MOLECULAR-FEATURES, PROCESSING AND IMPORT OF THE RIESKE IRON-SULFUR PROTEIN FROM POTATO MITOCHONDRIA

The mitochondrial iron-sulfur protein (also termed Rieske iron-sulfur protein) of cytochrome c reductase was purified from potato tubers and identified with heterologous antibodies. The sequences of the N-termi nus of this 25 kDa protein and of an internal peptide were determined to design oligonucleotide mixtures for screening a cDNA library. One c lass of cDNA clones containing an open reading frame of 265 amino acid s was isolated. The encoded protein contains the peptide sequences of the 25 kDa protein and shares about 50% sequence identity with the Rie ske iron-sulfur proteins from fungi and around 43% with those from mam mals. In vine transcription and translation of the cDNA reveals that t he iron-sulfur protein is made as a larger precursor of 30 kDa which i s processed by the cytochrome c reductase/processing peptidase complex from potato. The processing product obtained after in vitro processin g has the same size as the mature protein imported into isolated mitoc hondria. The presequence, which targets the protein to the organelle, is 53 amino acids long and has molecular features different from those found in presequences of fungal iron-sulfur proteins, which are proce ssed in two steps. Our results indicate that, unlike in yeast and Neur ospora, the presequence of the iron-sulfur protein from potato is remo ved by a single processing enzyme in one step.