Background: The Hsp90 chaperone complex functions in assembly, folding and activation of numerous substrates. The two vertebrate homologues encoded by hsp90 and hsp90 genes are differentially expressed in embryonic and adult tissues and during stress, however, it is not known if they possess identical functional activities in chaperone complexes. This question was addressed by examining potential differences between the Hsp90 isoforms with respect to both co-chaperone and substrate interactions.

Materials and Methods: Epitope-tagged proteins were expressed in mammalian cells or Xenopus oocytes and subjected to immunoprecipitation with an array of co-chaperones.

Results: Both isoforms were shown to participate equally in multi-chaperone complexes and no significant difference in co-chaperone distribution was observed. The substrates Raf-1, HSF1, Cdc37 and Mek interacted with both Hsp90 and Hsp90, and the relative patterns of these interactions were not affected by heat shock. The substrates kinases c-Src, CKIIB, A-raf, and Erk interacted with both isoforms, however, significantly more Hsp90 was recovered after heat shock.

Conclusion: The results demonstrate that the Hsp90 and Hsp90 exhibit similar interactions with co-chaperones, but significantly different behaviors with respect to substrate interactions under stress conditions.