Genetic and biochemical basis for viability of yeast lacking mitochondrial genomes

Abstract

Yme1p, an ATP-dependent protease localized in the mitochondrial inner membrane, is required for the growth of yeast lacking an intact mitochondrial genome. Specific dominant mutations in the genes encoding the α- and γ-subunits of the mitochondrial F1F0-ATPase suppress the slow-growth phenotype of yeast that simultaneously lack Yme1p and mitochondrial DNA. F1F0-ATPase activity is reduced in yeast lacking Yme1p and is restored in yme1 strains bearing suppressing mutations in F1-ATPase structural genes. Mitochondria isolated from yme1 yeast generated a membrane potential upon the addition of succinate, but unlike mitochondria isolated either from wild-type yeast or from yeast bearing yme1 and a suppressing mutation, were unable to generate a membrane potential upon the addition of ATP. Nuclear-encoded F0 subunits accumulate in yme1 yeast lacking mitochondrial DNA; however, deletion of genes encoding those subunits did not suppress the requirement of yme1 yeast for intact mitochondrial DNA. In contrast, deletion of INH1, which encodes an inhibitor of the F1F0-ATPase, partially suppressed the growth defect of yme1 yeast lacking mitochondrial DNA. We conclude that Yme1p is in part responsible for assuring sufficient F1F0-ATPase activity to generate a membrane potential in mitochondria lacking mitochondrial DNA and propose that Yme1p accomplishes this by catalyzing the turnover of protein inhibitors of the F1F0-ATPase.