Triage model of the protein quality control system: the interaction between molecular chaperones and the UPP. This model predicts that most, if not all, proteins have intrinsic signals for interaction with molecular chaperones or the ubiquitination system. These signals (red) are hidden in properly folded native proteins and they are not recognized by the protein quality control systems. Upon environmental stress, such as heat or oxidation, proteins could be unfolded with exposure of the recognition signals, such as hydrophobic patches. The unfolded proteins are recognized by Hsp90 or Hsp70. With the help of other chaperones or co-factors, Hsp70 can refold the denatured proteins in an ATP-dependent manner. If the denatured proteins cannot be refolded rapidly, CHIP, a U-box E3, which interacts with Hsp90 and Hsp70 with its TPR domain, triggers the ubiquitination of Hsp90/hsp70-bound substrates. The ubiquitinated substrates will be recognized and degraded by the 26S proteasome. If the ubiquitinated proteins were deubiquitinated by isopeptidases, the denatured proteins would have a second chance to be refolded by molecular chaperones. The parallel/competitive functional relationship between the UPP and molecular chaperones assures the efficiency of the protein quality control systems to get rid of abnormal proteins.