CHARACTERIZATION OF POSTTRANSLATIONAL MODIFICATIONS OF HUMAN A33 ANTIGEN, A NOVEL PALMITOYLATED SURFACE GLYCOPROTEIN OF HUMAN GASTROINTESTINAL EPITHELIUM

Monoclonal antibody (mAb) A33 recognizes a differentiation antigen (A3 3) expressed in normal human gastrointestinal epithelium and in 95% of human colon cancers. Murine mAb A33 shows specific targeting of colon cancer in humans and a humanized A33 antibody is currently being eval uated in the clinic. The cDNA far the human A33 antigen has recently b een cloned, and sequence comparison indicated that the A33 antigen is a novel human cell surface molecule of the immunoglobulin superfamily. Because mAb A33 recognizes a conformational epitope, only a partial c haracterization of the A33 antigen has been carried out to date. In th is report we show that the A33 antigen is (I) N-glycosylated, containi ng approximately 8 K of N-linked carbohydrate and there is no evidence for O-glycosylation, sialylation or glycophosphatidylinositol, and (i i) S-acylated in vitro, incorporating [H-3] palmitic acid linked throu gh a hydroxylamine-sensitive thioester bond. The S-palmitoylation may be involved in regulating the internalization process initiated by bin ding of mAb A33 to cell surface A33 antigen. (C) 1997 Academic Press.