Sequence analysis of a cDNA clone encoding the liver cell adhesion molecule, L-CAM

Abstract

The liver cell adhesion molecule (L-CAM) appears on non-neural epithelial tissues and mediates calcium-dependent adhesion in these tissues both in the embryo and in the adult. It appears on cell surfaces as a glycoprotein of Mr 124,000 but is synthesized as a precursor of Mr 135,000. We have isolated and determined the nucleic acid sequence of a cDNA clone (lambda L320) encoding chicken L-CAM. The 5' end of this clone has an open reading frame extending for 2520 base pairs, followed by an 850-base-pair untranslated region terminating with a polyadenylylation site at its 3' end. Protein sequence analysis of intact L-CAM and of cyanogen bromide fragments of the protein confirmed the reading frame and indicated that lambda L320 encodes the complete sequence of L-CAM as it is expressed on the cell surface as well as the bulk of the precursor. The sequence includes a hydrophobic segment of 31 amino acids, supporting our earlier conclusion that L-CAM is an intrinsic membrane protein. There are five potential asparagine glycosylation sites on the extracellular part of the molecule and an intracellular domain that is phosphorylated in vivo. The mature L-CAM polypeptide consists of 727 amino acids, with a calculated Mr of 79,900 for the carbohydrate-free protein. The L-CAM sequence is not homologous to other known protein sequences, including those of the neural cell adhesion molecule (N-CAM) and other members of the immunoglobulin superfamily, but the L-CAM molecule does contain three contiguous segments (113 amino acids each) that are homologous to each other. The similarities among these segments suggest that at least part of the L-CAM molecule arose by gene duplication.