With assistance from co-chaperone partner proteins, Hsp90 plays an essential positive role in supporting the structure and function of numerous client proteins in vivo. Hsp90's co-chaperone partnerships are believed to regulate and/or target its function. Here we describe associations between Hsp90 chaperone machinery and another chaperone, the 97-kDa valosin-containing protein VCP. Coimmunoadsorption assays indicate that VCP occurs in one or more native heterocomplexes containing Hsp90 and the Hsp90 partner proteins Cdc37, FKBP52, and p23. Functional characterizations indicate that VCP,is not an Hsp90 substrate, but rather demonstrate the biochemical hallmarks of an Hsp90 co-chaperone. Potential roles for a collaboration between for Hsp90 and VCP are discussed. (c) 2005 Elsevier Inc. All rights reserved.