In eukaryotes, the 90-kDa heat shock proteins (Hsp90s) are profusely studied chaperones that, together with 70-kDa heat shock proteins (Hsp70s), control protein homeostasis. In bacteria, however, the function of Hsp90 (HtpG) and its collaboration with Hsp70 (DnaK) remains poorly characterized. To uncover physiological processes that depend on HtpG and DnaK, we performed comparative quantitative proteomic analyses of insoluble and total protein fractions from unstressed wild-type (WT) Escherichia coli and from knockout mutants dnaKdnaJ ( KJ), htpG ( G), and dnaKdnaJ htpG ( KJG). Whereas the G mutant showed no detectable proteomic differences with wild-type, KJ expressed more chaperones, proteases and ribosomes and expressed dramatically less metabolic and respiratory enzymes. Unexpectedly, we found that the triple mutant KJG showed higher levels of metabolic and respiratory enzymes than KJ, suggesting that bacterial Hsp90 mediates the degradation of aggregationprone Hsp70-Hsp40 substrates. Further in vivo experiments suggest that such Hsp90mediated degradation possibly occurs through the HslUV protease.