Mutually exclusive splicing generates two distinct isoforms of pig heart succinyl-CoA synthetase

We have identified two distinct cDNAs encoding the alpha-subunit of pig heart succinyl-CoA synthetase. The derived amino acid sequence of one of these, PHalpha57, is highly similar to the alpha-subunit of the rat liver precursor enzyme. The second cDNA, PHalpha108, was identical throughout its sequence with PHalpha57 except for a stretch of 108 nucleotides which replaced a 57 nucleotide sequence in PHalpha57. Coexpression of either alpha-subunit cDNA with a common pig heart beta-subunit cDNA produced isozymes with GTP-specific enzyme activity. The enzyme produced by the combination of PHalpha57 and the beta-subunit cDNA resembled the "native" enzyme purified from pig heart tissue. In contrast, the expressed enzyme from the combination with PHalpha108 was clearly distinguishable from the native enzyme by, for example, hydroxyapatite chromatography. Moreover, it was now apparent that this isoform had been observed in previous preparations of the native enzyme, but always in very low amounts and, thus, disregarded. We have shown further that the two mRNA transcripts arise from a single gene and are generated by mutually exclusive splicing. The production of the PHalpha108 message involves the use of a non-canonical splice site pair, AT-AA. Finally, we provide evidence for tissue specific regulation in the splicing of the PHalpha108 message.