Dominant negative (DN) mutations provide valuable tools for investigating protein mechanisms but can be difficult to isolate because of their toxic effects. We used a mutational scanning approach to identify DN mutations in yeast Hsp90. In a previous mutational scan of the ATPase domain of Hsp90, we noticed that many mutations were at very low frequency after outgrowth in cells coexpressing wildtype Hsp90. Most of these depleted variants were located at the hinge of a lid that closes over ATP. To quantify toxic effects in the hinge regions, we performed mutational scanning using an inducible promoter and identified 113 variants with strong toxic effects. We analyzed individual DN mutations in detail and found that addition of the E33A mutation that prevents ATP hydrolysis by Hsp90 abrogated the DN phenotype. FRET assays performed on individual DN mutants indicate the linkage between ATPase activity and formation of the closed structure is disrupted. DN Hsp90 decreased the expression level of two model Hsp90 clients, glucocorticoid receptor (GR) and v-src kinase. Using MG132, we found that GR was rapidly destabilized in a proteasome-dependent manner. Biochemical analyses indicate that ATP hydrolysis by Hsp90 from open conformations can lead to ubiquitin-dependent client degradation.