Periplasmic Chaperones and their Function in the Folding of Outer Membrane Protein A into Lipid Membranes

Abstract

In Gram-negative bacteria, outer membrane proteins (OMPs) are translocated across the cytoplasmic membrane in unfolded form via the Sec translocon. Prior to insertion and folding into the outer membrane, the OMPs have to traverse the periplasm. Molecular chaperones prevent OMP aggregation before the OMPs reach the outer membrane. OMPs like outer membrane protein A (OmpA) have been shown to interact with chaperones, like Skp (1-3) and SurA (4-6).OmpA folds spontaneously into lipid bilayers from a urea-unfolded form when the denaturant urea is strongly diluted. We have examined the kinetics of folding of OmpA and how it is affected by the chaperones Skp, SurA, FkpA. In all experiments, either in the absence or in the presence of any of these chaperones, the kinetics are well-described by two parallel kinetic processes of OmpA folding and insertion into lipid bilayers. Both processes were of first order. All examined chaperones increased the contribution of the faster folding process. However, only SurA caused an increase of the rate constant of the fast folding process. The temperature dependence of OmpA folding into lipid bilayers indicated that SurA lowers the activation energy of folding for the faster process.