Steroid receptors and their associated proteins

Abstract

Those investigators involved in the purification and characterization of steroid receptors have been confronted with a number of difficulties, one being the expanding list of cellular proteins that can bind and copurify with these receptors. Research over the past 8-10 yr has established a high degree of acceptance for the concept that most inactive steroid receptors are sequestered in a complex with the 90-kilodalton (kDa) heat shock protein, hsp90 (see refs. 1, 2 for review). Initial observations on the copurification of this 90-kDa protein were confusing, and in some cases it was thought to be a contaminant or to actually be the receptor. Clarification of a receptor:hsp90 complex was provided by several approaches which mainly involved the use of antibodies to rapidly and gently isolate receptor complexes. That such a complex exists in the cell is supported primarily by three types of experiments The metabolic labeling studies of Howard and Distelhorst (3) argued that hsp90 binding was not simply occurring after cell lysis. Yeast mutants having an abnormally low level of hsp90 have been shown to be inefficient for the expression of functional steroid receptors (4). Most recently, Gehring and coworkers (5) have used chemical cross-linking in intact cells to show an association of glucocorticoid receptor with hsp90 and a second protein, p59. While a role of hsp90 in receptor structure is strongly supported, the topic of receptor-associated proteins is still quite confusing and uncertain. Most more recent members of this list (see Table 1) suffer from a lack of information regarding their identity and their presence or absence in a variety of receptor systems. Whether they truly represent intracellular receptor-associated proteins of significance to hormone action is unknown, and this represents a key topic for future research in this area. In this brief review, we will attempt to consolidate studies from a variety of systems, clarify some of the confusing terminology about receptor-associated proteins, and suggest some rationale for their participation