Evolutionary diversity and structural comparison of the FimA adhesin in Porphyromonas gingivalis

Porphyromonas gingivalis is a heterogeneous oral pathogen whose major fimbrial subunit FimA occurs as genotypes linked to adhesion differences. We asked whether fimA diversification tracks the phylogeny, carries selection signatures and varies structurally. We analyzed 99 Porphyromonas assemblies: genotypes were mapped onto the core-genome phylogeny, nucleotide diversity compared with 1248 core genes, recombination screened with GARD, and codon-level selection tested with FEL and MEME, with and without multiple-hit models. Genotype I and IV precursors were characterized by mass spectrometry, SEC-MALS and far-UV circular dichroism; rigid docking used β1-derived peptides and a truncated β1 construct. Genotype was only partially coupled to lineage: 27 tree changes against five expected under clade structure. fimA had Tajima's D = 1.29, 13th of 1247 families, against a negative background; under lineage-balanced sampling of reconstructed alignments its median position fell to the 96th percentile, and within one lineage it was negative. GARD placed two breakpoints and the multiple-hit model fitted better. Ten codons met p ≤ 0.1 for diversifying selection in two or more analyses, none survived Benjamini-Hochberg correction, while 119 to 132 codons met the same nominal threshold for purifying selection and 12 to 18 of those held at q ≤ 0.05. The best supported were V193 and A231, with episodic MEME signals at D52, S55, A76, T227 and A306, and all ten lie outside the mapped epithelial cell-binding region. Both behaved as monomers of 38.3 and 38.0 kDa, their predicted structures superposed at 0.73 Å, and docking gave no convergent interface. The position of fimA depends on the sampling scheme, so these data demonstrate neither balancing nor host-associated selection, and genotype-dependent β1-integrin recognition requires experimental testing.