THE MITOCHONDRIAL CLPB HOMOLOG HSP78 COOPERATES WITH MATRIX HSP7O IN MAINTENANCE OF MITOCHONDRIAL-FUNCTION

The mitochondrial heat shock protein Hsp78 is a member of the Hsp104/C lp family with unknown function. Saccharomyces cerevisiae deletion mut ants of HSP78 show wild-type like growth. We report that deletion of t he HSP78 gene in yeast strains with point mutations in the SSC1 gene ( encoding matrix Hsp70) led to loss of mitochondrial DNA, indicating th at at least one of the heat shock proteins Hsp78 and mt-Hsp70 is neede d to maintain a rho(+) state of the mitochondrial genome. Mitochondria isolated from these double mutants had a strongly reduced membrane po tential, explaining defects in the rate of preprotein import. The lack of Hsp78 led to aggregation of the mutant mt-Hsp70s while other matri x chaperones stayed soluble. We conclude that Hsp78 is required to kee p mutant forms of mt-Hsp70 soluble and suggest a cooperation of Hsp78 and mt-Hsp70 in maintenance of essential mitochondrial functions. (C) 1995 Academic Press Limited