IDENTIFICATION OF A NEW ISOFORM OF CELL-CELL ADHESION MOLECULE-105 (C-CAM), C-CAM4 - A SECRETORY PROTEIN WITH ONLY ONE IG DOMAIN

A series of Southern blot hybridization experiments using probes deriv ed from different regions of the rat liver cell-cell adhesion molecule 105 (C-CAM) cDNA revealed the presence of a 9.6 kb EcoRI genomic frag ment that seemed to encode a unique C-CAM isoform. An RNase protection study showed that this C-CAM transcript was expressed in placenta, sp leen, lung and large intestine. In contrast, the other C-CAM isoforms, C-CAM1 and C-CAM2, are expressed in liver and small intestine. This r esult also suggests that the new isoform, which we named C-CAM4, was i ndeed encoded by a new C-CAM gene. A rat placenta cDNA library was the n screened and the full-length cDNA coding for C-CAM4 was isolated. Th e deduced protein contained 142 amino acids and had a calculated molec ular mass of 15 kDa. C-CAM4 was composed of a leader sequence and the first V-like Ig domain typical of C-CAM-family proteins. However, C-CA M4 lacked the C-like Ig domains, the transmembrane domain, and the cyt oplasmic domain found in other C-CAM isoforms. Thus, C-CAM4 is differe nt from the other known C-CAMs in that it is a secreted protein. We ha ve previously shown that the first Ig domain of C-CAM1 is crucial for its adhesion function, The V-like Ig domain of C-CAM4 had 92% and 89% sequence identity with the corresponding regions of C-CAM1 and C-CAM2 respectively. Together these results suggest that C-CAM4 may play a ro le in regulating the function of other C-CAM family proteins.