytoplasmic tail (GenBank(TM) access ion no. AF286113), MUC13 mRNA is expressed most highly in the large intesti ne and trachea, and at moderate levels in the kidney, small intestine, appe ndix, and stomach, In situ hybridization in murine tissues revealed express ion in intestinal epithelial and lymphoid cells. Immunohistochemistry demon strated the human MUC13 protein on the apical membrane of both columnar and goblet cells in the gastrointestinal tract, as well as within goblet cell thecae, indicative of secretion in addition to presence on the cell surface . MUC13 is cleaved, and the beta -subunit containing the cytoplasmic tail u ndergoes homodimerization, Including MUC13, there are at least five cell su rface mucins expressed in the gastrointestinal tract.