Molecular characterisation of agn43 and its encoded protein, antigen 43, the major phase-variable outer membrane protein of Escherichia coli

Abstract

A number of year ago workers in this laboratory identified antigen 43 (Ag43) during a systematic immunochemical analysis of the envelopes of Escherichia coli ML308-225 O13:O68:H . This Escherichia-specific outer membrane protein, which can be present in copy numbers exceeding 50,000 per cell, is composed of two chemically and immunologically distinct protein subunits termed α43 (Mr 49,800) and β43 (Mr 51,641) which are present in 1:1 stoichiometry, α43 is surface expressed, extends beyond the O-side chains of smooth lipopolysaccharide, and is bound to the cell surface via a non-covalent interaction with a heat-modifiable integral outer membrane protein. The gene encoding Ag43 (agn43) has recently been sequenced in both the K12 and ML308-225 strains of E. coli. agn43, which is present in a single copy within E. coli K-12, is subject to phasevariable regulation where OxyR and Dam methylase function as repressor and derepressor, respectively. Studies have indicated the protein to be involved in autoaggregation and biofilm formation of E. coli K-12 strains, while further lines of evidence suggest the protein may function as an adhesin. The initial aim of the present study was to conclusively establish a function for Ag43.