LIGHT-STIMULATED DEGRADATION OF AN UNASSEMBLED RIESKE FES PROTEIN BY A THYLAKOID-BOUND PROTEASE - THE POSSIBLE ROLE OF THE FTSH PROTEASE

Unassembled subunits of the cytochrome b(6)f complex as well as compon ents of other unassembled chloroplastic complexes are rapidly degraded within the organelle, However, the mechanisms involved in these prote olytic processes are obscure. When the Rieske FeS protein (RISP) is im ported into isolated chloroplasts in vitro, some of the protein does n ot properly assemble with the cytochrome complex, as determined by its sensitivity to exogenous protease, When assayed in intact, lysed, or fractionated chloroplasts, the imported RISP was found to be sensitive to endogenous proteases as well. The activity responsible for degrada tion of the unassembled protein was localized to the thylakoid membran e and characterized as a metalloprotease requiring zinc ions for its a ctivity. The degradation rate was stimulated by light, but no involvem ent of ATP or redox control was observed. Instead, when the RISP that was attached to thylakoid membranes was first illuminated on ice, degr adation proceeded in either light or darkness at equal rates, suggesti ng a light-induced conformational change making the protein prone to d egradation. Antibodies raised against native FtsH, a bacterial, membra ne-bound, ATP-dependent, zinc-stimulated protease, effectively inhibit ed degradation of the unassembled RISP, suggesting a role for the chlo roplastic FtsH in this process.