The mechanism by which lipopolysaccharide (LPS) is synthesized in the inner membrane of Gram-negative bacteria has long been understood. However, how it is transported to the outer membrane has remained unknown. Now, reporting in Proceedings of the National Academy of Sciences, Martine Bos and co-workers have identified an outer membrane protein, Imp (for increased membrane permeability), as responsible.

Previous work has shown that, unlike Escherichia coli, LPS is not an essential component of Neisseria meningitidis outer membranes, and for this reason, the authors used N. meningitidis to study the effects of Imp on LPS transport. Using the completed genome sequence of N. meningitidis, the authors identified the N. meningitidis imp gene and constructed an Imp-deficient mutant. SDS–PAGE analysis showed that, although the cellular content of LPS was reduced in these mutants relative to the wild type, the synthesized LPS was structurally unaltered.

The authors demonstrated that, when grown in the presence of CMP-NANA (which sialylates LPS) and treated with neuramidinase (which removes the sialic acid group), only a small fraction of the LPS of Imp-deficient cells is desialylated, indicating that LPS is not accessible on the surface of these mutants. This was further supported by expression in the mutant cells of the enzyme PagL, which covalently modifies LPS within the outer membrane; electrophoretic analysis of LPS showed that it had not been modified by this enzyme.

Although the authors have, as yet, been unable to identify the precise location of LPS in imp mutants, they have shown that it is not localized at the outer leaflet of the outer membrane, and they postulate that Imp is the transporter that mediates transport of LPS over the outer membrane.