STUDIES WITH PURIFIED CHAPERONES ADVANCE THE UNDERSTANDING OF THE MECHANISM OF GLUCOCORTICOID RECEPTOR HSP90 HETEROCOMPLEX ASSEMBLY

The study of the 9S, untransformed state of steroid receptors has led to the discovery of a multiprotein chaperone system that assembles het erocomplexes between hsp90 and a variety of proteins involved in signa l transduction. Using the formation of glucocorticoid receptor (GR)-hs p90 heterocomplexes as a model, we have reconstituted a fully function al heterocomplex assembly system from purified components. The basic a ssembly system requires four proteins - hsp90, hsp70, p60/Hop and hsp4 0 - to assemble GR-hsp90 heterocomplexes, which are then stabilized by the hsp90-interacting protein p23. The four proteins can self-assembl e into an hsp90-p60/Hop-hsp70-hsp40 complex that we call a foldosome. Foldosomes isolated from reticulocyte lysate or formed from purified p roteins open up a steroid-binding pocket to create a high-affinity ste roid-binding state of the GR. We describe here the systematic reconsti tution of the hsp90-based chaperone machinery and develop a model of t he receptor-hsp90 heterocomplex assembly mechanism.