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Chaperone-Mediated Repression

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Science  16 Nov 2001:
Vol. 294, Issue 5546, pp. 1421
DOI: 10.1126/science.294.5546.1421c

Chaperone proteins help to fold functional signaling proteins and also control the activity of their protein targets. In particular, the Hsp90- and Hsp70-type chaperones are involved in the assembly and control of transcriptional nuclear receptor complexes.

Hon et al. now present evidence for similar, yet distinct, roles for chaperones in the control of the transcription factor heme activator protein (Hap1), in Saccharomyces cerevisiae. Yeast respond to oxygen with increased production of heme, which activates Hap1 to promote transcription of genes involved in respiration and protection from oxygen damage. In the absence of heme, the activity of Hap1 is repressed by interactions with Hsp90 and the Hsp70 proteins Ssa and Sro9. Decreased expression of Ssa (and, to a lesser extent, decreased expression of Sro or its partner Ydj1) enhanced activity of Hap1 in the absence of heme but did not affect Hap1 activity in the presence of high concentrations of heme. This contrasts with the effect of Hsp90 on Hap1 or on nuclear receptors. Loss of Hsp90 reduces the transcriptional activity of its partners even in the presence of activators (steroid hormones or heme). These distinct modes of Hap1 regulation by different chaperones provide a means for precise regulation of Hap1 and suggest versatility in the control of transcription in complexes that contain multiple chaperones.—LBR

Mol. Cell. Biol.21, 7923 (2001).

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