The Rieske Fe/S protein of the cytochrome b(6)/f complex in chloroplasts -Missing link in the evolution of protein transport pathways in chloroplasts?

The Rieske Fe/S protein, a nuclear-encoded subunit of the cytochrome b(6)/f complex in chloroplasts, is retarded in the stromal space after import int o the chloroplast and only slowly translocated further into the thylakoid m embrane system. As shown by the sensitivity to nigericin and to specific co mpetitor proteins, thylakoid transport takes place by the Delta pH-dependen t TAT pathway. The Rieske protein is an untypical TAT substrate, however. I t is only the second integral membrane protein shown to utilize this pathwa y, and it is the first authentic substrate without a cleavable signal pepti de. Transport is instead mediated by the NH2-terminal membrane anchor, whic h lacks, however, the twin-arginine motif indicative of Delta pH/TAT-depend ent transport signals. Furthermore, transport is affected by sodium azide a s well as by competitor proteins for the See pathway in chloroplasts, demon strating for the first time some cross-talk of the two pathways. This might take place in the stroma where the Rieske protein accumulates after import in several complexes of high molecular mass, among which the cpn60 complex is the most prominent. These untypical. features suggest that the Rieske p rotein represents an intermediate or early state in the evolution of the th ylakoidal protein transport pathways.