Mitochondrial DNA instability mutants of the bifunctional protein Ilv5p have altered organization in mitochondria and are targeted for degradation by Hsp78 and the Pim1p protease.

Ilv5p is a bifunctional mitochondrial protein in Saccharomyces cerevisiae required for branched-chain amino acid biosynthesis and for the stability of wild-type (rho(+)) mitochondrial DNA (mtDNA). Mutant forms of Ilv5p defective in mtDNA stability (a(+)D(-)) are present as 5-10 punctate structures in mitochondria, whereas mutants lacking enzymatic function (a(-)D(+)) show a reticular distribution, as does wild-type Ilv5p. a(+)D(-) ilv5 mutations are recessive, and the mutant protein is redistributed to a reticular form when co-expressed with wild-type Ilv5p. Ilv5p proteins that are punctate in vivo are also less soluble in detergent extracts of isolated mitochondria, suggesting that the punctate foci in a(+)D(-) Ilv5p mutants are aggregates of the protein. a(+)D(-) Ilv5p proteins are selectively degraded in cells lacking a functional mitochondrial genome, but only in cells grown under derepressing conditions. The targeted degradation of a(+)D(-) Ilv5p, which occurs even when co-expressed with wild-type Ilv5p, is mediated by the glucose-repressible chaperone, Hsp78, and by the ATP-dependent Pim1p protease, whose activity may be modulated by rho(+) mtDNA.