CYC2 ENCODES A FACTOR INVOLVED IN MITOCHONDRIAL IMPORT OF YEAST CYTOCHROME-C

The gene CYC2 from the yeast Saccharomyces cerevisiae was previously s hown to affect levels of mitochondrial cytochrome c by acting at a pos ttranslational step in cytochrome c biosynthesis. We report here the c loning and identification of the CYC2 gene product as a protein involv ed in import of cytochrome c into mitochondria. CYC2 encodes a 168-ami no-acid open reading frame with at least two potential transmembrane s egments. Antibodies against a synthetic peptide corresponding to the c arboxyl terminus of the predicted sequence were raised. These antibodi es recognize multiple bands on immunoblots of mitochondrial extracts. The intensities of these bands vary according to the gene dosage of CY C2 in various isogenic strains. Immunoblotting of subcellular fraction s suggests that the CYC2 gene product is a mitochondrial protein. Dele tion of CYC2 leads to accumulation of apocytochrome c in the cytoplasm . However, strains with deletions of this gene still import low levels of cytochrome c into mitochondria. The effects of cyc2 mutations are more pronounced in rho- strains than in rho' strains, even though rho- strains that are CYC2+ contain normal levels of holocytochrome c. cyc 2 mutations affect levels of iso-1-cytochrome c more than they do leve ls of iso-2-cytochrome c, apparently because of the greater susceptibi lity of apo-iso-1-cytochrome c to degradation in the cytoplasm. We pro pose that CYC2 encodes a factor that increases the efficiency of cytoc hrome c import into mitochondria.