The cytoplasmic domain of C-CAM is required for C-CAM-mediated adhesion function: studies of a C-CAM transcript containing an unspliced intron

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Abstract
Cell-CAM105 (also named C-CAM) is a cell surface glycoprotein involved in intercellular adhesion of rat hepatocytes. It has four extracellular immunoglobulin (Ig) domains, a transmembrane domain and a cytoplasmic domain and therefore is a member of the Ig supergene family. We have characterized multiple cDNAs of the C-CAM genes in rat intestine. Sequence analyses showed that rat intestine contained not only the previously reported L-form and S-form C-CAMs (renamed C-CAM1 and C-CAM2 respectively) but also a new isoform, C-CAM3. The C-CAM3 transcript codes for a polypeptide with a truncated C-terminus that lacks 65 amino acids from the previously reported C-CAM1 cytoplasmic domain. Unlike C-CAM1, C-CAM3 did not mediate cell adhesion when expressed in insect cells using the baculoviral expression system. Thus the extra 65 amino acids in the cytoplasmic domain of C-CAM1 are important for adhesion phenotype when expressed in insect cells. Although C-CAM1 and C-CAM2 are encoded by different genes, sequence analysis suggests that C-CAM3 is probably derived from alternative splicing of the C-CAM1 gene. To examine this possibility, we have determined the exon organization of the C-CAM1 gene. C-CAM3 differed from C-CAM1 by the presence of a single unspliced intron which contained a stop codon immediately after the regular splice junction. As a result, translation of C-CAM3 terminates at the point where C-CAM1 and C-CAM3 sequences diverge. To investigate the expression of C-CAM1, C-CAM2 and C-CAM3 in different tissues, we used an RNAase-protection assay to simultaneously assess the levels of expression of these transcripts. Using total RNA prepared from various tissues, we showed that expression of C-CAM3 was tissue-specific, and the C-CAM3 transcript accounted for about 25% of the transcripts derived from the C-CAM1 gene. However, further analysis revealed that C-CAM3 transcript was not present in cytosolic RNA, rather it was enriched in nuclear RNA prepared from hepatocytes. Although C-CAM3 cDNA contains the polyadenylation signal and is polyadenylated, these results indicate that C-CAM3 is probably an incomplete spliced product of C-CAM1 gene.