vaginalis Is a Soluble Hydrogenosomal Protein with an Amino-Terminal Sequence That Resembles

We describe studies directed toward understanding the biogenesis and origin of the hydrogenosome, an unusual organelle found exclusively in certain anaerobic eukaryotes that lack mitochondria. Hydrogenosomes are involved in fermentative carbohydrate metabolism and are proposed to have arisen through conversion of mitochondria or via endosymbiosis with an anaerobic bacterium. We cloned a gene encoding the beta subunit of the hydrogenosomal protein succinyl-coenzyme A synthetase (13-SCS) and isolated the protein from Trichomonas vaginalis. The T. vaginalis 13-SCS gene encodes a protein with a calculated molecular mass of 43,980 Da that has 43 % amino acid identity (65 % similarity) with (-SCS from Escherichia coli. The trichomonad protein partitions into the soluble fraction of hydrogenosomes treated with sodium carbonate at high pH, consistent with a matrix localization within the organelle. The protein is encoded by a multigene family composed of at least three members. Amino-terminal sequencing of 13-SCS purified from T. vaginalis hydrogenosomes shows that the mature protein lacks the first nine amino acids encoded in the gene. This apparent amino-terminal leader sequence is strikingly similar to that of another hydrogenosomal protein and to mitochondrial presequences. Tichomonas vaginalis is an ancient eukaryote that lacks mitochondria (32, 39). This anaerobic protozoan parasit