ROLE OF THE CHAPERONIN COFACTOR HSP10 IN PROTEIN-FOLDING AND SORTING IN YEAST MITOCHONDRIA

Protein folding in mitochondria is mediated by the chaperonin Hsp60, t he homologue of E. coli GroEL. Mitochondria also contain a homologue o f the cochaperonin GroES, called Hsp10, which is a functional regulato r of the chaperonin. To define the in vivo role of the co-chaperonin, we have used the genetic and biochemical potential of the yeast S. cer evisiae. The HSP10 gene was cloned and sequenced and temperature-sensi tive lethal hsp10 mutants were generated. Our results identify Hsp10 a s an essential component of the mitochondrial protein folding apparatu s, participating in various aspects of Hsp60 function. Hsp10 is requir ed for the folding and assembly of proteins imported into the matrix c ompartment, and is involved in the sorting of certain proteins, such a s the Rieske Fe/S protein, passing through the matrix en route to the intermembrane space. The folding of the precursor of cytosolic dihydro folate reductase (DHFR), imported into mitochondria as a fusion protei n, is apparently independent of Hsp10 function consistent with observa tions made for the chaperonin-mediated folding of DHFR in vitro. The t emperature-sensitive mutations in Hsp10 map to a domain (residues 25-4 0) that corresponds to a previously identified mobile loop region of b acterial GroES and result in a reduced binding affinity of hsp10 for t he chaperonin at the non-permissive temperature.