Linear organization of the liver cell adhesion molecule L-CAM

Abstract

A linear model of the liver cell adhesion molecule L-CAM from embryonic chickens is proposed in terms of its orientation on the cell surface, the number, type, and distribution of carbohydrate moieties, and sites of phosphorylation. L-CAM is isolated from cell membranes as a glycoprotein of Mr = 124,000. A soluble fragment (Ft1) of Mr = 81,000 can be released from cells by digestion with trypsin in the presence of calcium. Radiochemical amino acid sequence analyses indicated that both polypeptides have the same sequence for the first 10 amino acids, suggesting that fragment Ft1 contains the amino terminus of the L-CAM molecule and that the carboxyl-terminal portion of the peptide chain is associated with the cell. Digestions with endoglycosidase H and endoglycosidase F indicated that Ft1 has all of the N-linked carbohydrate groups associated with the larger species, including one high mannose oligosaccharide and three complex oligosaccharides. When hepatocytes were grown in the presence of 32PO4, 32P was detected in phosphoserine and phosphothreonine residues of intact L-CAM, but little or no 32P was detected in Ft1, suggesting that L-CAM is phosphorylated in the carboxyl-terminal region. On CNBr cleavage, the bulk of the 32P was detected in a single fragment of Mr = 20,000. The overall features of the L-CAM molecule incorporated in the model provide a basis for correlating its structure with its cell-cell binding activity and for detailed comparisons with similar molecules described in mammalian species.