Molecular Cloning of the Human β1,4 N-Acetylgalactosaminyltransferase Responsible for the Biosynthesis of the Sda Histo-Blood Group Antigen: The Sequence Predicts a Very Long Cytoplasmic Domain

The Sda antigen is a carbohydrate determinant expressed on erythrocytes, the colonic mucosa and other tissues. This epitope, whose structure is Siaα2,3[GalNAcβ1,4]Gal β1,4GlcNAc, is synthesized by a β1,4 N-acetylgalactosaminyltransferase (β4GalNAc-T) that transfers a β1,4-linked GalNAc to the galactose residue of an α2,3-sialylated chain. We have cloned from human colon carcinoma Caco2 cells a cDNA whose transfection in COS cells induces a GalNAc-T active on sialylated but not on asialylated fetuin and putatively represents the human Sda β4GalNAc-T. The cDNA predicts a 566 aa protein showing 66.6% and 39% identity with mouse CT β4GalNAc-T and human GM2/GD2 synthase, respectively, with a typical type II glycosyltransferase organization, no potential N-glycosylation sites and a 67 aa cytoplasmic tail, which is probably the longest among the glycosyltransferases cloned to date. The gene maps in chromosome 17q23, and is composed of at least 11 exons. Exons 2–11 are homologous to exons 2–11 of the previously cloned CT β4GalNAc-T from murine cytotoxic T lymphocytes while exons 1 of the two enzymes are totally different. The mRNA is expressed at a high level in differentiated Caco2 cells and in colonic mucosa and at a much lower level in lymphocytes and other colon cancer cell lines.