The mitochondrial F1ATPase alpha-subunit is necessary for efficient import of mitochondrial precursors

The mitochondrial import and assembly of the F1ATPase subunits requires, respectively, the participation of the molecular chaperones hsp70SSA1 and hsp70SSC1 and other components operating on opposite sides of the mitochondrial membrane. In previous studies, both the homology and the assembly properties of the F1ATPase alpha-subunit (ATP1p) compared to the groEL homologue, hsp60, have led to the proposal that this subunit could exhibit chaperone-like activity. In this report the extent to which this subunit participates in protein transport has been determined by comparing import into mitochondria that lack the F1ATPase alpha-subunit (delta ATP1) versus mitochondria that lack the other major catalytic subunit, the F1ATPase beta-subunit (delta ATP2). Yeast mutants lacking the alpha-subunit but not the beta-subunit grow much more slowly than expected on fermentable carbon sources and exhibit delayed kinetics of protein import for several mitochondrial precursors such as the F1 beta subunit, hsp60MIF4 and subunits 4 and 5 of the cytochrome oxidase. In vitro and in vivo the F1 beta-subunit precursor accumulates as a translocation intermediate in absence of the F1 alpha-subunit. In the absence of both the ATPase subunits yeast grows at the same rate as a strain lacking only the beta-subunit, and import of mitochondrial precursors is restored to that of wild type. These data indicate that the F1 alpha-subunit likely functions as an "assembly partner" to influence protein import rather than functioning directly as a chaperone. These data are discussed in light of the relationship between the import and assembly of proteins in mitochondria.