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Many biological processes are initiated by cooperative assembly of large multicomponent complexes; however, mechanisms for modulating or terminating the actions of these complexes are not well understood. For example, hormone-bound intracellular receptors (IRs) nucleate formation of transcriptional regulatory complexes whose actions cease promptly upon hormone withdrawal. Here, we show that the p23 molecular chaperone localizes in vivo to genomic response elements in a hormone-dependent manner, disrupting receptor-mediated transcriptional activation in vivo and in vitro; Hsp90 weakly displayed similar activities. Indeed, p23 and Hsp90 also disrupted the activities of some non-IR-containing transcriptional regulatory complexes. We suggest that molecular chaperones promote disassembly of transcriptional regulatory complexes, thus enabling regulatory machineries to detect and respond to signaling changes.
Many physiological signal transduction pathways culminate in selective modulation of specific gene transcription rates. For example, glucocorticoid or thyroid hormones in the bloodstream enter target cells and associate with the glucocorticoid receptor (GR) or thyroid hormone receptor (TR), respectively. These receptor-hormone complexes bind selectively to specific genomic sites termed hormone response elements (BREs) and trigger formation of large, heterotypic transcriptional regulatory complexes, which in turn activate or repress transcription from nearby promoters (1, 2). The effects of hormonal signaling are quite rapid: Rates of transcription at inducible genes increase with a half-life (t1/2) of ~5 to 10 min (3, 4). Similarly, hormone withdrawal leads promptly to hormone release from the receptor and cessation of activation (5).
How does an intracellular receptor (IR), bound to hormone and in the cell nucleus as part of a large regulatory complex, detect and respond to a decline in extracellular hormone levels? Recent studies suggest that certain factors, when assembled into regulatory complexes, may become targets for proteasomal degradation (6). However, IRs do not appear to be degraded upon hormone withdrawal; indeed, unliganded IRs commonly display longer intracellular half-lives than their liganded counterparts (7, 8). Rather, we have proposed that IR-containing regulatory complexes may be actively and reversibly disassembled by molecular chaperones (9).
We tested directly whether the p23 molecular chaperone may be involved in the disassembly of transcriptional regulatory complexes, by (i) assessing the actions of p23 on functional regulatory complexes in vitro, (ii) developing a general approach to determine in vivo the effects of chaperone components on regulatory complexes containing IRs or...