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

The outer membrane (OM) of gram-negative bacteria consists of a lipid bilayer, which is composed of phospholipids in the periplasmic leaflet and of lipopolysaccharide in the outer leaflet. Integral membrane proteins facilitate transport, for example, of nutrients, across this hydrophobic barrier.After their biosynthesis, outer membrane proteins (OMPs) are targeted to the cytoplasmic membrane in complex with SecB/SecA and they are then transported through the SecY/E/G translocon of the cytoplasmic membrane into the periplasm. In the periplasm, the signal sequences of the OMPs are cleaved by a leader peptidase and the proteins cross the periplasm in an unfolded form prior to their assembly into the OM. Misfolding of OMPs leads to their proteolysis and to the activation of extracytoplasmic stress response, which results in expression of periplasmic and outer membrane folding factors, such as chaperones, isomerases, and proteases. In the periplasm, the OMPs are kept soluble in largely unfolded form by periplasmic chaperones. A complex of several proteins in the OM is involved in targeting and assembly of the integral membrane proteins into the OM. OMPs insert and fold into lipid bilayers by a highly concerted mechanism, in which secondary and tertiary structure formation is synchronized and coupled to lipid bilayer insertion. This chapter summarizes the current knowledge about the transport of OMPs through the periplasm and about their assembly into membranes. Biological membranes are needed for maintaining the integrity of cells and cell organelles as well as their shape.All membranes including the OM of bacteria are composed of a lipid bilayer that has a hydrophobic core and polar surfaces.The lipid bilayer prevents the arbitrary passage of solutes across the membrane. Integral membrane proteins, also called transmembrane proteins (TMPs), are embedded into the lipid bilayers to perform many different functions, including the transport of solutes and signals across the hydrophobic barrier.Therefore, TMPs are essential for the survival of the cell. The biogenesis of membranes is of fundamental interest to basic research from the perspectives of cell biologists, structural biologists, biochemists, and biophysicists. Important for the understanding of membrane biogenesis is how integral membrane proteins are inserted and folded into the lipid bilayer host matrix. For insertion and folding into the OM,TMPs cross the cytoplasmic membrane in an unfolded Jörg H. Kleinschmidt, Fachbereich Biologie, Fach M 694, Universität Konstanz, D-78457 Konstanz, Germany.