Folding of outer membrane proteins

Outer membrane proteins (OMPs) represent a large group of β-barrel proteins found both in the membranes of both bacteria and eukaryotes. Their general ease of expression and refolding and straightforward methods to monitor their degree of folding conspire to make OMPs excellent model systems to investigate how the membrane environment and other biological factors modulating the membrane insertion and folding of OMPs influence the folding pathway. This review attempts to provide an overview of how these proteins are studied in vitro and what kind of information can reliably be extracted. Numerous choices have to be made in setting the conditions for successful folding/unfolding, and here a major challenge remains to identify conditions that lead to completely reversible folding without any hysteresis. Recent progress indicates that this is possible through rigorous optimization, such as the use of relatively extreme pH and phospholipids with short chain lengths. OMPs are generally kinetically very stable, which means that they both fold and unfold very slowly. Many OMPs cannot even unfold when embedded in lipid vesicles, but recent work has demonstrated that surfactants can provide a useful alternative which can lead to a complete description of the kinetics of folding and unfolding of an OMP. The recent report of the first protein engineering study of an OMP has demonstrated that it may soon be possible to have almost atomic-level resolution of an OMP folding mechanism. Combining this insight with the biological complexity of the membrane environment constitutes an exciting new frontier in membrane protein science.

► OMPs represent attractive and tractable models for membrane protein folding.
► It is important to establish conditions for complete folding reversibility.
► Surfactants can be used to obtain complete chevron plots for folding/unfolding.
► Complexities include pre-folding collapse, surfactant stripping and domain swapping.
► Future challenges are to reproduce the composite effect of the membrane environment.