Hsp90/Hsp70 chaperone machine regulation of the Saccharomyces MAL-activator as determined in vivo using noninducible and constitutive mutant alleles

The Hsp90/Hsp70 chaperone machine is an essential regulator of cell growth and division. It is required for activation of select client proteins, chiefly protein kinases and transcription activators and thus plays a major role in regulating intracellular signaling and gene expression. This report demonstrates, in vivo, the association of the Saccharomyces cerevisiae maltose-responsive transcription activator Mal63 (MAL-activator) with the yeast Hsp70 (Ssal), Hsp90 (Hsp82), and Hop (Stil) homologs, using a collection of inducible, constitutive, and noninducible alleles. Each class of mutant activator forms a distinctly different stable multichaperone complex in the absence of maltose. Inducible Mal63p associates with Ssal, Hsp82, and Stil and is released in the presence of maltose. Noninducible Mal63 mutant proteins bind to Ssal alone and do not stably associate with Hsp82 or Stil. Constitutive MAL-activators bind well to Hsp82 and poorly to Ssal and Stil, but deletion of STIl restores Ssal binding. Taken together, Mal63p regulation requires the formation of Hsp90/Hsp70 subcomplexes comparable to, yet distinct from those observed with previously characterized Hsp90 clients including glucocorticoid receptor and yeast Haplp. Thus, comparative studies of different client proteins highlight functional diversity in the operation of the Hsp90/Hsp70 chaperone machine.