Identification of two inner-membrane proteins required for the transport of lipopolysaccharide to the outer membrane of Escherichia coli

doi:10.1073/pnas.0801196105

Identification of two inner-membrane proteins required for the transport of lipopolysaccharide to the outer membrane of Escherichia coli

*Department of Molecular Biology, Princeton University, Princeton, NJ 08544;
^‡Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138; and
^§Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, MA 02115

Contributed by Thomas J. Silhavy, February 5, 2008 (received for review January 17, 2008)

Abstract

The outer membrane (OM) of most Gram-negative bacteria contains lipopolysaccharide (LPS) in the outer leaflet. LPS, or endotoxin, is a molecule of important biological activities. In the host, LPS elicits a potent immune response, while in the bacterium, it plays a crucial role by establishing a barrier to limit entry of hydrophobic molecules. Before LPS is assembled at the OM, it must be synthesized at the inner membrane (IM) and transported across the aqueous periplasmic compartment. Much is known about the biosynthesis of LPS but, until recently, little was known about its transport and assembly. We applied a reductionist bioinformatic approach that takes advantage of the small size of the proteome of the Gram-negative endosymbiont Blochmannia floridanus to search for novel factors involved in OM biogenesis. This led to the discovery of two essential Escherichia coli IM proteins of unknown function, YjgP and YjgQ, which are required for the transport of LPS to the cell surface. We propose that these two proteins, which we have renamed LptF and LptG, respectively, are the missing transmembrane components of the ABC transporter that, together with LptB, functions to extract LPS from the IM en route to the OM.

Footnotes

^†To whom correspondence may be addressed at:
Department of Molecular Biology, Lewis Thomas Laboratory, Princeton University, Washington Road, Princeton, NJ 08544.
E-mail: nruiz{at}princeton.edu
^¶To whom correspondence may be addressed. E-mail: tsilhavy{at}princeton.edu
Author contributions: N.R., L.S.G., D.K., and T.J.S. designed research; N.R. and L.S.G. performed research; N.R., L.S.G., D.K., and T.J.S. analyzed data; and N.R., L.S.G., D.K., and T.J.S. wrote the paper.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/cgi/content/full/0801196105/DCSupplemental.