The suspension was disrupted with a MSK Cell Homogenizer

(Braun Biotech, Goettingen, Germany) using glass beads (five 1-min cycles). A pellet collected by GKT137831 solubility dmso centrifugation at 20 000  g for 10 min at 4 °C was suspended as above and treated for 1 h at 37 °C with DNase (25 μL, 5 U μL−1) and RNase A (25 μL, 100 mg mL−1) and 1 h at 37 °C with trypsin (250 μg mL−1). All hydrolytic enzymes were from Sigma-Aldrich (Milan, Italy). Following ultracentrifugation (100 000  g , 60 min, 4 °C; Beckman L7-65 Instruments, Gagny, France), the pellet was suspended in 3 mL of phosphate buffer and sonicated for 10 min in an ice bath. Residual cell wall-associated proteins were removed by papain treatment (3 μL, 50 mg mL−1 solution; Sigma-Aldrich) for 1 h at 37 °C, followed by ultracentrifugation. For the cell wall breakdown assay, the pellet from ultracentrifugation was suspended in 6 mL of 10 mM phosphate (pH 7) and divided into four aliquots that were treated with vancomycin (100 μg mL−1), lysozyme (100 μg mL−1),

sakacin A (80 AU mL−1, 100 μg mL−1), or left untreated. The absorbance at 600 nm was measured after incubation at 30 °C (30 min and 24 h). Samples were frozen, lyophilized, and used as such for subsequent MS analysis of released products. In a separate set of experiments, aliquots of the same cell wall preparations were treated overnight (16 h) at 30 °C with increasing amounts of sakacin A (from 0 to 300 μg, equivalent to 0–240 AU) and analyzed by MS. All experiments were performed in triplicate. Statistical analysis was carried out using a Tukey’s multiple comparison test (Minitab 15v, State College, PA) and differences PFT�� molecular weight considered significant at P < 0.05. Sakacin A was purified through a sequence of chromatographic steps from L. sakei cultures propagated in an inexpensive broth (Trinetta et al., 2008a). Sakacin A was eluted at c. 0.45 M NaCl from a cation exchanger at pH 4.5, confirming its cationic character and was further purified through RP and gel-permeation HPLC. A final RP-HPLC step eliminated a minor contaminant (Supporting Information, Fig. PLEKHM2 S1) and gave 1.7 mg

of purified sakacin A L−1 of the original culture (Table S1). The highly purified material showed a single band in SDS-PAGE, with a molecular mass of c. 4000 Da (Fig. 1a). The band retained antimicrobial activity against L. ivanovii (Fig. 1b), highlighting a peculiar resistance of the protein to denaturation as suggested also by activity retention at the high acetonitrile concentrations used in RP-HPLC. Purity and identity of the isolated material and correspondence to a published sequence (Holck et al., 1992) were established by MALDI-TOF MS (Figure S2). The observed molecular mass (4302.36 Da) agrees with the sequence-calculated monoisotopic (4302.89 Da) and average isotopic (4306.89 Da) values. The effects of sakacin A on the individual components of the proton motive force (PMF) (namely, ΔΨ and ΔpH) on Listeria cells were studied.