FOLDING IN-VIVO OF A NEWLY TRANSLATED YEAST CYTOSOLIC ENZYME IS MEDIATED BY THE SSA CLASS OF CYTOSOLIC YEAST HSP70 PROTEINS

The nature of chaperone action in the eukaryotic cytosol that assists newly translated cytosolic proteins to reach the native state has rema ined poorly defined. Actin, tubulin, and G(alpha) transducin are assis ted by the cytosolic chaperonin, CCT, but many other proteins, for exa mple, ornithine transcarbamoylase (OTC), a cytosolic homotrimeric enzy me of yeast, do not require CCT action. Here, we observe that yeast cy tosolic OTC is assisted to its native state by the SSA class of yeast cytosolic Hsp70 proteins. In vitro, refolding of OTC diluted from dena turant was assisted by crude yeast cytosol and ATP and found to be dir ected by SSA1/2, In vivo, when OTC was induced in a temperature-sensit ive SSA-deficient strain, it exhibited reduced specific activity, and nonnative subunits were detected in the soluble fraction, These findin gs indicate that, in vivo, the Hsp70 system assists in folding at leas t some newly translated cytosolic enzymes, most likely functioning in a posttranslational manner.