MITOCHONDRIAL IMPORT OF SUBUNIT VA OF CYTOCHROME-C-OXIDASE CHARACTERIZED WITH YEAST MUTANTS - INDEPENDENCE FROM RECEPTORS, BUT REQUIREMENT FOR MATRIX HSP70 TRANSLOCASE FUNCTION

We have investigated the unusual import pathway of cytochrome c oxidas e subunit Va (COXVa) into the yeast mitochondrial inner membrane by us e of mutants that lack import receptors or are defective in matrix hsp 70. (i) Mitochondria lacking the receptor MOM72 are not impaired in im port of COXVa. Mitochondria lacking the main receptor MOM19 are modera tely reduced in import of COXVa; this, however, is caused by a reducti on of the inner membrane potential and not by a lack of specific recep tor functions. (ii) Mitochondria defective in the unfoldase function o f matrix hsp70 efficiently import COXVa, whereas mitochondria defectiv e in the translocase function of the hsp70 are blocked in import of CO XVa. A COXVa construct where the internal hydrophobic sorting signal i s placed close to the presequence does not require either hsp70 functi on. These results demonstrate that import of COXVa does not require MO M19 or MOM72, but they unexpectedly reveal a strong dependence on the translocase function of matrix hsp70. Two important implications about the characterization of mitochondrial protein import in general are o btained. First, the interpretation of import results with mutants lack ing MOM19 have to consider effects on the membrane potential. Second, the distance between a matrix targeting sequence and a hydrophobic sor ting sequence within a precursor appears to determine if the inner mem brane sorting machinery can substitute for the translocase function of hsp70 or not.