Structure and characterization of AgfB from Salmonella enteritidis thin aggregative fimbriae

The agfBAC operon of Salmonella enteritidis encodes thin aggregative fimbriae, fibrous, polymeric structures primarily composed of AgfA fimbrins. Although uncharacterized, AgfB shows a 51 % overall amino acid sequence similarity to AgfA. Using AgfB epitope-specific antiserum, AgfB was detected as a minor component of whole, purified fimbriae. Like AgfA, AgfB was released from purified fimbriae by >70 % formic acid, whereupon both AgfA-AgfA and AgfA-AgfB dimers as well as monomers were detected. This suggested that AgfB may form specific, highly stable, structural associations with AgfA in native fimbrial filaments, associations that were weakened in structurally unstable fibers derived from AgfA chimeric fimbrial mutants. Detailed sequence comparisons between AgfA and AgfB showed that AgfB harbored a similar fivefold repeated sequence pattern (x(6)QxGx(2)NxAx(3)Q), and contained structural motifs similar to the parallel beta helix model proposed for AgfA. Molecular modeling of AgfB revealed a 3D structure remarkably similar to that of AgfA, the structures differing principally in the surface disposition of non-conserved, basic, acidic and non-polar residues. Thus AgfB is a fimbrin-like structural homologue of AgfA and an integral, minor component of native thin aggregative fimbrial fibers. AgfB from an agfA deletion strain was detected as a non-fimbrial, SDS-insoluble form in the supernatant and was purified. AgfA from an agfB deletion strain was found in both SDS-soluble and insoluble, non-fimbrial forms. No AgfA-AgfA dimers were detected in the absence of AgfB. Fimbriae formation by intercellular complementation between agfB and agfA deletion strains could not be shown under a variety of conditions, indicating that AgfA and AgfB are not freely diffusible in S. enteritidis. This has important implications on the current assembly hypothesis for thin aggregative fimbriae.