Functional Characterization of a Putative Sortase FA1364 in Filifactor alocis.

Gram-positive bacteria covalently anchor specific proteins to the peptidoglycan cell wall via sortase, a cysteine transpeptidase that targets proteins with a cell wall sorting signal. Sortase enzymes are critical for bacterial pathogenesis, and their inhibitors have become promising therapeutic targets for infection management. Filifactor alocis, a Gram-positive anaerobic bacterium, is now proposed as a diagnostic indicator of periodontal disease. Unlike other bacteria, F. alocis encodes a single putative sortase, FA1364. In this study, we functionally characterized the putative sortase FA1364 and found that it belongs to the class A family (SrtA). The SrtA-anchored surface proteins (FA1006, FA1336, FA1424, and FA1750) were identified, and MS/MS analysis confirmed that SrtA is required for their cell-surface localization. The recombinant SrtA protein could recognize and cleave the LPKTG sorting motif with cysteine 191 and arginine 200 as essential catalytic residues. F. alocis FLL101 (ΔFA1364::ermF) showed reduced ability to coaggregate and form biofilm, along with decreased collagen binding and survival in epithelial cells. Additionally, the FA1364-defective mutant exhibited increased sensitivity to air exposure. Collectively, these results suggest that the F. alocis SrtA protein is an important virulence factor and may represent a novel therapeutic target for the control of periodontal diseases.