Evolution and function of diverse Hsp90 homologs and cochaperone proteins.

Members of the Hsp90 molecular chaperone family are found in the cytosol, ER, mitochondria and chloroplasts of eukaryotic cells, as well as in bacteria. These diverse family members cooperate with other proteins, such as the molecular chaperone Hsp70, to mediate protein folding, activation and assembly into multiprotein complexes. All examined Hsp90 homologs exhibit similar ATPase rates and undergo similar conformational changes. One of the key differences is that cytosolic Hsp90 interacts with a large number of cochaperones that regulate the ATPase activity of Hsp90 or have other functions, such as targeting clients to Hsp90. Diverse Hsp90 homologs appear to chaperone different types of client proteins. This difference may reflect either the pool of clients requiring Hsp90 function or the requirement for cochaperones to target clients to Hsp90. This review discusses known functions, similarities and differences between Hsp90 family members and how cochaperones are known to affect these functions. This article is part of a Special Issue entitled: Heat Shock Protein 90 (HSP90).