Inhibition of GR-mediated transcription by p23 requires interaction with Hsp90

p23 is a regulatory co-chaperone of heat shock protein (Hsp) 90, but can also act as a general molecular chaperone by itself. Using novel point mutations of p23 that disrupt its interaction with Hsp90 we found its co-chaperone function to be required for its inhibitory effect on glucocorticoid receptor (GR). The C-terminal region of p23, which is required for its chaperone activity, is dispensable for inhibition of GR. Importantly, similar results were obtained with a constitutively active GR. Thus, the action of p23 on the nuclear stage of GR regulation requires its Hsp90 co-chaperone function, but not its chaperone activity.