Mutually Exclusive Splicing Generates Two Distinct Isoforms of Pig Heart Succinyl-CoA Synthetase

We have identified two distinct cDNAs encoding the α-subunit of pig heart succinyl-CoA synthetase. The derived amino acid sequence of one of these, PHα57, is highly similar to the α-subunit of the rat liver precursor enzyme. The second cDNA, PHα108, was identical throughout its sequence with PHα57 except for a stretch of 108 nucleotides which replaced a 57 nucleotide sequence in PHα57. Coexpression of either α-subunit cDNA with a common pig heart β-subunit cDNA produced isozymes with GTP-specific enzyme activity. The enzyme produced by the combination of PHα57 and the β-subunit cDNA resembled the “native” enzyme purified from pig heart tissue. In contrast, the expressed enzyme from the combination with PHα108 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 PHα108 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 PHα108 message.