Identification and characterization of harc, a novel Hsp90-associating relative of Cdc37

Although little is known about the precise mechanisms by which the molecular chaperone Hsp90 recognizes its client proteins, Cdc37 has been shown to play a critical role in the targeting of Hsp90 to client protein kinases. Described here is the identification and characterization of a novel 35-kDa human protein that is 31% identical to Cde37. We have named this novel protein Rare ((H) under bar sp90-(a) under bar ssociating (r) under bar elative of (C) under bar dc37). Northern blot analysis revealed the presence of Hare mRNA in several human tissues, including liver, skeletal muscle, and kidney. Biochemical fractionation and immunofluorescent localization of epitope-tagged Hare (ie. FLAG-Hare) indicated that it is present in the cytoplasm of cells. FLAG-Harc binds Hsp90 but unlike Cdc37 does not bind Src family kinases or Raf-l. Mapping experiments indicate that the central 120 amino acids of both Hare and Cdc37 constitute a Hsp90-binding domain not described previously. FLAG-Harc is basally serine-phosphorylated and hyperphosphorylated when co-expressed with an activated mutant of the Src family kinase Hck. Notably, FLAG-Harc forms complexes with Hsp90, Hsp70, p60Hop, immunophilins, and an unidentified p22 protein but not with the Hsp90 co-chaperone p23. Thus Hare likely represents a novel participant in Hsp90-mediated protein folding, potentially targeting Hsp90 to Hsp70-client protein heterocomplexes.