INTRAMITOCHONDRIAL SORTING OF THE PRECURSOR TO YEAST CYTOCHROME-C-OXIDASE SUBUNIT VA

We have continued our studies on the import pathway of the precursor t o yeast cytochrome c oxidase subunit Va (pVa), a mitochondrial inner m embrane protein. Previous work on this precursor demonstrated that imp ort of pVa is unusually efficient, and that inner membrane localizatio n is directed by a membrane-spanning domain in the COOH-terminal third of the protein. Here we report the results of studies aimed at analyz ing the intramitochondrial sorting of pVa, as well as the role played by ancillary factors in import and localization of the precursor. We f ound that pVa was efficiently imported and correctly sorted in mitocho ndria prepared from yeast strains defective in the function of either mitochondrial heat shock protein (hsp)60 or hsp70. Under identical con ditions the import and sorting of another mitochondrial protein, the p recursor to the beta subunit of the F1ATPase, was completely defective . Consistent with previous results demonstrating that the subunit Va p recursor is loosely folded, we found that pVa could be efficiently imp orted into mitochondria after translation in wheat germ extracts. This result suggests that normal levels of extramitochondrial hsp70 are al so not required for import of the protein. The results of this study e nhance our understanding of the mechanism by which pVa is routed to th e mitochondrial inner membrane. They suggest that while the NH2 termin us of pVa is exposed to the matrix and processed by the matrix metallo protease, the protein remains anchored to the inner membrane before be ing assembled into a functional holoenzyme complex.