The ability of Gram-negative bacteria to survive
in harsh environments is partly attributed to the asymmetric outer membrane
(OM) lipid bilayer that hinders the entry of toxic compounds. Lipid asymmetry in
the OM is established by having phospholipids (PLs) confined to the inner
leaflet of the membrane and lipopolysaccharides (LPS) to the outer leaflet. Perturbation
of lipid asymmetry, characterized by PL accumulation in the outer leaflet of
the OM, disrupts proper LPS packing and results in increased membrane
permeability. A multi-component Mla (maintenance of OM lipid asymmetry)
system that prevents PL accumulation in the outer leaflet of the OM has
recently been identified. This system is widely conserved and is known to be
important in the virulence of several pathogens. How the Mla system maintains
OM lipid asymmetry is not known. Neither has its role in bacterial pathogenesis
been elucidated. Here, we demonstrate that in Escherichia coli, the Mla system maintains OM lipid asymmetry with
the help of osmoporin OmpC. We show that OmpC interacts specifically with the
OM lipoprotein MlaA. This interaction is sufficient to localize MlaA without
its lipid anchor to the OM. Removing OmpC from cells causes accumulation of PLs
in the outer leaflet of the OM when cells are in stationary phase. Therefore,
we conclude that OmpC is an additional component of the Mla system; the OmpC-MlaA
complex functions to remove PLs from the outer leaflet of the OM to maintain
lipid asymmetry. A possible role of this two-protein complex in pathogenesis
will be discussed.