Background: Actinomyces naeslundii genospecies 1 and 2 express type-2 fimbriae (FimA subunit polymers) with variant Galbeta binding specificities and Actinomyces odontolyticus a sialic acid specificity to colonize different oral surfaces. However, the fimbrial nature of the sialic acid binding property and sequence information about FimA proteins from multiple strains are lacking.
Results: Here we have sequenced fimA genes from strains of A.naeslundii genospecies 1 (n = 4) and genospecies 2 (n = 4), both of which harboured variant Galbeta-dependent hemagglutination (HA) types, and from A.odontolyticus PK984 with a sialic acid-dependent HA pattern. Three unique subtypes of FimA proteins with 63.8-66.4% sequence identity were present in strains of A. naeslundii genospecies 1 and 2 and A. odontolyticus. The generally high FimA sequence identity (> 97.2%) within a genospecies revealed species specific sequences or segments that coincided with binding specificity. All three FimA protein variants contained a signal peptide, pilin motif, E box, proline-rich segment and an LPXTG sorting motif among other conserved segments for secretion, assembly and sorting of fimbrial proteins. The highly conserved pilin, E box and LPXTG motifs are present in fimbriae proteins from other Gram-positive bacteria. Moreover, only strains of genospecies 1 were agglutinated with type-2 fimbriae antisera derived from A. naeslundii genospecies 1 strain 12104, emphasizing that the overall folding of FimA may generate different functionalities. Western blot analyses with FimA antisera revealed monomers and oligomers of FimA in whole cell protein extracts and a purified recombinant FimA preparation, indicating a sortase-independent oligomerization of FimA.
Conclusion: The genus Actinomyces involves a diversity of unique FimA proteins with conserved pilin, E box and LPXTG motifs, depending on subspecies and associated binding specificity. In addition, a sortase independent oligomerization of FimA subunit proteins in solution was indicated.