Bowl-shaped oligomeric structures on membranes as DegP's new functional forms in protein quality control.

In the periplasm of Escherichia coli, DegP (also known as HtrA), which has both chaperone-like and proteolytic activities, prevents the accumulation of toxic misfolded and unfolded polypeptides. In solution, upon binding to denatured proteins, DegP forms large cage-like structures. Here, we show that DegP forms a range of bowl-shaped structures, independent of substrate proteins, each with a 4-, 5-, or 6-fold symmetry and all with a DegP trimer as the structural unit, on lipid membranes. These membrane-bound DegP assemblies have the capacity to recruit and process substrates in the bowl chamber, and they exhibit higher proteolytic and lower chaperone-like activities than DegP in solution. Our findings imply that DegP might regulate its dual roles during protein quality control, depending on its assembly state in the narrow bacterial envelope.