THE CHLOROPLASTIC PROTEIN IMPORT MACHINERY CONTAINS A RIESKE-TYPE IRON-SULFUR CLUSTER AND A MONONUCLEAR IRON-BINDING PROTEIN

Transport of precursor proteins across the chloroplastic envelope memb ranes requires the interaction of protein translocons localized in bot h the outer and inner envelope membranes, Analysis by blue native gel electrophoresis revealed that the translocon of the inner envelope mem branes consisted of at least six proteins with molecular weights of 36 , 45, 52, 60, 100 and 110 kDa, respectively, Tic110 and ClpC, identifi ed as components of the protein import apparatus of the inner envelope membrane, were prominent constituents of this complex, The amino acid sequence of the 52 kDa protein, deduced from the cDNA, contains a pre dicted Rieske-type iron-sulfur cluster and a mononuclear iron-binding site, Diethylpyrocarbonate, a Rieske-type protein-modifying reagent, i nhibits the translocation of precursor protein across the inner envelo pe membrane, whereas binding of the precursor to the outer envelope me mbrane is still possible. In another independent experimental approach , the 52 kDa protein could be copurified with a trapped precursor prot ein in association with the chloroplast protein translocon subunits To c86, Toc75, Toc34 and Tic110, Together, these results strongly suggest that the 52 kDa protein, named Tic55 due to its calculated molecular weight, is a member of the chloroplastic inner envelope protein transl ocon.