The common 90-kd protein component of non-transformed ‘8S’ steroid receptors is a heat-shock protein.

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Abstract
Non‐transformed steroid receptors have an approximately 8S sedimentation coefficient that corresponds to an oligomeric structure of 250‐300 kd which includes a non‐hormone binding 90‐kd protein. A monoclonal antibody BF4 raised against the purified, molybdate‐stabilized, 8S progesterone receptor (8S‐PR) from chick oviduct, recognizes 8S forms of all steroid hormone receptors. BF4 was found specific for a 90‐kd protein present in great abundance in all chicken tissues, including that present in 8S‐forms of steroid receptors. Here, using immunological and biochemical techniques, we demonstrate that this ubiquitous BF4‐positive 90‐kd protein is in fact the chicken 90 kd heat‐shock protein (hsp 90): it increased in heat‐shocked chick embryo fibroblasts, and displayed identical migration in two‐dimensional gel electrophoresis and the same V8 peptide map as the already described hsp 90. We discuss the possibility that the interaction between hsp 90 and steroid hormone‐binding subunits may play a role in keeping the receptor in an inactive form.