High-molecular-weight FK506-binding proteins are components of heat-shock protein 90 heterocomplexes in wheat germ lysate

In animal cell lysates the multiprotein heat-shock protein 90 (hsp90)-based chaperone complexes consist of hsp70, hsp40, and p60. These complexes act to convert steroid hormone receptors to their steroid-binding state by asse mbling them into heterocomplexes with hsp90, p23, and one of several immuno philins. Wheat germ lysate also contains a hsp90-based chaperone system tha t can assemble the glucocorticoid receptor into a functional heterocomplex with hsp90. However, only two components of the heterocomplex-assembly syst em, hsp90 and hsp70, have thus far been identified. Recently, purified mamm alian p23 preadsorbed with JJ3 antibody-protein A-Sepharose pellets was use d to isolate a mammalian p23-wheat hsp90 heterocomplex from wheat germ lysa te (J.K. Owens-Grille, L.F. Stancato, K. Hoffmann, W.B. Pratt, and P. Krish na [1996] Biochemistry 35: 15249-15255). This heterocomplex was found to co ntain an immunophilin(s) of the FK506-binding class, as judged by binding o f the radiolabeled immunosuppressant drug [H-3]FK506 to the immune pellets in a specific manner. In the present study we identified the immunophilin c omponents of this heterocomplex as FKBP73 and FKBP77, the two recently desc ribed high-molecular-weight FKBPs of wheat. In addition, we present evidenc e that the two FKBPs bind hsp90 via tetratricopeptide repeat domains. Our r esults demonstrate that binding of immunophilins to hsp90 via tetratricopep tide repeat domains is a conserved protein interaction in plants. Conservat ion of this protein-to-protein interaction in both plant and animal cells s uggests that it is important for the biological action of the high-molecula r-weight immunophilins.