Drug-resistant bacteria have caused serious medical problems in recent years, and the need for new antibacterial agents is undisputed. Transglycosylase, a multidomain membrane protein essential for cell wall synthesis, is an excellent target for the development of new antibiotics. Here, we determined the X-ray crystal structure of the bifunctional transglycosylase penicillin-binding protein 1b (PBP1b) from Escherichia coli in complex with its inhibitor moenomycin to 2.16-Å resolution. In addition to the transglycosylase and transpeptidase domains, our structure provides a complete visualization of this important antibacterial target, and reveals a domain for protein–protein interaction and a transmembrane helix domain essential for substrate binding, enzymatic activity, and membrane orientation.

The lipid cubic phase (LCP) continues to grow in popularity as a medium in which to generate crystals of membrane (and soluble) proteins for high-resolution X-ray crystallographic structure determination. To date, the PDB includes 227 records attributed to the LCP or in meso method. Among the listings are some of the highest profile membrane proteins, including the β₂-adrenoreceptor–G_s protein complex that figured in the award of the 2012 Nobel Prize in Chemistry to Lefkowitz and Kobilka. The most successful in meso protocol to date uses glass sandwich crystallization plates. Despite their many advantages, glass plates are challenging to harvest crystals from. However, performing in situ X-ray diffraction measurements with these plates is not practical. Here, an alternative approach is described that provides many of the advantages of glass plates and is compatible with high-throughput in situ measurements. The novel in meso in situ serial crystallography (IMISX) method introduced here has been demonstrated with AlgE and PepT (alginate and peptide transporters, respectively) as model integral membrane proteins and with lysozyme as a test soluble protein. Structures were solved by molecular replacement and by experimental phasing using bromine SAD and native sulfur SAD methods to resolutions ranging from 1.8 to 2.8 Å using single-digit microgram quantities of protein. That sulfur SAD phasing worked is testament to the exceptional quality of the IMISX diffraction data. The IMISX method is compatible with readily available, inexpensive materials and equipment, is simple to implement and is compatible with high-throughput in situ serial data collection at macromolecular crystallography synchrotron beamlines worldwide. Because of its simplicity and effectiveness, the IMISX approach is likely to supplant existing in meso crystallization protocols. It should prove particularly attractive in the area of ligand screening for drug discovery and development.

Here, a method for presenting crystals of soluble and membrane proteins growing in the lipid cubic or sponge phase forin situdiffraction data collection at cryogenic temperatures is introduced. The method dispenses with the need for the technically demanding and inefficient crystal-harvesting step that is an integral part of the lipid cubic phase orin mesomethod of growing crystals. Crystals are dispersed in a bolus of mesophase sandwiched between thin plastic windows. The bolus contains tens to hundreds of crystals, visible with an in-line microscope at macromolecular crystallography synchrotron beamlines and suitably disposed for conventional or serial crystallographic data collection. Wells containing the crystal-laden boluses are removed individually from hermetically sealed glass plates in which crystallization occurs, affixed to pins on goniometer bases and excess precipitant is removed from around the mesophase. The wells are snap-cooled in liquid nitrogen, stored and shipped in Dewars, and manually or robotically mounted on a goniometer in a cryostream for diffraction data collection at 100 K, as is performed routinely with standard, loop-harvested crystals. The method is a variant on the recently introducedin meso in situserial crystallography (IMISX) method that enables crystallographic measurements at cryogenic temperatures where crystal lifetimes are enormously enhanced whilst reducing protein consumption dramatically. The new approach has been used to generate high-resolution crystal structures of a G-protein-coupled receptor, α-helical and β-barrel transporters and an enzyme as model integral membrane proteins. Insulin and lysozyme were used as test soluble proteins. The quality of the data that can be generated by this method was attested to by performing sulfur and bromine SAD phasing with two of the test proteins.

5-hydroxytryptamine (5-HT, also known as serotonin) regulates many physiological processes through the 5-HT receptor family. Here we report the crystal structure of 5-HT_1Bsubtype receptor (5-HT_1BR) bound to the psychotropic serotonin receptor inverse agonist methiothepin (MT). Crystallization was facilitated by replacing ICL3 with a novel optimized variant of BRIL (OB1) that enhances the formation of intermolecular polar interactions, making OB1 a potential useful tool for structural studies of membrane proteins. Unlike the agonist ergotamine (ERG), MT occupies only the conserved orthosteric binding pocket, explaining the wide spectrum effect of MT on serotonin receptors. Compared with ERG, MT shifts toward TM6 and sterically pushes residues W327^6.48, F330^6.50and F331^6.51from inside the orthosteric binding pocket, leading to an outward movement of the extracellular end and a corresponding inward shift of the intracellular end of TM6, a feature shared by other reported inactive G protein-coupled receptor (GPCR) structures. Together with the previous agonist-bound serotonin receptor structures, the inverse agonist-bound 5-HT_1BR structure identifies a basis for the ligand-mediated switch of 5-HT_1BR activity and provides a structural understanding of the inactivation mechanism of 5-HT_1BR and some other class A GPCRs, characterized by ligand-induced outward movement of the extracellular end of TM6 that is coupled with inward movement of the cytoplasmic end of this helix.

Moenomycin inhibits bacterial growth by blocking the transglycosylase activity of class A penicillin-binding proteins (PBPs), which are key enzymes in bacterial cell wall synthesis. We compared the binding affinities of moenomycin A with various truncated PBPs by using surface plasmon resonance analysis and found that the transmembrane domain is important for moenomycin binding. Full-length class A PBPs from 16 bacterial species were produced, and their binding activities showed a correlation with the antimicrobial activity of moenomycin against Enterococcus faecalis and Staphylococcus aureus . On the basis of these findings, a fluorescence anisotropy-based high-throughput assay was developed and used successfully for identification of transglycosylase inhibitors.

Bacterial transpeptidase and transglycosylase on the surface are essential for cell wall synthesis, and many antibiotics have been developed to target the transpeptidase; however, the problem of antibiotic resistance has arisen and caused a major threat in bacterial infection. The transglycosylase has been considered to be another excellent target, but no antibiotics have been developed to target this enzyme. Here, we determined the crystal structure of the Staphylococcus aureus membrane-bound transglycosylase, monofunctional glycosyltransferase, in complex with a lipid II analog to 2.3 Å resolution. Our results showed that the lipid II-contacting residues are not only conserved in WT and drug-resistant bacteria but also significant in enzymatic activity. Mechanistically, we proposed that K140 and R148 in the donor site, instead of the previously proposed E156, are used to stabilize the pyrophosphate-leaving group of lipid II, and E100 in the acceptor site acts as general base for the 4-OH of GlcNAc to facilitate the transglycosylation reaction. This mechanism, further supported by mutagenesis study and the structure of monofunctional glycosyltransferase in complex with moenomycin in the donor site, provides a direction for antibacterial drugs design.