Summary of the NOW'98 Phenomenology Working Group

Summary of the Phenomenology Working Group at the Europhysics Neutrino Oscillation Workshop (NOW'98), 7-9 September 1998, Amsterdam.Comment: 66 page

Magnetic structure and charge ordering in Fe3BO5 ludwigite

The crystal and magnetic structures of the three-leg ladder compound Fe3BO5 have been investigated by single crystal x-ray diffraction and neutron powder diffraction. Fe3BO5 contains two types of three-leg spin ladders. It shows a charge ordering transition at 283 K, an antiferromagnetic transition at 112 K, ferromagnetism below 70 K and a weak ferromagnetic behavior below 40K. The x-ray data reveal a smooth charge ordering and an incomplete charge localization down to 110K. Below the first magnetic transition, the first type of ladders orders as ferromagnetically coupled antiferromagnetic chains, while below 70K the second type of ladders orders as antiferromagnetically coupled ferromagnetic chains

Catalysis by hen egg-white lysozyme proceeds via a covalent intermediate

Hen egg-white lysozyme (HEWL) was the first enzyme to have its three-dimensional structure determined by X-ray diffraction techniques(1). A catalytic mechanism, featuring a long-lived oxo-carbenium-ion intermediate, was proposed on the basis of model-building studies(2). The `Phillips' mechanism is widely held as the paradigm for the catalytic mechanism of beta -glycosidases that cleave glycosidic linkages with net retention of configuration of the anomeric centre. Studies with other retaining beta -glycosidases, however, provide strong evidence pointing to a common mechanism for these enzymes that involves a covalent glycosyl-enzyme intermediate, as previously postulated(3). Here we show, in three different cases using electrospray ionization mass spectrometry, a catalytically competent covalent glycosyl-enzyme intermediate during the catalytic cycle of HEWL. We also show the three-dimensional structure of this intermediate as determined by Xray diffraction. We formulate a general catalytic mechanism for all retaining beta -glycosidases that includes substrate distortion, formation of a covalent intermediate, and the electrophilic migration of C1 along the reaction coordinate

Crystal structure of A3B3 complex of V-ATPase from Thermus thermophilus

Vacuolar-type ATPases (V-ATPases) exist in various cellular membranes of many organisms to regulate physiological processes by controlling the acidic environment. Here, we have determined the crystal structure of the A3B3 subcomplex of V-ATPase at 2.8 Å resolution. The overall construction of the A3B3 subcomplex is significantly different from that of the α3β3 sub-domain in FoF1-ATP synthase, because of the presence of a protruding ‘bulge' domain feature in the catalytic A subunits. The A3B3 subcomplex structure provides the first molecular insight at the catalytic and non-catalytic interfaces, which was not possible in the structures of the separate subunits alone. Specifically, in the non-catalytic interface, the B subunit seems to be incapable of binding ATP, which is a marked difference from the situation indicated by the structure of the FoF1-ATP synthase. In the catalytic interface, our mutational analysis, on the basis of the A3B3 structure, has highlighted the presence of a cluster composed of key hydrophobic residues, which are essential for ATP hydrolysis by V-ATPases

Targeting Mycobacterium tuberculosis Biotin Protein Ligase (MtBPL) with Nucleoside-Based Bisubstrate Adenylation Inhibitors

Mycobacterium tuberculosis (Mtb), responsible for both latent and symptomatic tuberculosis (TB), remains the second leading cause of mortality among infectious diseases worldwide. Mycobacterial biotin protein ligase (MtBPL) is an essential enzyme in Mtb and regulates lipid metabolism through the post-translational biotinylation of acyl coenzyme A carboxylases. We report the synthesis and evaluation of a systematic series of potent nucleoside-based inhibitors of MtBPL that contain modifications to the ribofuranosyl ring of the nucleoside. All compounds were characterized by isothermal titration calorimetry (ITC) and shown to bind potently with KDs ≤ 2 nM. Additionally, we obtained high-resolution cocrystal structures for a majority of the compounds. Despite fairly uniform biochemical potency, the whole-cell Mtb activity varied greatly with minimum inhibitory concentrations (MIC) ranging from 0.78 to >100 μM. Cellular accumulation studies showed a nearly 10-fold enhancement in accumulation of a C-2'-α analogue over the corresponding C-2'-β analogue, consistent with their differential whole-cell activity

Bacterial β-Glucosidase Reveals the Structural and Functional Basis of Genetic Defects in Human Glucocerebrosidase 2 (GBA2)

Human glucosylcerebrosidase 2 (GBA2) of the CAZy family GH116 is responsible for the breakdown of glycosphingolipids on the cytoplasmic face of the endoplasmic reticulum and Golgi apparatus. Genetic defects in GBA2 result in spastic paraplegia and cerebellar ataxia, while cross-talk between GBA2 and GBA1 glucosylceramidases may affect Gaucher disease. Here, we report the first three-dimensional structure for any GH116 enzyme, Thermoanaerobacterium xylanolyticum TxGH116 β-glucosidase, alone and in complex with diverse ligands. These structures allow identification of the glucoside binding and active site residues, which are shown to be conserved with GBA2. Mutagenic analysis of TxGH116 and structural modeling of GBA2 provide a detailed structural and functional rationale for pathogenic missense mutations of GBA2

The PHCCEx domain of Tiam1/2 is a novel protein- and membrane-binding module

Tiam1 and Tiam2 (Tiam1/2) are guanine nucleotide-exchange factors that possess the PH–CC–Ex (pleckstrin homology, coiled coil and extra) region that mediates binding to plasma membranes and signalling proteins in the activation of Rac GTPases. Crystal structures of the PH–CC–Ex regions revealed a single globular domain, PHCCEx domain, comprising a conventional PH subdomain associated with an antiparallel coiled coil of CC subdomain and a novel three-helical globular Ex subdomain. The PH subdomain resembles the β-spectrin PH domain, suggesting non-canonical phosphatidylinositol binding. Mutational and binding studies indicated that CC and Ex subdomains form a positively charged surface for protein binding. We identified two unique acidic sequence motifs in Tiam1/2-interacting proteins for binding to PHCCEx domain, Motif-I in CD44 and ephrinB's and the NMDA receptor, and Motif-II in Par3 and JIP2. Our results suggest the molecular basis by which the Tiam1/2 PHCCEx domain facilitates dual binding to membranes and signalling proteins

Collective-coordinate analysis of inhomogeneous nonlinear Klein-Gordon field theory

Two different sets of collective-coordinate equations for solitary solutions of Nonlinear Klein-Gordon (NKG) model is introduced. The collective-coordinate equations are derived using different approaches for adding the inhomogeneities as exrernal potentials to the soliton equation of motion. Interaction of the NKG field with a local inhomogeneity like a delta function potential wall and also delta function potential well is investigated using the presented collective-coordinate equations and the results of two different models are compared. Most of the characters of the interaction are derived analytically. Analytical results are also compared with the results of numerical simulations.Comment: 16 pages, 8 figures. Accepted for publication in Volume 43 of the Brazilian Journal of Physic

The Structure of the NPC1L1 N-Terminal Domain in a Closed Conformation

NPC1L1 is the molecular target of the cholesterol lowering drug Ezetimibe and mediates the intestinal absorption of cholesterol. Inhibition or deletion of NPC1L1 reduces intestinal cholesterol absorption, resulting in reduction of plasma cholesterol levels.Here we present the 2.8 Å crystal structure of the N-terminal domain (NTD) of NPC1L1 in the absence of cholesterol. The structure, combined with biochemical data, reveals the mechanism of cholesterol selectivity of NPC1L1. Comparison to the cholesterol free and bound structures of NPC1(NTD) reveals that NPC1L1(NTD) is in a closed conformation and the sterol binding pocket is occluded from solvent.The structure of NPC1L1(NTD) reveals a degree of flexibility surrounding the entrance to the sterol binding pocket, suggesting a gating mechanism that relies on multiple movements around the entrance to the sterol binding pocket

A transient homotypic interaction model for the influenza A virus NS1 protein effector domain

Influenza A virus NS1 protein is a multifunctional virulence factor consisting of an RNA binding domain (RBD), a short linker, an effector domain (ED), and a C-terminal 'tail'. Although poorly understood, NS1 multimerization may autoregulate its actions. While RBD dimerization seems functionally conserved, two possible apo ED dimers have been proposed (helix-helix and strand-strand). Here, we analyze all available RBD, ED, and full-length NS1 structures, including four novel crystal structures obtained using EDs from divergent human and avian viruses, as well as two forms of a monomeric ED mutant. The data reveal the helix-helix interface as the only strictly conserved ED homodimeric contact. Furthermore, a mutant NS1 unable to form the helix-helix dimer is compromised in its ability to bind dsRNA efficiently, implying that ED multimerization influences RBD activity. Our bioinformatical work also suggests that the helix-helix interface is variable and transient, thereby allowing two ED monomers to twist relative to one another and possibly separate. In this regard, we found a mAb that recognizes NS1 via a residue completely buried within the ED helix-helix interface, and which may help highlight potential different conformational populations of NS1 (putatively termed 'helix-closed' and 'helix-open') in virus-infected cells. 'Helix-closed' conformations appear to enhance dsRNA binding, and 'helix-open' conformations allow otherwise inaccessible interactions with host factors. Our data support a new model of NS1 regulation in which the RBD remains dimeric throughout infection, while the ED switches between several quaternary states in order to expand its functional space. Such a concept may be applicable to other small multifunctional proteins