Molecular cloning of a murine N-acetylgalactosamine transferase cDNA that determines expression of the T lymphocyte-specific CT oligosaccharide differentiation antigen.

Two monoclonal antibodies termed CT1 and CT2 define a cell surface oligosaccharide molecule expressed on restricted populations of murine lymphocytes. This oligosaccharide structure is largely associated with the extracellular domain of the CD45 family of tyrosine phosphatases, molecules required for lymphocytes to proliferate in response to antigen stimulation. Previous work has shown that the oligosaccharide structure recognized by the CT antibodies is identical, at least in part, to that of the human Sda blood group, a structure formed through enzymatic addition of N-acetylgalactosamine in beta 1,4-linkage to a sub-terminal galactose substituted with an alpha 2,3-linked N-acetylneuraminic acid residue. We have used a mammalian transient expression cloning approach to isolate a murine cDNA that determines expression of an oligosaccharide structure recognized by the CT antibodies as well as human anti-Sda serum. The nucleotide sequence predicts a 510-amino acid type II transmembrane protein characteristic of other mammalian glycosyltransferases. Enzymatic characterization of the protein expressed by this cDNA demonstrates that it encodes a beta 1,4-N-acetylgalactosamine transferase activity that can add to the low molecular weight acceptor 3'-sialyl-N-acetyllactosamine, to form the nonreducing terminal tetrasaccharide Sda blood group structure. This cDNA shares 51% nucleotide sequence identity with a cDNA encoding the human GM2/GD2 synthase, particularly throughout the regions encoding their putative catalytic domains. Southern blot analysis demonstrates that these two cDNA's represent distinct loci in the murine genome. The CT-GalNAc transferase cDNA isolated here represents a tool with which to define the role(s) of lymphocyte cell surface CT determinants, and may facilitate the isolation of the human Sda blood group locus through cross-hybridization approaches.