Sorting of lipoproteins to the outer membrane in E-coli

Escherichia coli lipoproteins are anchored to the periplasmic surface of the inner or outer membrane depending on the sorting signal. An ATP-binding cassette (ABC) transporter, Lo1CDE, releases outer membrane-specific lipoproteins from the inner membrane, causing the formation of a complex between the released lipoproteins and the periplasmic molecular chaperone Lo1A. When this complex interacts with outer membrane receptor LoIB, the lipoproteins are transferred from Lo1A to Lo1B and then localized to the outer membrane. The structures of Lo1A and Lo1B are remarkably similar to each other. Both have a hydrophobic cavity consisting of an unclosed beta-barrel and an alpha-helical lid. Structural differences between the two proteins reveal the molecular mechanisms underlying the energy-independent transfer of lipoproteins from Lo1B to Lo1B. Strong inner membrane retention of lipoprotems occurs with Asp at position 2 and a few limited residues at position 3. The inner membrane retention signal functions as a Lo1 avoidance signal and inhibits the recognition of lipoproteins by Lo1CDE, thereby causing their retention in the inner membrane. The positive charge of phosphatidylethanolamine and the negative charge of Asp at position 2 are essential for Lo1 avoidance. The Lo1 avoidance signal is speculated to cause the formation of a tight lipoprotein-phosphatidylethanolamine complex that has five acyl chains and therefore cannot be recognized by LoICDE. (C) 2004 Elsevier B.V. All rights reserved.