ANIMAL AND PLANT-CELL LYSATES SHARE A CONSERVED CHAPERONE SYSTEM THATASSEMBLES THE GLUCOCORTICOID RECEPTOR INTO A FUNCTIONAL HETEROCOMPLEXWITH HSP90

The hormone-binding domain of the glucocorticoid receptor must be boun d to heat shock protein (hsp) 90 for it to have a high-affinity steroi d-binding conformation. Cell-free assembly of a glucocorticoid recepto r-hsp90 heterocomplex is brought about in reticulocyte lysate by a pre formed protein-folding complex containing hsp90, hsp70, and other prot eins [Hutchison, K. A., Dittmar, K. D., st Pratt, W. B. (1994) J. Biol . Chem. 269, 27894-27899]. In this ''foldosome'' system, hsp70 is requ ired for assembly of the receptor-hsp90 complex and concomitant activa tion of steroid-binding activity [Hutchison, K. A., Dittmar, K. D., Cz ar, M. J., & Pratt, W. B. (1994) J. Biol. Chem. 269, 22157-22161]. Ail previous experiments involving cell-free assembly of both receptor-hs p90 and protein kinase-hsp90 heterocomplexes have been carried out wit h the protein-folding system in rabbit reticulocyte lysate. In this wo rk, we show that concentrated lysates of receptor-free mouse (L cells) and insect (Sf9) cells and also a plant (wheat germ) lysate fold the immunopurified glucocorticoid receptor into a functional (i.e., steroi d binding) heterocomplex with hsp90. Receptor heterocomplex formation in animal lysates and in the plant lysate are not identical in that th e dynamics of complex assembly are different, but both systems produce a functional complex that binds steroid. Also, in contrast to animal and insect complexes, receptor-plant hsp90 complexes are not stabilize d by molybdate. When added to the other lysate, purified plant and ani mal hsp90s show partial complementarity, in that a receptor-hsp90 comp lex is formed but the receptor is not converted to the steroid-binding conformation. When added to rabbit reticulocyte lysate that has been depleted of endogenous hsp70, purified wheat germ and mouse hsp70's an equally active in promoting both assembly of receptor-hsp90 heterocom plexes and conversion of receptor to the steroid-binding conformation. Thus, hsp70 from the plant kingdom has conserved the ability to inter act functionally with chaperone proteins of the animal kingdom to coop erate in protein folding as evidenced by formation of a functional rec eptor-hsp90 heterocomplex.