Protein K (porin)

Protein K is a porin expressed in some pathogenic strains of E. coli bacteria. It has a molecular weight of about 40 kDa and is localized to the outer membrane, through which it allows both inorganic and organic ions to pass.

The strains in which protein K has been identified are encapsulated, or surrounded by a poly-sialic acid capsule that renders them more resistant to phagocytosis by cells in the immune system.