Hsp90-containing multiprotein complexes in the eukaryotic microbe Achlya

In the oomycete fungus Achlya ambisexualis, hyphae of the male strain undergo sexual differentiation in the presence of the steroid hormone antheridiol. Earlier studies demonstrated that antheridiol binds with high affinity to a 9S multiprotein complex from A. ambisexualis cytosols. Although these complexes were found to contain the heat shock protein Hsp90, the other components were not known. It was of interest to determine if any of the other protein components in the Achlya Hsp90-heterocomplexes would be homologous to those found in the steroid receptor-Hsp90-heterocomplexes of vertebrates. Cytosolic proteins of 110 kDa, 74 kDa, 64 kDa, 61 kDa, 56 kDa, 47 kDa, 27 kDa and 23 kDa, were found in repeated trials, to co-immunoprecipitate with Achlya Hsp90. The 74 kDa protein was identified as the heat shock protein Hsp70, the 23 kDa protein was found to be related to the vertebrate protein p23 and the 56 kDa protein was found to be related to immunophilin FKBP51. All three of these proteins are components of the vertebrate receptor heterocomplexes. The 110 kDa, 61 kDa and 27 kDa proteins appeared to be unique to the Achlya complexes. Unlike the seven other proteins co-immunoprecipitating with Hsp90, the 61 kDa protein was observed only in the co-immunoprecipitates produced from in vitro translates of RNA isolated from antheridiol-treated mycelia.