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 β1,4-1inked 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.