Mechanisms underlying energy-independent transfer of lipoproteins from LolA to LolB, which have similar unclosed β-barrel structures

The Lol system, comprising five Lol proteins, transfers lipoproteins from the inner to the outer membrane of Escherichia coli. Periplasmic LolA accepts lipoproteins from LolCDE in the inner membrane and immediately transfers them to LolB, a receptor anchored to the outer membrane. The unclosed beta-barrel structures of LolA and LolB are very similar to each other and form hydrophobic cavities for lipoproteins. The lipoprotein transfer between these similar structures is unidirectional and very efficient, but requires no energy input. To reveal the mechanisms underlying this lipoprotein transfer, Arg and Phe at positions 43 and 47, respectively, of LolA were systematically mutagenized. The two residues were previously found to affect abilities to accept and transfer lipoproteins. Substitution of Phe-47 with polar residues inhibited the ability to accept lipoproteins from the inner membrane. No derivatives caused periplasmic accumulation of lipoproteins. In contrast, many Arg-43 derivatives caused unusual periplasmic accumulation of lipoproteins to various extents. However, all derivatives, except one having Leu instead of Arg, supported the growth of cells. All Arg-43 derivatives retained the ability to accept lipoproteins from the inner membrane, whereas their abilities to transfer associated lipoproteins to LolB were variously reduced. Assessment of the intensity of the hydrophobic interaction between lipoproteins and Arg-43 derivatives revealed that the LolA-lipoprotein interaction should be weak, otherwise lipoprotein transfer to LolB is inhibited, causing accumulation of lipoproteins in the periplasm.