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

Although little is known about the precise mechanisms by which the molecula r chaperone Hsp90 recognizes its client proteins, Cdc37 has been shown to p lay a critical role in the targeting of Hsp90 to client protein kinases. De scribed here is the identification and characterization of a novel 35-kDa h uman protein that is 31% identical to Cde37. We have named this novel prote in 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 Har e mRNA in several human tissues, including liver, skeletal muscle, and kidn ey. 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 k inases or Raf-l. Mapping experiments indicate that the central 120 amino ac ids of both Hare and Cdc37 constitute a Hsp90-binding domain not described previously. FLAG-Harc is basally serine-phosphorylated and hyperphosphoryla ted when co-expressed with an activated mutant of the Src family kinase Hck . Notably, FLAG-Harc forms complexes with Hsp90, Hsp70, p60Hop, immunophili ns, 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 heterocomplex es.