Nat Struct Mol Biol. that PU-H71 is able to access a broader range of N domain undimerized Hsp90 conformations than is geldanamycin and is less affected by Hsp90 phosphorylation, consistent with its broader and more potent anti-tumor activity. A more complete understanding of the impact of the cellular milieu on small molecule inhibitor binding to Hsp90 should facilitate their more effective use in the clinic. reconstituted systems have shed light on several aspects of Hsp90-inhibitor interaction, such studies do not recapitulate the presentation of Hsp90 in the human cancer cell, including the potentially complex impact of the chaperone’s numerous and dynamic posttranslational modifications on Rabbit Polyclonal to ME1 drug binding[13-20]. To begin to better understand these parameters, we have investigated the impact of the cellular milieu on the binding preferences and consequences of two chemically unrelated Hsp90 inhibitors, the benzoquinone ansamycin geldanamycin (GA) and the purine analog PU-H71 (PU). Further, we queried the ability of these inhibitors to access similar Hsp90 conformational states. Our data suggest that PU samples a more diverse repertoire of Hsp90 conformations compared to GA, and these differences are amplified in a cellular context. However, both inhibitors prevent the N domain dimerization that is necessary for a productive chaperone cycle. We also find that inhibitor binding is not uniformly affected by Hsp90 phosphorylation. These data suggest that additional posttranslational modifications may differentially affect drug binding and influence their cellular Hsp90 inhibitory activity in ways not predicted by analysis. RESULTS AND DISCUSSION GA and PU recognize overlapping but not identical cellular Hsp90 populations In order to investigate the binding preferences of GA and PU in cancer cells, we used drug-conjugated agarose as an investigative tool. Meclofenoxate HCl Consistent with a recent study , we found that repeated challenge of a tumor cell protein lysate with GA- or PU-conjugated agarose beads Meclofenoxate HCl could not capture the entire Hsp90 population, although PU-beads were able to capture a larger fraction of Hsp90 compared to GA-beads (Figure ?(Figure1A).1A). The unbound Hsp90 fraction retained affinity for Hsp90-specific antibody and ATP-conjugated agarose (, and data not shown), suggesting that although it was not accessible to drugs, this population of Hsp90 maintained a native conformation. Repeated challenge of recombinant Hsp90 protein with drug-beads yielded a qualitatively similar result, although the discrepancy between GA- and PU-beads was less apparent (Fig. ?(Fig.1B).1B). To discern whether the Hsp90 populations isolated by each inhibitor were mutually exclusive, we subjected a tumor cell lysate to several rounds of GA-agarose followed by several rounds of PU-agarose, and vice versa. We found that, after depletion of the GA-bindable population, there remained a significant fraction of Hsp90 with affinity for PU (Fig. ?(Fig.1C,1C, top panel). However, the reverse was not true, suggesting that the GA-bindable cellular Hsp90 pool is contained within a more abundant PU-bindable fraction (Fig. ?(Fig.1C,1C, bottom panel). When we performed a similar analysis using recombinant Hsp90 protein, we observed less divergence Meclofenoxate HCl between GA-bindable and PU-bindable populations (Fig. ?(Fig.1D),1D), suggesting that cell-dependent modifications of Hsp90 contribute to this discrepancy. Treatment of whole cells with excess soluble drug support this interpretation, as PU-agarose was able to capture Hsp90 not quenched by soluble GA, but soluble PU completely inhibited Hsp90 interaction with immobilized GA (data not shown). Open in a separate window Figure 1 GA and PU recognize overlapping but distinct Hsp90 populations which are not equally sensitive to protease cleavage(A and B) Serial challenge of SkBr3 cell lysate (A) or recombinant Hsp90 protein (B) with GA and PU beads. (C and D) Serial challenge of SkBr3 cell lysate (C) or recombinant Hsp90 (D) with GA-beads followed by PU-beads (top) and vice versa (bottom). The GA-bindable Hsp90 population is contained within a larger PU-bindable population. (E) Cartoon of major tryptic cleavage sites in Hsp90, including a site 75 kD from the N-terminus Meclofenoxate HCl (*) and another that is 40 kD from the N-terminus (**). (F, G) Purified recombinant Hsp90 (F) or Hsp90 from SkBr3 cell lysate (G) were captured by GA- or PU-beads. Samples were digested with increasing concentrations of trypsin, separated by SDS-PAGE, and subjected to.