Origin of a chloroplast protein importer

During evolution, chloroplasts have relinquished the majority of their gene s to the nucleus. The products of transferred genes are imported into the o rganelle with the help of an import machinery that is distributed across th e inner and outer plastid membranes. The evolutionary origin of this machin ery is puzzling because, in the putative predecessors, the cyanobacteria, t he outer two membranes, the plasma membrane, and the lipopolysaccharide lay er lack a functionally similar protein import system. A 75-kDa protein-cond ucting channel in the outer envelope of pea chloroplasts, Toc75, shares app roximate to 22% amino acid identity to a similarly sized protein, designate d SynToc75, encoded in the Synechocystis PCC6803 genome, Here we show that SynToc75 is located in the outer membrane (lipopolysaccharide layer) of Syn echocystis PCC6803 and that SynToc75 forms a voltage-gated, high conductanc e channel with a high affinity for polyamines and peptides in reconstituted liposomes, These findings suggest that a component of the chloroplast prot ein import system, Toc75, was recruited from a preexisting channel-forming protein of the cyanobacterial outer membrane. Furthermore, the presence of a protein in the chloroplastic outer envelope homologous to a cyanobacteria l protein provides support for the prokaryotic nature of this chloroplastic membrane.