The evolutionary origin of the protein-translocating channel of chloroplastic envelope membranes: Identification of a cyanobacterial homolog

The known envelope membrane proteins of the chloroplastic protein import ap paratus lack sequence similarity to proteins of other eukaryotic or prokary otic protein transport systems. However, we detected a putative homolog of the gene encoding Toc75, the protein-translocating channel from the outer e nvelope membrane of pea chloroplasts, in the genome of the cyanobacterium S ynechocystis sp, PCC 6803, We investigated whether the low sequence identit y of 21% reflects a structural and functional relationship between the two proteins. We provide evidence that the cyanobacterial protein is also local ized in the outer membrane. From this information and the similarity of the predicted secondary structures, we conclude that Toc75 and the cyanobacter ial protein, referred to as SynToc75, are structural homologs, synToc75 is essential, as homozygous null mutants were not recovered after directed mut agenesis, Sequence analysis indicates that SynToc75 belongs to a family of outer membrane proteins from Cram-negative bacteria whose function is not y et known. However, we demonstrate that these proteins are related to a spec ific group of prokaryotic secretion channels that transfer virulence factor s, such as hemolysins and adhesins, across the outer membrane.