CALDESMON MESSENGER-RNA SPLICING AND ISOFORM EXPRESSION IN MAMMALIAN SMOOTH-MUSCLE AND NONMUSCLE TISSUES

The recent determination of the genomic sequence of human caldesmon in dicates that eight caldesmon mRNA species could be generated by select ion of exon 1 or 1', exon 3a or 3ab and/or exon 4. We used reverse tra nscriptase PCR to determine which transcripts were produced in human, rabbit and sheep artery, vein, lung, intestine, kidney and liver. In a ll tissues the same three transcripts were present: exons 1'-2-3a-5-6. ..13, exons 1'-2-3a3b-5-6-, 13 and exons 1'-2-3a3b-4-5-6...13. Exon 1 was not present and exon 4 was only present when exon 3b was also pres ent. Three protein isoforms of caldesmon can be distinguished by elect rophoresis on high-porosity 6% polyacrylamide gel: 130 kDa, 120 kDa an d 70 kDa. The 70 kDa isoform lacks the sequence encoded by exon 3b. We investigated whether the two high-molecular-mass isoforms correspond to the presence and absence of exon 4 using an antiserum specific to t he sequence encoded by exon 4. Western-blotting and immunoprecipitatio n experiments showed that both the 130 kDa and the 120 kDa isoforms we re expressed with and without the exon 4 sequence. We therefore propos e that the molecular-mass heterogeneity arises from additional first e xons, possibly with separate promoter regions, which have not yet been characterized in the genomic sequence.