Abstract
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 into the chloroplast and only slowly translocated further into the thylakoid membrane system. As shown by the sensitivity to nigericin and to specific competitor proteins, thylakoid transport takes place by the DeltapH-dependent TAT pathway. The Rieske protein is an untypical TAT substrate, however. It is only the second integral membrane protein shown to utilize this pathway, and it is the first authentic substrate without a cleavable signal peptide. Transport is instead mediated by the NH(2)-terminal membrane anchor, which lacks, however, the twin-arginine motif indicative of DeltapH/TAT-dependent transport signals. Furthermore, transport is affected by sodium azide as well as by competitor proteins for the Sec pathway in chloroplasts, demonstrating 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 protein represents an intermediate or early state in the evolution of the thylakoidal protein transport pathways.
Highlights
The Rieske Fe/S protein, a nuclear-encoded subunit of the cytochrome b6/f complex in chloroplasts, is retarded in the stromal space after import into the chloroplast and only slowly translocated further into the thylakoid membrane system
In experiments with intact chloroplasts isolated from spinach, only about 30% of the Rieske protein that is imported into the organelle reaches the thylakoids during the incubation period of 20 min (Fig. 1)
The Rieske protein represents the first link between the Sec- and the ⌬pH-dependent transport routes in chloroplasts that so far were considered to operate completely independently from each other
Summary
The thylakoid transport signal of the Rieske protein does not resemble ⌬pH/TATspecific targeting signals It is not a cleavable signal peptide but operates as a signal anchor domain [8], and it lacks the indicative twin-R motif [24], which even accounts for the denomination of the entire pathway. Translocated by the ⌬pH/TAT-dependent pathway across the thylakoid membrane, the targeting process is characterized by a number of unusual features including the involvement of components from the Sec-dependent protein transport route. This suggests that the Rieske protein might represent an ancient or intermediate state in the evolution of transport pathways at the thylakoid membrane in chloroplasts
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