ISSN:
1574-695X
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Biology
,
Medicine
Notes:
Abstract Escherichia coli contains at least two phase-variable proteins in its outer membrane. One, termed antigen 43 (Ag43), is the product of the metastable flu gene located at min 43.6 on the E. coli chromosome and is responsible for colony form variation and for autoaggregation in liquid media. Ag43 is composed of two proteinaceous subunits, α43 and β43 in 1:1 stoichiometry. α43 (apparent Mr 60,000) is surface expressed, extends beyond the O-side chains of smooth lipopolysaccharide and is bound to the cell surface through an interaction with β43 (apparent Mr 53,000), itself an integral, heat-modifiable, outer membrane protein. α43 shows limited N-terminal sequence homology with certain enterobacterial adhesins, and notable sequence homology with AIDA-1, an adhesin of diffuse-adhering E. coli. In addition, α43 contains an RGD motif and a consensus sequence for an (autoproteolytic?) aspartyl protease active site. Expression of Ag43 is subject to reversible phase variation — in liquid minimal medium, the rates of variation from Ag43+ to Ag43− states and from Ag43− to Ag43+ states being ≈2.2 × 10−3 and ≈1 × 10−3, respectively. Phase switching of Ag43 is regulated by DNA methylation (deoxyadenosine methylase (dam) mutants being ‘locked OFF’) and by OxyR (oxyR mutants being ‘locked ON’). It is proposed that OxyR acts as a repressor of Ag43 transcription by binding to unmethylated GATC sites in the regulatory region of the gene. In some strains, Ag43 may also undergo antigenic variation. A 94 kDa immunocrossreactive outer membrane protein, showing similar rates of phase variation, has additionally been detected for some enteropathogenic and uropathogenic strains of E. coli. This 94 kDa protein can be proteolytically cleaved in situ with trypsin to yield two membrane-bound products with Mrs and properties similar to those of α43 and β43. Results suggest that Ag43 may represent one of a family of antigenically-related high-Mr adhesins which are synthesized as polyprotein precursors. Some members may be processed and presented on the cell surface as bipartite protein complexes (as Ag43). Others can remain uncleaved.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1574-695X.1996.tb00124.x
Permalink
