Mutational analysis of Hsp90 function: interactions with a steroid receptor and a protein kinase

Hsp90 is a protein chaperone whose functions are focused on a specific set of target proteins. The nature of Hsp90's interactions with these proteins is poorly understood. To provide tools for examining these interactions, we have isolated eight broadly distributed temperature-sensitive (ts) point mutations in the Hsp90 gene (HSP82) of Saccharomyces cerevisiae. The mutants fall into two distinct classes. One has a classic ts phenotype, with nearly wild-type activity at 25 degrees C and a precipitous loss of function at 34 degrees C. The remaining seven mutants, in contrast, cause a general reduction in Hsp90 function and are ts because they do not provide the high level of function required for growth at high temperatures. The effects of these mutants on two target proteins, a transcription factor (glucocorticoid receptor) and a tyrosine kinase (pp60v-src), provided several insights on Hsp90 function. First, Hsp90 is not only required to help the glucocorticoid receptor achieve a hormone-activable state, it is continuously required to maintain that state. Second, Hsp90's function in the maturation of pp60v-src involves separable roles in protein accumulation and kinase activation. Thus, Hsp90 is an integral component of both the steroid receptor and kinase signaling pathways. Finally, all eight point mutants affect the activation of both the glucocorticoid receptor and pp60v-src, indicating that Hsp90 promotes the activity of these very different target proteins through common mechanisms.