A variety of signaling proteins form heterocomplexes with and are regulated by the heat shock protein chaperone hsp90. These complexes are formed by a multiprotein machinery, including hsp90 and hsp70 as essential and abundant components and Hop, hsp40, and p23 as nonessential cochaperones that are present in much lower abundance in cells. Overexpression of signaling proteins can overwhelm the capacity of this machinery to properly assemble heterocomplexes with hsp90. Here, we show that the

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... ting component of this assembly machinery in vitro in reticulocyte lysate and in vivo in Sf9 cells is p23. Only a fraction of glucocorticoid receptors (GR) overexpressed in Sf9 cells are in heterocomplex with hsp90 and have steroid binding activity, with the majority of the receptors present as both insoluble and cytosolic GR aggregates. Coexpression of p23 with the GR increases the proportion of cytosolic receptors that are in stable GR⅐hsp90 heterocomplexes with steroid binding activity, a strictly hsp90-dependent activity for the GR. Coexpression of p23 eliminates the insoluble GR aggregates and shifts the cytosolic receptor from very large aggregates without steroid binding activity to ϳ600-kDa heterocomplexes with steroid binding activity. These data lead us to conclude that p23 acts in vivo to stabilize hsp90 binding to client protein.