Outer membrane proteins as surface display systems.

Outer membrane proteins (OMPs) of gram-negative bacteria can be used as carrier proteins to present foreign peptide epitopes on the bacterial cell surface. They all have a common structural motif of a beta-barrel that is composed of a variable number of transmembrane beta-strands connected at the periplasmic side with short turns and at the outside with long surface-accessible loops. Outer membrane proteins occur as monomers like OmpA, or assemble into trimers like the porins. Foreign gene products have been fused to surface-accessible regions of several outer membrane proteins including the porins OmpC, PhoE and LamB, lipoproteins as well as the OmpA protein. Short epitopes that are inserted into outer membrane proteins induce epitope-specific antibody responses, and are thus appealing candidates for live recombinant vaccines. Also large insertions, of more than 100 amino acids, are in some cases tolerated and do not affect the overall conformation of the carrier protein. The possible applications for outer membrane display include recombinant vaccines, peptide library screening, development of biocatalysts or whole-cell adsorbents, and adhesin-receptor interaction studies. It is expected that in the near future, development of new display systems will still increase the utilization of this emerging exciting technology.