Hsp90/Hsp70 chaperone machine regulation of the Saccharomyces MAL-activator as determined in vivo using noninducible and constitutive mutant alleles
Fulai Ran 1, Mehtap Bali 1 and Corinne A. Michels 2*
1 Graduate School of CUNY
2 Queens College of CUNY
* To whom correspondence should be addressed. E-mail: corinne.michels{at}qc.cuny.edu.
Submitted on November 23, 2007
Revised on December 19, 2007
Accepted on 8 March 2008
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Abstract |
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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 (Ssa1), Hsp90 (Hsp82), and Hop (Sti1) homologs using a collection of inducible, constitutive, and noninducible alleles. Each class of mutant activator forms a distinctly different stable multi-chaperone complex in the absence of maltose. Inducible Mal63p associates with Ssa1, Hsp82, and Sti1 and is released in the presence of maltose. Noninducible mal63 mutant proteins bind to Ssa1 alone and do not stably associate with Hsp82 or Sti1. Constitutive MAL-activators bind well to Hsp82 and poorly to Ssa1 and Sti1, but deletion of STI1 restores Ssa1 binding. Taken together, Mal63p regulation requires the formation of Hsp90/Hsp70 sub-complexes comparable to yet distinct from those observed with previously characterized Hsp90 clients including glucocorticoid receptor and yeast Hap1p. Thus, comparative studies of different client proteins highlight functional diversity in the operation of the Hsp90/Hsp70 chaperone machine.
Key Words:
Hsp90/Hsp70 chaperone machine, MAL genes, MAL-activator mutations, MAL-activator regulation, Saccharomyces
