Assembly of Integral Membrane Proteins from the Periplasm into the Outer Membrane

Abstract

Integral membrane proteins facilitate transport, for example, of nutrients, across a hydrophobic barrier. This chapter summarizes the current knowledge about the transport of outer membrane proteins (OMPs) through the periplasm and about their assembly into membranes. How insertion and folding of OMPs into the outer membrane (OM) takes place is largely unknown, and the chapter gives an overview about our current knowledge on the insertion and folding of OMPs from the periplasm into the OM. Therefore, some of the structures and properties of OMPs are described in the chapter, followed by an overview of the currently known periplasmic folding factors of OMPs. The chapter focuses on membrane insertion and assembly of the porins that form single-chain transmembrane β-barrels. It is now clear that periplasmic chaperones, such as Skp and SurA, help to keep OMPs unfolded in the periplasm and prevent their aggregation without requiring ATP as an energy source. The identification of the integral OMP YaeT and outer membrane lipoproteins as factors involved in targeting and/or insertion of OMPs into the OM on one side and the spontaneous assembly of OMPs into lipid bilayers in vitro on the other side raises several interesting questions. Furthermore, the accumulation of misfolded OMPs in the periplasm upon deletion of yaeT (omp85) suggests that the properties of the OM lipid bilayer differ from the properties of the phospholipid bilayers into which OMPs successfully fold in vitro.