DIFFERENT ISOFORMS AND STOCK-SPECIFIC VARIANTS OF THE CELL-ADHESION MOLECULE C-CAM (CELL-CAM 105) IN RAT-LIVER

C-CAM is a cell adhesion molecule of the immunoglobulin superfamily with homophilic binding properties. Here we used the polymerase chain reaction to isolate clones of C-CAM from a rat liver cDNA library. Sequence analyses identified two major isoforms, C-CAM1 and C-CAM2, which differed in their 3' ends. C-CAM2 lacked a sequence of 53 nucleotides that was present in C-CAM1. This causes a frame shift and new stop codons, which gives rise to cytoplasmic domains of different sizes in the two isoforms (10 versus 71 amino-acid residues). In addition, all the clones had a different nucleotide and deduced amino-acid sequence (variant b) in the most N-terminal of the four immunoglobulin-like domains, compared to a previously published C-CAM sequence (variant a). Northern-blot analyses with specific oligonucleotide probes demonstrated that two different rat stocks expressed either variant a or variant b. Northern-blot analyses of rat liver and lung also showed that at least five different C-CAM transcripts are produced. Two major mRNA size classes of 4.0 kb and 6.0 kb, and one minor class of 3.0 kb were found. Both the 4.0-kb and 3.0-kb messenger classes reacted with two different probes that could distinguish between C-CAM1 and C-CAM2, while the 6.0-kb population only reacted with the probe selective for C-CAM1. Taken together these data demonstrate the existence of four different protein-coding sequences of rat liver C-CAM (C-CAM1 a and b, and C-CAM2 a and b). We suggest that both allelic variation and alternative splicing may contribute to the isoform-expression pattern of C-CAM in rats.