The MraY transferase is an integral membrane protein that catalyzes an essential step of peptidoglycan biosynthesis, namely the transfer of the phospho-N-acetylmuramoyl-pentapeptide motif onto the undecaprenyl phosphate carrier lipid. It belongs to a large superfamily of eukaryotic and prokaryotic prenyl sugar transferases. No 3D structure has been reported for any member of this superfamily, and to date MraY is the only protein that has been successfully purified to homogeneity. Nineteen polar residues located in the five cytoplasmic segments of MraY appeared as invariants in the sequences of MraY orthologues. A certain number of these invariant residues were found to be conserved in the whole superfamily. To assess the importance of these residues in the catalytic process, site-directed mutagenesis was performed using the Bacillus subtilis MraY as a model. Fourteen residues were shown to be essential for MraY activity by an in vivo functional complementation assay using a constructed conditional mraY mutant strain. The corresponding mutant proteins were purified and biochemically characterized. None of these mutations did significantly affect the binding of the nucleotidic and lipidic substrates, but the k cat was dramatically reduced in almost all cases. The important residues for activity therefore appeared to be distributed in all the cytoplasmic segments, indicating that these five regions contribute to the structure of the catalytic site. Our data show that the D98 residue that is invariant in the whole superfamily should be involved in the deprotonation of the lipid substrate during the catalytic process.

During spore formation in Bacillus subtilis a transenvelope complex is assembled across the double membrane that separates the mother cell and forespore. This complex (called the "A-Q complex") is required to maintain forespore development and is composed of proteins with remote homology to components of type II, III, and IV secretion systems found in Gram-negative bacteria. Here, we show that one of these proteins, SpoIIIAG, which has remote homology to ring-forming proteins found in type III secretion systems, assembles into an oligomeric ring in the periplasmic-like space between the two membranes. Three-dimensional reconstruction of images generated by cryo-electron microscopy indicates that the SpoIIIAG ring has a cup-and-saucer architecture with a 6-nm central pore. Structural modeling of SpoIIIAG generated a 24-member ring with dimensions similar to those of the EM-derived saucer. Point mutations in the predicted oligomeric interface disrupted ring formation in vitro and impaired forespore gene expression and efficient spore formation in vivo. Taken together, our data provide strong support for the model in which the A-Q transenvelope complex contains a conduit that connects the mother cell and forespore. We propose that a set of stacked rings spans the intermembrane space, as has been found for type III secretion systems.sporulation | SpoIIIAG | type III secretion system | EscJ/PrgK/FliF | SigG

Fluorescent proteins are particularly susceptible to photobleaching, the permanent loss of fluorescence emission resulting from photodestruction of the chromophore. In the case of Reversibly Switchable Fluorescent Proteins (RSFPs), which can be switched back and forth between a non-fluorescent and a fluorescent state, the achievable number of switching cycles is limited by photobleaching, a process known as photofatigue. Photofatigue has become a crucial limitation in a number of advanced applications based on repeated photoswitching of RSFPs, notably in the field of super-resolution fluorescence microscopy. Here, based on our previous structural investigation of photobleaching mechanisms in IrisFP, an RSFP also capable of green-to-red photoconversion, we present the rational design of a single-mutant IrisFP-M159A that displays considerably enhanced photostability. The results suggest that, under moderate illumination intensities, photobleaching of IrisFP-like Anthozoan fluorescent proteins such as EosFP, Dendra or Dronpa derivatives is mainly driven by an oxygen-dependent mechanism resulting in the irreversible sulfoxidation of methionine 159. The photofatigue decay profiles of IrisFP and its photoresistant mutant IrisFP-M159A were investigated in different experimental conditions, in vitro and in cellulo. Although the performance of the mutant was found to be always superior, the results showed switching behaviors strongly dependent on the nanoenvironment. Thus, in general, assessment of photostability and switching properties of RSFPs should be carried out in real experimental conditions.

Ceftobiprole is a new cephalosporin that exhibits a high level of affinity for methicillin-resistant Staphylococcus aureus PBP 2a. It was reported that ceftobiprole did not interact with a mutated form of the low-affinity protein Enterococcus faecium PBP 5 (PBP 5fm) that, when overexpressed, confers a ␤-lactam resistance phenotype to the bacterium. Our results show that ceftobiprole binds to unmutated PBP 5fm to form a stable acyl-enzyme and that ceftobiprole is able to efficiently kill a penicillin-resistant Enterococcus faecium strain that produces this protein.

The opportunistic human pathogen Enterococcus faecium overproduces the low-affinity PBP5. In clinical strains, mutations in PBP5 further reduce its acylation rate by ␤-lactams. Previous studies have reported that ceftaroline had poor inhibitory activity against ␤-lactam-resistant E. faecium strains. In this study, we show that ceftaroline exhibits killing activity against our laboratory-derived ampicillin-resistant E. faecium mutant that overproduces a wild-type PBP5 and that ceftaroline inactivates PBP5 much faster than benzylpenicillin and faster than ceftobiprole. ␤ -Lactam antibiotics (penicillins, cephalosporins, carbapenems, and monobactams) represent the most important group of drugs prescribed to treat bacterial infections. They form stable acyl-enzymes with their targets, the membrane-bound D, D-transpeptidases, which are essential enzymes in peptidoglycan biosynthesis. These proteins are usually referred to as penicillin-binding proteins (PBPs) (1-3). The presence or overproduction of loweraffinity PBPs is responsible for the resistance of Gram-positive cocci against ␤-lactam antibiotics. It is known that the opportunistic human pathogen Enterococcus faecium overproduces the low-affinity PBP5 and that mutations further reduce the acylation rate of PBP5 by altering its amino acid sequence (4, 5). Ceftaroline is a cephalosporin with broad-spectrum activity against Grampositive organisms, including methicillin-resistant Staphylococcus aureus (MRSA), and many common Gram-negative bacteria except those producing extended-spectrum ␤-lactamase (ESBL) or AmpC ␤-lactamase (6-8). The Staphylococcus aureus low-affinity PBP2a is closely related to PBP5 (9). It is inhibited by ceftaroline, the active metabolite of the prodrug ceftaroline fosamil, which was recently approved by the U.S. Food and Drug Administration (FDA) for use in adult patients with acute bacterial skin and skinstructure infections and community-acquired bacterial pneumonia and more recently approved by the European Medicines Agency (EMA) for similar indications (10, 11). Previous studies have reported that ceftaroline had poor inhibitory activity against ␤-lactam-resistant E. faecium strains, all of which were expected to possess a PBP5 protein (6,12). To better understand these differences, we have characterized the interaction between a wild-type form of PBP5 and ceftaroline.Ceftaroline MIC values determined by the microdilution method (13) for E. faecium D63r and D63 strains (5) were 2 and 0.25 mg · liter Ϫ1 , respectively. These values contrasted with those reported by others (10-12), who have found that most ampicillinresistant E. faecium clinical isolates were resistant to ceftaroline and concluded that ceftaroline had little activity against most isolates of E. faecium. The killing effects (14) of ceftaroline on E. faecium D63 and D63r strains were studied by exposing exponentially growing cultures of both strains to increasing concentrations of antibiotics corresponding to 1 and 4 times their respective MICs (Fig. 1). Ceftaroline showed k...

The study of ion channel activity and the screening of possible inhibitor molecules require reliable methods for production of active channel proteins, their insertion into artificial membranes and for the measurement of their activity. Here we report on cell-free expression of soluble and active K1.1 and K1.3 channels and their efficient insertion into liposomes. Two complementary methods for the determination of the electrical activity of the proteoliposome-embedded channels were compared using K1.1 as a model system: (1) single channel recordings in droplet interface bilayers (DIB) and (2) measurement of the membrane voltage potential generated by a potassium ion diffusion potential using the voltage-sensitive fluorescent dye oxonol VI. Single channel recordings in DIBs proved unreliable because of the non-reproducible fusion of proteoliposomes with an artificial membrane. Therefore, the use of the optical indicator oxonol VI was adapted for 96 well microtiter plates using the ionophore valinomycin as a positive control. The activity of K1.1 and K1.3 channels was then monitored in the absence and presence of different venom toxins, demonstrating that fluorescent dyes can be used very efficiently when screening small molecules for their channel blocking activity.