Acta crystallographica. Section F, Structural biology communications最新文献

RND-type multidrug-efflux pumps are major contributors to multidrug resistance in Gram-negative bacteria, with MexY from Pseudomonas aeruginosa playing a central role in aminoglycoside resistance. Unlike other RND transporters, MexY exhibits unusually large open clefts in the binding and extrusion states. To determine whether this feature is intrinsic to its drug-recognition porter domain, we created a chimeric protein, MexBYB, by replacing the funnel-like and transmembrane domains of MexY with those of the homologous transporter MexB, and determined its structures by cryoEM under apo and kanamycin-supplemented conditions. Under both conditions, MexBYB was reported to adopt symmetric-like and asymmetric conformations. Structural comparisons reveal that the unusually large open clefts are retained in MexBYB, indicating that this feature is intrinsic to the MexY porter domain.

Monobodies, engineered protein scaffolds derived from the fibronectin type III domain, are powerful alternatives to conventional antibodies. While the native scaffold is robust, engineering the variable loops can often compromise solubility and promote aggregation. Here, we report the crystallization and structure determination at 2.57 Å resolution of a monobody (Mb-P') engineered to bind the synthetic small molecule HPPU [1-(4-hydroxyphenyl)-3-phenylurea] with nanomolar affinity. Although Mb-P' exhibited severe polydispersity and heterogeneous oligomerization in solution, N-terminal fusion with maltose-binding protein (MBP) using an optimized linker successfully yielded monodisperse species and diffraction-quality crystals. The crystal structure exhibited pseudo-D₃ symmetry in the asymmetric unit, in which the MBP moiety interacts with and partially covers the F and G β-strands of the monobody. This steric masking suggests that MBP acts as a solubility enhancer by shielding the aggregation-prone surface patches generated by loop engineering. Our results demonstrate that this fusion strategy effectively enables structural studies of aggregation-prone proteins obtained from engineered scaffolds.

Carbon-nitrogen hydrolases (CNHs) are members of the diverse nitrilase superfamily of enzymes that facilitate cellular adaptation to environmental stress by metabolizing nitrogen, detoxifying xenobiotics and catabolizing environmentally derived metabolites. Helicobacter pylori CNH (HpCNH) may contribute to metabolic flexibility under acid stress, detoxification of reactive nitrogen species or nutrient scavenging in the nutrient-limited gastric environment. Here, we report the 2.1 Å resolution crystal structure of a CNH from H. pylori strain G27 (PDB entry 6mg6). HpCNH adopts the characteristic nitrilase-superfamily αββα-sandwich core and contains the conserved catalytic cysteine typical of enzymatically active CNHs. The overall structure and active site of HpCNH are most similar to those of carbamoylputrescine amidohydrolase from the plant Medicago truncatula. Despite structural variations in loop regions, including near the active site, HpCNH retains the key residues required to bind putrescine and the prototypical N-carbamoylputrescine amidase active site.

The crystal structure of endo-β-N-acetylglucosaminidase HSα (Endo HSα) was determined at 1.8 Å resolution, revealing that the enzyme is composed of five distinct domains. Domains I to III adopt a fold that is conserved among GH85 enzymes, with catalytic residues Asn216, Glu218 and Tyr252 corresponding to conserved positions, while Tyr282 is newly implicated in catalysis based on the Endo HSα structure. A long loop unique to Endo HSα constricts the active site in domain I. Domain IV represents a novel structural element that is not observed in other GH85 enzymes. Its glycan-binding model and structural similarity to known sugar-binding domains play a role in substrate recognition. The minimal contacts with other domains allow it to remain flexible, accommodating bulky substrates at the active site. These features provide insights into the structural basis for substrate specificity and expand the structural diversity of the GH85 family.

Despite the theoretical advantages of phosphorus single-wavelength anomalous diffraction (P-SAD) for nucleic acid phasing, its application remains limited due to high atomic displacement parameters and an unfavourable ratio of unique reflections to anomalous scatterers. In this study, we report the crystal structure of an RNA complex composed of four strands, which was solved by experimental phasing after AlphaFold3 failed to produce reliable models. Bromine single-wavelength anomalous diffraction (Br-SAD) data were collected at 0.916 Å on beamline I04 at Diamond Light Source, while phosphorus anomalous data were obtained at 3.024 Å on beamline I23. The structure was successfully phased using bromine anomalous scattering, and phosphorus anomalous peaks corroborated the backbone positions and validated the model. Attempts to phase the structure directly from phosphorus data failed, consistent with theoretical predictions that successful SAD phasing requires a significantly higher reflection-to-scatterer ratio. The final models reveal an RNA complex stabilized by Watson-Crick and Hoogsteen base pairing, forming a pseudo-helical complex instead of the anticipated hairpin stem-loop, likely reflecting crystallization artefacts. This work demonstrates the complementary use of bromine and phosphorus anomalous signals in RNA crystallography.

The striking blue hue of live Homarus americanus, known as the American lobster, arises from the interaction of astaxanthin with the carotenoprotein α-crustacyanin. The mechanism underlying the bathochromic shift of the chromophore from its unbound red form (λ_max = 472 nm) to the blue protein-bound form (λ_max = 631 nm) is the subject of attention from the food and nutraceutical industries for the development of versatile food colourants. Here, we present sample purification and characterization of the α- and β-crustacyanin pigments from the American lobster for crystallographic studies and cryo-EM. Moving from H. gammarus to H. americanus for complex isolation, together with an integrated biophysical characterization, resulted in the production, for the first time, of α-crustacyanin crystals that diffracted to 6.3 Å resolution at ⟨I/σ(I)⟩ = 1.0 and high-quality negative-stain and cryo-EM images.

The XPD helicase plays a critical role in DNA repair and serves as a model for structural studies of superfamily 2 (SF2) helicases. We report a novel orthorhombic crystal form of DNA-free Thermoplasma acidophilum XPD (TaXPD) obtained under high ionic strength conditions generated by sodium potassium tartrate and NaCl-based vapor-diffusion conditions, in contrast to earlier previously reported conditions that used polyols (PEG) or diols (MPD). The crystals belonged to space group P2₁2₁2₁ (a = 59.53, b = 96.00, c = 159.09 Å) and diffracted to 2.13 Å resolution, yielding the highest resolution TaXPD structure to date. Structural analysis showed that this crystal form contains fewer intermolecular interfaces than the previously reported hexagonal lattice, as supported by Protein Interfaces, Surfaces and Assemblies (PISA) analysis. This supports the determination of the 510–514 loop in the long-form DNA-free TaXPD, which was previously disordered in other structures. This work highlights how crystallization conditions influence lattice organization, structural completeness and diffraction quality. In this structure, Tyr425 adopts a conformation that may regulate DNA access in the DNA-free state.

Here, we report the X-ray structural analysis of dihydroorotase from Methanococcus jannaschii co-crystallized with dihydroorotate at pH 6.5. The crystals are isomorphous with the crystals of the apoenzyme, with space group P3₂21 and unit-cell dimensions a = b = 111.4, c = 101.2 Å. The structure was refined to R = 0.169 and R_free = 0.186 at a resolution of 1.87 Å at room temperature. During crystallization the degradative reaction took place, and the electron density in the active site corresponds to the substrate carbamoyl aspartate. Carbamoyl aspartate interacts with the protein in the active site in a manner similar to that observed for Escherichia coli and human dihydroorotases. However, the flexible loop (residues 140-151) adopts both conformations in the crystal, loop-out and loop-in, with the loop-out conformation having higher occupancy. This contrasts with our expectations for the flexible loop to be exclusively in the loop-in conformation, which is the conformation that it adopts to stabilize the binding of the substrate in the known systems. Additional studies with substrate analogs that resemble carbamoyl aspartate and different crystallization conditions would provide further insight into the conformation of the flexible loop in this system.