Nucelotide binding states of hsp70 and hsp90 during assembly of heterocomplexes between the glucocorticoid receptor and hsp90.

Abstract

The hsp90-based chaperone system is essential for the conversion of the glucocorticoid receptor (GR) to a steroid binding state. The two essential components of the system, hsp70 and hsp90, have nucleotide binding sites and intrinsic ATPase activity. This thesis presents new results on the nucleotide requirements of hsp70 and hsp90 during the GR-hsp90 heterocomplex assembly process and during the opening of the steroid binding cleft of the GR to access by steroid. BAG-1, which binds to hsp70 and accelerates its conversion to the ATP-bound form, has differential effects on GR.hsp90 heterocomplex assembly and steroid binding. I show that at physiologic concentrations, BAG-1 does not affect GR activation but does promote the dissociation of Hop from the GR&middot;hsp90 heterocomplex. At high concentrations of BAG-1 that are stoichiometric with respect to hsp70, a concentration-dependent inhibition of GR&middot;hsp90 heterocomplex assembly and steroid-binding is produced. Hip stabilizes the ADP-bound form of hsp70, and I demonstrate that Hip has no affect on GR&middot;hsp90 heterocomplex assembly by the chaperone machinery. However, I found that Hip is able to reverse the inhibition of assembly by BAG-1 <italic>in vitro</italic>, and transfection of Hip attenuates the inhibition of steroid binding activity caused by BAG-1. I conclude that Hip and BAG-1 are not essential for GR folding or GR&middot;hsp90 heterocomplex assembly, but act as co-chaperones involved in regulation of hsp70 <italic>in vivo</italic>. Using a stepwise approach, the chaperone machinery has been assembled on the GR in two sequential ATP-dependent steps. In the first step, the GR is primed by hsp70 to a state that can bind hsp90. In the second step, hsp90 binds to the GR&middot;hsp70 complex and is converted to its ATP-bound conformation. I determined that hsp70 initially interacts with the GR in an ATP-bound form. During the priming step, hsp70 is converted to the ADP-bound form on the receptor. Hsp90 binds rapidly to the GR&middot;hsp70 complex and is converted to the ATP-dependent conformation. Both steps are potassium-dependent, suggesting a requirement for hsp70 ATPase activity during both steps. This thesis shows the first demonstration that the two essential components of the chaperone machinery, hsp70 and hsp90, participate in a series of ordered conversions between their ATP- and ADP-dependent conformations during heterocomplex assembly and cleft opening.