Defining the roles of the periplasmic chaperones SurA, Skp, and DegP in <i>Escherichia coli</i>

TLDR

Outer membrane β‑barrel proteins are abundant in Gram‑negative bacteria, synthesized in the cytoplasm, translocated across the inner membrane by Sec, and assembled into the outer membrane through a YaeT‑dependent pathway that requires the periplasmic chaperones SurA, DegP, and Skp. SurA is the primary chaperone that delivers most OMPs to YaeT, as its depletion markedly reduces outer membrane density, whereas DegP and Skp alone have little effect but compensate when SurA is absent by rescuing mis‑translocated OMPs, indicating that the chaperones act in parallel with relative importance shifting under stress.

Abstract

Integral β-barrel proteins (OMPs) are a major class of outer membrane proteins in Gram-negative bacteria. In Escherichia coli , these proteins are synthesized in the cytoplasm, translocated across the inner membrane via the Sec machinery, and assembled in the outer membrane through an unknown mechanism that requires the outer membrane YaeT complex and the periplasmic chaperones SurA, DegP, and Skp. Here, we have established the relationship between these three chaperones providing insight into the mechanism of OMP biogenesis using depletion analysis. Depletion of SurA alone results in a marked decrease in outer membrane density, while the loss of DegP and Skp has no effect on outer membrane composition. Furthermore, we demonstrate that SurA and YaeT interact directly in vivo. Based on these results, we suggest that SurA is the primary chaperone responsible for the periplasmic transit of the bulk mass of OMPs to the YaeT complex. The role of Skp and DegP is amplified in the absence of SurA. Evidence presented suggests that DegP/Skp function to rescue OMPs that fall off the SurA pathway. The seemingly redundant periplasmic chaperones do function in parallel, but the relative importance of the primary function of each pathway depends on whether or not cells are under stress.