Identification of SSF1, CNS1, and HCH1 as multicopy suppressors of a Saccharomyces cerevisiae Hsp90 loss-of-function mutation

Hsp90 functions in a multicomponent chaperone system to promote the maturat ion and maintenance of a diverse, but specific, set of target proteins that play key roles in the regulation of cell growth and development. To identi fy additional components of the Hsp90 chaperone system and its targets, we searched for multicopy suppressors of various temperature-sensitive mutatio ns in the yeast Hsp90 gene, HSP82. Three suppressors were isolated for one Hsp90 mutant (glutamate --> lysine at amino acid 381). Each exhibited a uni que, allele-specific pattern of suppression with other Hsp90 mutants and ha d unique structural and biological properties. SSF1 is a member of an essen tial gene family and functions in the response to mating pheromones. CNS1 i s an essential gene that encodes a component of the Hsp90 chaperone machine ry. The role of HCH1 is unknown; its sequence has no strong homology to any protein of known function. SSF1 and CNS1 were weak suppressors, whereas HC H1 restored wild-type growth rates at all temperatures tested to cells expr essing the E381K mutant. Overexpression of CNS1 or HCH1, but not SSI;I, enh anced the maturation of a heterologous Hsp90 target protein, p60(v-src). Th ese results suggest that like Cns1p, Hch1p is a general modulator of Hsp90 chaperone functions, whereas Ssf1p likely is either an Hsp90 target protein or functions in the same pathway as an Hsp90 target protein.