Cells use molecular chaperones and proteases to implement the essential quality control mechanism of proteins. The DegP (HtrA) protein, essential for the survival of Escherichia coli cells at elevatedtemperatures with homologues found in almost all organisms uniquely has both functions. Here we report a mechanism for DegP to activate both functions via formation of large cage-like 12-and 24-mers after binding to substrate proteins. Cryo-electron microscopic and biochemical studies revealed that both oligomers are consistently assembled by blocks of DegP trimers, via pairwise PDZ1–PDZ2 interactions between neighboring trimers. Such interactions simultaneously eliminate the inhibitory effects of the PDZ2domain. Additionally, both DegP oligomers were also observed in extracts of E. coli cells, strongly  mplicating their physiological importance.

Jiang JS, Zhang X, Chen Y, Wu Y, Zhou ZH, Chang ZY, Sui SF. Activation of DegP Chaperone-Protease via Formation of Large Cage-like Oligomers Upon Binding to Substrate Proteins. Proc. Natl. Acad. Sci. USA.2008, 106:11939-11944.