Different isoforms and stock‐specific variants of the cell adhesion molecule C‐CAM (cell‐CAM 105) in rat liver

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Abstract
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.