STRUCTURAL PREDICTIONS OF AGFA, THE INSOLUBLE FIMBRIAL SUBUNIT OF SALMONELLA THIN AGGREGATIVE FIMBRIAE

The unusually stable and multifunctional, thin aggregative fimbriae common to all Salmonella spp. are principally polymers of the fimbrin subunit, AgfA. AgfA of Salmonella enteritidis consists of two domains: a protease-sensitive, 22 amino acid residue N-terminal region and a protease-resistant, 109 residue C-terminal core. The unusual amino acid sequence of the AgfA core region comprises two-, five- and tenfold internal sequence homology patterns reflected in five conserved, 18-residue tandem repeats. These repeats have the consensus sequence, Sx5QxGx2NxAx3Q and are linked together by four or five residues, (x)xAx2. The predicted secondary structure for this unusual arrangement of tandem repeats in AgfA indicates mainly extended conformation with the β strands linked by four to six residues. Candidate proteins of known structure with motifs of alternating β strands and short loops were selected from folds described in SCOP as a source of coordinates for AgfA model construction. Three all-β class motifs selected from the Serratia marcescens metalloprotease, myelin P2 protein or vitelline membrane outer protein I were used for initial AgfA homology build-up procedures ultimately resulting in three structural models; β barrel, β prism and parallel β helix. The β barrel model is a compact, albeit irregular structure, with the β strands arranged in two antiparallel β sheet faces. The β prism model does not reflect the 5 or 10-fold symmetry of the AgfA primary sequence. However, the favored, parallel β helix model is a compact coil of ten helically arranged β strands forming two parallel β sheet faces. This arrangement predicts a regular, potentially stable, C-terminal core region consistent with the observed tandem repeat sequences, protease-resistance and strong tendency of this fimbrin to oligomerize and aggregate. Positional conservation of amino acid residues in AgfA and the Escherichia coli AgfA homologue, CsgA, provides strong support for this model. The parallel β helix model of AgfA offers an interesting solution to a multifunctional fimbrin molecular surface having solvent exposed areas, regions for major and minor subunit interactions as well as fiber-fiber interactions common to many bacterial fimbriae.