tokens: Using purified proteins , we have prepared a five-protein heterocomplex assembly system consisting of two proteins essential for heterocomplex assembly- hsp90 and hsp70 -and three proteins that act as co-chaperones to enhance assembly- Hop , hsp40 , p23 [ Morishima , Y. , Kanelakis , K. C. , Silverstein , A. M. , Dittmar , K. D. , Estrada , L. , and Pratt , W. B. ( 2000 ) J. Biol. Chem. 275 , 6894-6900 ] .