Cloning and Expression of the α Subunit of Succinyl-CoA Synthetase from Dictyostelium discoideum

We have isolated and characterized a genomic fragment and a cDNA clone for the α subunit of succinyl-CoA synthetase (SCS) from Dictyostelium discoideum. The coding region predicts a protein of 310 amino acids. Direct amino acid sequence data indicate that the first 16 amino acids encoded by the cDNA comprise a mitochondrial targeting sequence. The predicted molecular mass of the mature protein is 31 kDa on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The deduced amino acid sequence of the α subunit from D. discoideum shows 75% identity to that from rat liver, which represents a GTP-utilizing form of SCS, and 62% identity to the Escherichia coli protein, which is an ATP-utilizing form. We have previously shown that the enzyme purified from growing D. discoideum is a ATP-utilizing enzyme. In D. discoideum, the α subunit is a single copy gene that is developmentally regulated. Highest levels of its mRNA are seen in growing cells, which is followed by a relatively dramatic decline at the time cells complete their aggregation program. In contrast, the level of the protein, as determined by Western blots, remains unchanged throughout development. This indicates that any developmental changes in SCS activity reflect changes in the availability of its substrates or effecters and not changes in the level of enzyme.