Journal of Structural Biology: X最新文献

Human serum high-density lipoproteins (HDLs) are a population of small, dense protein-lipid aggregates that are crucial for intravascular lipid trafficking and are protective against cardiovascular disease. The spheroidal HDL subfraction can be separated by size and density into five major subpopulations with distinct molecular compositions and unique biological functionalities: HDL_3c, HDL_3b, HDL_3a, HDL_2a and HDL_2b. Representative molecular models of these five subpopulations were developed and characterised for the first time in the presence of multiple copies of its primary protein component apolipoprotein A-I (apoA-I) using coarse-grained molecular dynamics simulations. Each HDL model exhibited size, morphological and compositional profiles consistent with experimental observables. With increasing particle size the separation of core and surface molecules became progressively more defined, resulting in enhanced core lipid mixing, reduced core lipid exposure at the surface, and the formation of an interstitial region between core and surface molecules in HDL_2b. Cholesterol molecules tended to localise around the central helix-5 of apoA-I, whilst triglyceride molecules predominantly interacted with aromatic, hydrophobic residues located within the terminal helix-10 across all subpopulation models. The three intermediate HDL models exhibited similar surface profiles despite having distinct molecular compositions. ApoA-I in trefoil, quatrefoil and pentafoil arrangements across the surface of HDL particles exhibited significant warping and twisting, but largely retained intermolecular contacts between adjacent apoA-I chains. Representative HDL subpopulations differed in particle size, morphology, intermolecular interaction profiles and lipid and protein dynamics. These findings reveal how different HDL subpopulations might exhibit distinct functional associations depending on particle size, form and composition.

Knowledge of three-dimensional protein structure is integral to most modern drug discovery efforts. Recent advancements have highlighted new techniques for 3D protein structure determination and, where structural data cannot be collected experimentally, prediction of protein structure. We have undertaken a major effort to use existing protein structures to collect, characterize, and catalogue the inter-atomic interactions that define and compose 3D structure by mapping hydropathic interaction environments as maps in 3D space. This work has been performed on a residue-by-residue basis, where we have seen evidence for relationships between environment character, residue solvent-accessible surface areas and their secondary structures. In this graphical review, we apply principles from our earlier studies and expand the scope to all common amino acid residue types in both soluble and membrane proteins. Key to this analysis is parsing the Ramachandran plot to an 8-by-8 chessboard to define secondary structure bins. Our analysis yielded a number of quantitative discoveries: 1) increased fraction of hydrophobic residues (alanine, isoleucine, leucine, phenylalanine and valine) in membrane proteins compared to their fractions in soluble proteins; 2) less burial coupled with significant increases in favorable hydrophobic interactions for hydrophobic residues in membrane proteins compared to soluble proteins; and 3) higher burial and more favorable polar interactions for polar residues now preferring the interior of membrane proteins. These observations and the supporting data should provide benchmarks for current studies of protein residues in different environments and may be able to guide future protein structure prediction efforts.

Although the tetanus neurotoxin (TeNT) delivers its protease domain (LC) across the synaptic vesicle lumen into the cytosol via its receptor binding domain (H_C) and translocation domain (H_N), the molecular mechanism coordinating this membrane translocation remains unresolved. Here, we report the high-resolution crystal structures of full-length reduced TeNT (rTeNT, 2.3 Å), TeNT isolated H_N (TeNT/iH_N, 2.3 Å), TeNT isolated H_C (TeNT/iH_C, 1.5 Å), together with the solution structures of TeNT/iH_N and beltless TeNT/iH_N (TeNT/blH_N). TeNT undergoes significant domains rotation of the H_N and LC were demonstrated by structural comparison of rTeNT and non-reduced-TeNT (nrTeNT). A linker loop connects the H_N and H_C is essential for the self-domain rotation of TeNT. The TeNT-specific C869-C1093 disulfide bond is sensitive to the redox environment and its disruption provides linker loop flexibility, which enables domain arrangement of rTeNT distinct from that of nrTeNT. Furthermore, the mobility of C869 in the linker loop and the sensitivity to redox condition of C1093 were confirmed by crystal structure analysis of TeNT/iH_C. On the other hand, the structural flexibility of H_N was investigated by crystallographic and solution scattering analyses. It was found that the region (residues 698–769), which follows the translocation region had remarkable change in TeNT/iH_N. Besides, the so-called belt region has a high propensity to swing around the upper half of TeNT/iH_N at acidic pH. It provides the first overview of the dynamics of the Belt in solution. These newly obtained structural information that shed light on the transmembrane mechanism of TeNT.

Euchromatic histone-lysine N-methyltransferase 1 (EHMT1; G9a-like protein; GLP) and euchromatic histone-lysine N-methyltransferase 2 (EHMT2; G9a) are protein lysine methyltransferases that regulate gene expression and are essential for development and the ability of organisms to change and adapt. In addition to ankyrin repeats and the catalytic SET domain, the EHMT proteins contain a unique cysteine-rich region (CRR) that mediates protein–protein interactions and recruitment of the methyltransferases to specific sites in chromatin. We have determined the structure of the CRR from human EHMT2 by X-ray crystallography and show that the CRR adopts an unusual compact fold with four bound zinc atoms. The structure consists of a RING domain preceded by a smaller zinc-binding motif and an N-terminal segment. The smaller zinc-binding motif straddles the N-terminal end of the RING domain, and the N-terminal segment runs in an extended conformation along one side of the structure and interacts with both the smaller zinc-binding motif and the RING domain. The interface between the N-terminal segment and the RING domain includes one of the zinc atoms. The RING domain is partially sequestered within the CRR and unlikely to function as a ubiquitin ligase.

The chitinolytic bacterium Paenibacillus sp. str. FPU-7 efficiently degrades chitin into oligosaccharides such as N-acetyl-D-glucosamine (GlcNAc) and disaccharides (GlcNAc)₂ through multiple secretory chitinases. Transport of these oligosaccharides by P. str. FPU-7 has not yet been clarified. In this study, we identified nagB1, predicted to encode a sugar solute-binding protein (SBP), which is a component of the ABC transport system. However, the genes next to nagB1 were predicted to encode two-component regulatory system proteins rather than transmembrane domains (TMDs). We also identified nagB2, which is highly homologous to nagB1. Adjacent to nagB2, two genes were predicted to encode TMDs. Binding experiments of the recombinant NagB1 and NagB2 to several oligosaccharides using differential scanning fluorimetry and surface plasmon resonance confirmed that both proteins are SBPs of (GlcNAc)₂ and (GlcNAc)₃. We determined their crystal structures complexed with and without chitin oligosaccharides at a resolution of 1.2 to 2.0 Å. The structures shared typical SBP structural folds and were classified as subcluster D-I. Large domain motions were observed in the structures, suggesting that they were induced by ligand binding via the “Venus flytrap” mechanism. These structures also revealed chitin oligosaccharide recognition mechanisms. In conclusion, our study provides insight into the recognition and transport of chitin oligosaccharides in bacteria.

Congenital cataract (CC) is the major cause of childish blindness, and nearly 50% of CCs are hereditary disorders. HSF4, a member of the heat shock transcription factor family, acts as a key regulator of cell growth and differentiation during the development of sensory organs. Missense mutations in the HSF4-encoding gene have been reported to cause CC formation; in particular, those occurring within the DNA-binding domain (DBD) are usually autosomal dominant mutations. To address how the identified mutations lead to HSF4 malfunction by placing adverse impacts on protein structure and DNA-binding specificity and affinity, we determined two high-resolution structures of the wild-type DBD and the K23N mutant of human HSF4, built DNA-binding models, conducted in silico mutations and molecular dynamics simulations. Our analysis suggests four possible structural mechanisms underlining the missense mutations in HSF4-DBD and cataractogenesis: (i), disruption of HSE recognition; (ii), perturbation of protein-DNA interactions; (iii), alteration of protein folding; (iv), other impacts, e.g. inhibition of protein oligomerization.

Knowledge of both apo and holo states of riboswitches aid in elucidating the various mechanisms of ligand-induced conformational “switching” that underpin their gene-regulating capabilities. Previous structural studies on the flavin mononucleotide (FMN)-binding aptamer of the FMN riboswitch, however, have revealed minimal conformational changes associated with ligand binding that do not adequately explain the basis for the switching behavior. We have determined a 2.7-Å resolution crystal structure of the ligand-free FMN riboswitch aptamer that is distinct from previously reported structures, particularly in the conformation and orientation of the P1 and P4 helices. The nearly symmetrical tertiary structure provides a mechanism by which one of two pairs of adjacent helices (P3/P4 or P1/P6) undergo collinear stacking in a mutually exclusive manner, in the absence or presence of ligand, respectively. Comparison of these structures suggests the stem-loop that includes P4 and L4 is important for maintaining a global conformational state that, in the absence of ligand, disfavors formation of the P1 regulatory helix. Together, these results provide further insight to the structural basis for conformational switching of the FMN riboswitch.

The flagellum of Trypanosoma brucei is a 20 µm-long organelle responsible for locomotion and cell morphogenesis. The flagellum attachment zone (FAZ) is a multi-protein complex whose function is to attach the flagellum to the cell body but also to guide cytokinesis. Cryo-transmission electron microscopy is a tool of choice to access the structure of the FAZ in a close-to-native state. However, because of the large dimension of the cell body, the whole FAZ cannot be structurally studied in situ at the nanometre scale in 3D using classical transmission electron microscopy approaches. In the present work, cryo-scanning transmission electron tomography, a new method capable of investigating cryo-fixed thick biological samples, has been used to study whole T. brucei cells at the bloodstream stage. The method has been used to visualise and characterise the structure and organisation of the FAZ filament. It is composed of an array of cytoplasmic stick-like structures. These sticks are heterogeneously distributed between the posterior part and the anterior tip of the cell. This cryo-STET investigation provides new insights into the structure of the FAZ filament. In combination with protein structure predictions, this work proposes a new model for the elongation of the FAZ.

Phosphotyrosine (pY) signaling is instrumental to numerous cellular processes. pY recognition occurs through specialized protein modules, among which the Src-homology 2 (SH2) domain is the most common. SH2 domains are small protein modules with an invariant fold, and are present in more than a hundred proteins with different function. Here we ask the question of how such a structurally conserved, small protein domain can recognize distinct phosphopeptides with the breath of binding affinity, specificity and kinetic parameters necessary for proper control of pY-dependent signaling and rapid cellular response. We review the current knowledge on structure, thermodynamics and kinetics of SH2–phosphopeptide complexes and conclude that selective phosphopeptide recognition is governed by both structure and dynamics of the SH2 domain, as well as by the kinetics of the binding events. Further studies on the thermodynamic and kinetic properties of SH2–phosphopeptide complexes, beyond their structure, are required to understand signaling regulation.

RTX adhesins are long, multi-domain proteins present on the outer membrane of many Gram-negative bacteria. From this vantage point, adhesins use their distal ligand-binding domains for surface attachment leading to biofilm formation. To expand the reach of the ligand-binding domains, RTX adhesins maintain a central extender region of multiple tandem repeats, which makes up most of the proteins’ large molecular weight. Alignments of the 10-15-kDa extender domains show low sequence identity between adhesins. Here we have produced and structurally characterized protein constructs of four tandem repeats (tetra-tandemers) from two different RTX adhesins. In comparing the tetra-tandemers to each other and already solved structures from Marinomonas primoryensis and Salmonella enterica, the extender domains fold as diverse beta-sandwich structures with widely differing calcium contents. However, all the tetra-tandemers have at least one calcium ion coordinated in the linker region between beta-sandwich domains whose role appears to be the rigidification of the extender region to help the adhesin extend its reach.