Characterization of Function and Role of FMTA in Staphylococcus Aureus

Staphylococcus aureus is one of the major reasons for infectious mortality and morbidity. Due to extensive use of antibiotics, S. aureus has acquired resistance to almost all antibiotics. FmtA is one of the additional factors for methicillin resistance in S. aureus which interacts with wall teichoic acid (WTA) and localizes in the cell division septum. Previous studies showed that FmtA has very weak D-Ala-D-Ala carboxypeptidase (Cpase) activity. In lieu of these findings, we hypothesized that FmtA may require WTA as an activator ligand or protein-protein interactions to become fully functional as Cpase in the division septum. Here we show that WTA is not an activator ligand for Cpase activity of FmtA, but FmtA has an esterase activity on WTA. FmtA can remove D-Ala from D-alanyl ester in WTA. Additionally, we show serine and lysine from sequence motif Ser127-X-X-Lys130 (conserved among penicillin binding proteins (PBPs), β-lactamase and family VIII esterase) of FmtA are involved in this catalytic activity. Mutation studies suggest that both Ser-X-X-Lys motif in FmtA, located at the position S63 and S127 are involved in catalysis. Our results suggest that FmtA’s esterase enzymatic activity depends on both enzyme and substrate concentrations and high concentration of substrate (10 mg/mL WTA) can inhibit the reaction. Beside WTA, FmtA can also remove D-Ala from lipoteichoic acid (LTA). Results from in-vivo studies of WTA from fmtA deletion and fmtA-CM (conditionally over expressed) in S. aureus were in agreement with in-vitro esterase activity. In conclusion, WTA is not an activator ligand for FmtA and that FmtA shows esterase activity towards D-alanyl ester of WTA and LTA. Both SXXK motifs in FmtA are involved in catalysis and Serine and lysine from SXXK motif are important for esterase catalysis. To date, biological function of most esterase is unknown. To our knowledge, this is the first study to report esterase from S. aureus that has sequence similarity to PBPs and β-lactamase and very selective to its biological substrate WTA and LTA compare to synthetic substrate.