A novel chaperone complex for steroid receptors involving heat shock proteins, immunophilins, and p23.

In the absence of hormone, the avian progesterone receptor exists in a large multiprotein complex that is inactive but able to bind and respond to progestins. This inactive complex can be reconstituted in vitro by incubation of receptor monomer in rabbit reticulocyte lysate in the presence of ATP and magnesium. This results in receptor binding to the two heat shock proteins, hsp90 and hsp70, the FK506 binding-proteins, FKBP54 and FKBP52, the cyclosporin A-binding protein, cyclophilin-40, and the recently characterized protein p23. Immune isolation of p23 from rabbit reticulocyte lysate in the absence of receptor reveals an ATP-dependent complex containing the major proteins associated with steroid receptors. Depletion of p23 from lysate prevents the assembly of progesterone receptor complexes, and the addition of purified p23 restores this activity, indicating that the p23 protein complex is an essential precursor to the formation of progesterone receptor complexes.