Outer membrane proteins

The outer membrane (OM) of Gram-negative bacteria is in direct contact with the external environment, and contains proteins, lipoproteins, phospholipids, and lipopolysaccharides (LPSs).  OM proteins (OMPs) are often directly involved in the virulence of pathogenic bacteria.  The Shigella flexneri OMP called IcsA protein is essential for bacterial motility inside infected human cells, while another S. flexneri OMP is IcsP which is a protease that cleaves IcsA.

IcsA

The IcsA protein is a large 116 kDa outer membrane protein which is able to interact with actin regulatory proteins  within the cytosol of host (human and animal) cells to trigger polymerisation of actin and hence actin-based motility.  IcsA is a member of the autotransporter family of proteins, and has a 85 kDa amino terminal passenger (or alpha domain) and a 37 kDa carboxy terminal transporter (or beta domain) that is located in the outer membrane. The IcsA protein is it is targeted to  and predominantly localised  at the old cell pole in strains that have intact LPS. Polar localisation is relaxed in strains unable to produce either LPS O antigens or the IcsP protease.