The steroid hormone antheridiol regulates sexual development in the fungus Achlya ambisexualis. Analyses of in vivo-labeled proteins from hormone-treated cells revealed that one of the characteristic antheridiolinduced proteins appeared to be very similar to the Achlya 85-kilodalton (kDa) heat shock protein. Analysis of in vitro translation products of RNA isolated from control, heat-shocked, or hormone-treated cells demonstrated an increased accumulation of mRNA encoding a similar 85-kDa protein in both the heat-shocked and hormone-treated cells. Northern (RNA) blot analyses with a Drosophila melanogaster hsp83 probe indicated that a mRNA species of approximately 2.8 kilobases was substantially enriched in both heat-shocked and hormone-treated cells. The monoclonal antibody AC88, which recognizes the non-hormone-binding component of the Achlya steroid receptor, cross-reacted with Achlya hsp85 in cytosols from heat-shocked cells. This monoclonal antibody also recognized both the hormone-induced and heat shock-induced 85-kDa in vitro translation products. Taken together, these data suggest that similar or identical 85-kDa proteins are independently regulated by the steroid hormone antheridiol and by heat shock and that this protein is part of the Achlya steroid receptor complex. Our results demonstrate that the association of hsp9O family proteins with steroid receptors observed in mammals and birds extends also to the eucaryotic microbes and suggest that this association may have evolved early in steroid-responsive systems.The water mold Achlya ambisexualis is a eucaryotic, filamentous fungus in which sexual reproduction is regulated by well-characterized steroid hormones (4,22,54). There are at least two mating types in A. ambisexualis, and these are usually referred to as male and female. Each of these mating types produces a specific fungal steroid hormone which is secreted into the medium and which mediates both chemotropic and developmental changes in cells of the opposite mating type. In this respect, the Achlya steroids act as both pheromones and hormones.The fungal steroid hormone antheridiol, normally produced by female strains of A. ambisexualis, has been isolated (30) and characterized (2). Studies in our laboratory have focused on the early developmental events which occur when antheridiol is added to vegetatively growing cultures of the male mating type, strain E87 (9-11, 48). In E87, one of the earliest developmental events seen at the morphological level after treatment of cells with the hormone is the formation of distinct structures called antheridial branches (3). Both the time of onset of branching and the percent of hyphae branched are directly related to the amount of hormone added (9, 30). Thus, the response is very synchronous, and a high percentage of the hyphae is target tissue for the hormone. For these reasons, A. ambisexualis represents an interesting system in which to investigate both the mechanism of action of steroid hormones and the evolution of steroid hormone systems.In previou...