2-STEP PROCESSING IS NOT ESSENTIAL FOR THE IMPORT AND ASSEMBLY OF FUNCTIONALLY ACTIVE IRON-SULFUR PROTEIN INTO THE CYTOCHROME BC(1) COMPLEXIN SACCHAROMYCES-CEREVISIAE

The iron-sulfur protein of the cytochrome be, complex is one of a smal l number of proteins that are processed in two Sequential steps by mat rix processing peptidase (MPP) and mitochondrial intermediate peptidas e (MIP) during import into Saccharomyces cerevisiae mitochondria. To t est whether two-step processing is necessary for import and assembly o f the iron-sulfur protein into the cytochrome bc(1) complex, we mutage nized the presequence of the iron-sulfur protein to eliminate the orig inal MPP site and replace the MIP site with a new MPP site. The mutate d presequence is cleaved and forms mature-sized protein in a single st ep, and the mature-sized iron-sulfur protein is correctly targeted to the outer side of the inner mitochondrial membrane in vitro. Mutant ir on-sulfur protein which is processed to mature size in one step comple ments the respiratory deficient phenotype of a yeast strain in which t he endogenous gene for the iron-sulfur protein is deleted, These resul ts establish that mature-sized iron-sulfur protein can be formed by si ngle-step processing and assembled into a functionally active form in the cytochrome be, complex in S. cerevisiae.