GENETIC AND BIOCHEMICAL-ANALYSIS OF P23 AND ANSAMYCIN ANTIBIOTICS IN THE FUNCTION OF HSP90-DEPENDENT SIGNALING PROTEINS

The ubiquitous molecular chaperone Hsp90 acts in concert with a cohort of associated proteins to facilitate the functional maturation of a n umber of cellular signaling proteins, such as steroid hormone receptor s and oncogene tyrosine kinases, The Hsp90-associated protein p23 is r equired for the assembly of functional steroid aporeceptor complexes i n cell lysates, and Hsp90-binding ansamycin antibiotics disrupt the ac tivity of Hsp90-dependent signaling proteins in cultured mammalian cel ls and prevent the association of p23 with Hsp90-receptor heterocomple xes; these observations have led to the hypotheses that p23 is require d for the maturation of Hsp90 target proteins and that ansamycin antib iotics abrogate the activity of such proteins by disrupting the intera ction of p23 with Hsp90, In this study, I demonstrate that ansamycin a ntibiotics disrupt the function of Hsp90 target proteins expressed in yeast cells; prevent the assembly of Sba1, a yeast p23-like protein, i nto steroid receptor-Hsp90 complexes; and result in the assembly of re ceptor-Hsp90 complexes that are defective for ligand binding. To asses s the role of p23 in Hsp90 target protein function, I show that the ac tivity of Hsp90 target proteins is unaffected by deletion of SBA1. Int erestingly, steroid receptor activity in cells lacking Sba1 displays i ncreased sensitivity to ansamycin antibiotics, and this phenotype is r escued by the expression of human p23 in yeast cells. These findings i ndicate that Hsp90-dependent signaling proteins can achieve a function al conformation in vivo in the absence of p23. Furthermore, while the presence of p23 decreases the sensitivity of Hsp90-dependent processes to ansamycin treatment, ansamycin antibiotics disrupt signaling throu gh some mechanism other than altering the Hsp90-p23 interaction.