Pjm. Murphy et al., Stoichiometry, abundance, and functional significance of the hsp90/hsp70-based multiprotein chaperone machinery in reticulocyte lysate, J BIOL CHEM, 276(32), 2001, pp. 30092-30098
Rabbit reticulocyte lysate contains a multiprotein chaperone system that as
sembles the glucocorticoid receptor (GR) into a complex with hsp90 and conv
erts the hormone binding domain of the receptor to its high affinity steroi
d binding state. This system has been resolved into five proteins, with hsp
90 and hsp70 being essential and Hop, hsp40, and p23 acting as co-chaperone
s that optimize assembly. Hop binds independently to hsp70 and hsp90 to for
m an hsp90(.)Hop(.)hsp70 complex that acts as a machinery to open up the GR
steroid binding site. Because purified hsp90 and hsp70 are sufficient for
some activation of GR steroid binding activity, some investigators have rej
ected any role for Hop in GR(.)hsp90 heterocomplex assembly. Here, we count
er that impression by showing that all of the Hop in reticulocyte lysate is
present in an hsp90(.)Hop(.)hsp70 complex with a stoichiometry of 2:1.1. T
he complex accounts for similar to 30% of the hsp90 and similar to9% of the
hsp70 in lysate, and upon Sephacryl S-300 chromatography the GR(.)hsp90 as
sembly activity resides in the peak containing Hop-bound hsp90. Consistent
with the notion that the two essential chaperones cooperate with each other
to open up the steroid binding site, we also show that purified hsp90 and
hsp70 interact directly with each other to form weak hsp90-hsp70 complexes
with a stoichiometry of 2:1.