Biogenesis of the outer membrane
- Jesus Arenas Busto
- Florian Putker
- Jan Tommassen
The constituents of the outer membrane, i.e. phospholipids, LPS and OMPs, are all synthesized in the cytoplasm or at the cytoplasmic side of the cytoplasmic membrane. To reach their destination, they have to cross the inner membrane and the periplasm and to insert into the outer membrane. We focus on the insertion of OMPs into this membrane and on the transport of LPS from its site of synthesis to the cell surface.
Recently, we made major progress in understanding the biogenesis of the bacterial outer membrane (1). First, we identified an OMP, designated Omp85 in N. meningitidis that is required for the insertion of OMPs into the outer membrane (2). We demonstrated that Omp85 is essential for the viability of the bacteria, and that, upon depleting the bacteria of Omp85, all other OMPs fail to insert into the outer membrane and accumulate as aggregates in the periplasm. Interestingly, Omp85 homologues are present not only in all other Gram-negative bacteria, but also in mitochondria, where they are also essential for the assembly of OMPs. Thus, the process of OMP assembly appears evolutionary conserved.
Secondly, by making use of the observation that Neisseria meningitidis is viable without LPS (3), we were able to identify a protein that is essential for the transport of LPS to its destination (4). This protein, designated Imp, is not essential for the viability of N. meningitidis. An imp mutant produced much less LPS than its parental strain, and the residual amounts of LPS produced were not accessible at the bacterial cell surface. Thus, Imp is most likely responsible for the flip-flop of the LPS across the outer membrane to the cell surface.
1) Bos MP, Tommassen J. 2004. Biogenesis of the Gram-negative bacterial outer membrane. Curr. Opn. Microbiol. 7:610-616.
2) Voulhoux R, Bos MP, Geurtsen J, Mols M, Tommassen J. 2003. Role of a highly conserved bacterial protein in outer membrane protein assembly. Science 299:262-265.
3) Steeghs L, de Cock H, Everts E, Zomer B, Tommassen J, van der Ley P. 2001. Outer membrane composition of a lipopolysaccharide-deficient Neisseria meningitidis mutant. EMBO J. 20:6937-6945.
4) Bos MP, Tefsen B, Geurtsen J, Tommassen J. 2004. Identification of an outer membrane protein required for the transport of lipopolysaccharide to the bacterial cell surface. Proc. Natl. Acad. Sci. USA 101:9417-9422.