Madhumati Sevvana

Endothelium-protective sphingosine-1-phosphate provided by HDL-associated apolipoprotein M

Protection of the endothelium is provided by circulating sphingosine-1-phosphate (S1P), which maintains vascular integrity. We show that HDL-associated S1P is bound specifically to both human and murine apolipoprotein M (apoM). Thus, isolated human ApoM+ HDL contained S1P, whereas ApoM− HDL did not. Moreover, HDL in Apom−/− mice contains no S1P, whereas HDL in transgenic mice overexpressing human apoM has an increased S1P content. The 1.7-Å structure of the S1P–human apoM complex reveals that S1P interacts specifically with an amphiphilic pocket in the lipocalin fold of apoM. Human ApoM+ HDL induced S1P1 receptor internalization, downstream MAPK and Akt activation, endothelial cell migration, and formation of endothelial adherens junctions, whereas apoM− HDL did not. Importantly, lack of S1P in the HDL fraction of Apom−/− mice decreased basal endothelial barrier function in lung tissue. Our results demonstrate that apoM, by delivering S1P to the S1P1 receptor on endothelial cells, is a vasculoprotective constituent of HDL.

A Ligand-Induced Switch in the Periplasmic Domain of Sensor Histidine Kinase Cita.

Sensor histidine kinases of two-component signal-transduction systems are essential for bacteria to adapt to variable environmental conditions. However, despite their prevalence, it is not well understood how extracellular signals such as ligand binding regulate the activity of these sensor kinases. CitA is the sensor histidine kinase in Klebsiella pneumoniae that regulates the transport and anaerobic metabolism of citrate in response to its extracellular concentration. We report here the X-ray structures of the periplasmic sensor domain of CitA in the citrate-free and citrate-bound states. A comparison of the two structures shows that ligand binding causes a considerable contraction of the sensor domain. This contraction may represent the molecular switch that activates transmembrane signaling in the receptor.

Serendipitous Fatty Acid Binding Reveals the Structural Determinants for Ligand Recognition in Apolipoprotein M

Apolipoprotein M (ApoM) is a 25-kDa HDL-associated apolipoprotein and a member of the lipocalin family of proteins. Mature apoM retains its signal peptide, which serves as a lipid anchor attaching apoM to the lipoproteins, thereby keeping it in the circulation. Studies in mice have suggested apoM to be antiatherogenic, but its physiological function is yet unknown. We have now determined the 1.95 A resolution crystal structure of recombinant human apoM expressed in Escherichia coli and made the unexpected discovery that apoM, although refolded from inclusion bodies, was in complex with fatty acids containing 14, 16 or 18 carbon atoms. ApoM displays the typical lipocalin fold characterised by an eight-stranded antiparallel beta-barrel that encloses an internal ligand-binding pocket. The crystal structures of two different complexes provide a detailed picture of the ligand-binding determinants of apoM. Additional fatty acid- and lipid-binding studies with apoM and the mutants apoM(W47F) and apoM(W100F) showed that sphingosine-1-phosphate is able to displace the bound fatty acids and efficiently quenched the intrinsic fluorescence with an IC(50) of 0.90 muM. Whereas the fatty acids bound in the crystal structure could be a mere consequence of recombinant protein production, the observed binding of sphingosine-1-phosphate might provide a key to a better understanding of the physiological function of apoM.

Crystal structure and functional analysis of the protein disulfide isomerase-related protein ERp29.

The protein disulfide isomerase-related protein ERp29 is a putative chaperone involved in processing and secretion of secretory proteins. Until now, however, both the structure and the exact nature of interacting substrates remained unclear. We provide for the first time a crystal structure of human ERp29, refined to 2.9 A, and show that the protein has considerable structural homology to its Drosophila homolog Wind. We show that ERp29 binds directly not only to thyroglobulin and thyroglobulin-derived peptides in vitro but also to the Wind client protein Pipe and Pipe-derived peptides, although it fails to process Pipe in vivo. A monomeric mutant of ERp29 and a D domain mutant in which the second peptide binding site is inactivated also bind protein substrates, indicating that the monomeric thioredoxin domain is sufficient for client protein binding. Indeed, the b domains of ERp29 or Wind, expressed alone, are sufficient for binding proteins and peptides. Interacting peptides have in common two or more aromatic residues, with stronger binding for sequences with overall basic character. Thus, the data allow a view of the two putative peptide binding sites of ERp29 and indicate that the apparent, different processing activity of the human and Drosophila proteins in vivo does not stem from differences in peptide binding properties.

Crystal Structure of the Human Cytomegalovirus pUL50-pUL53 Core Nuclear Egress Complex Provides Insight into a Unique Assembly Scaffold for Virus-Host Protein Interactions

Background: The conserved cytomegalovirus proteins pUL50 and pUL53 heterodimerize and form a core nuclear egress complex. Results: The crystal structure of pUL50-pUL53 was solved at 2.44 Å resolution, revealing an N-terminal hook-like extension of pUL53. Conclusion: Data unravel the core NEC architecture, providing a scaffold for viral-cellular NEC protein interactions. Significance: The identified NEC structure will stimulate the development of novel antiviral strategies. Nuclear replication of cytomegalovirus relies on elaborate mechanisms of nucleocytoplasmic egress of viral particles. Thus, the role of two essential and conserved viral nuclear egress proteins, pUL50 and pUL53, is pivotal. pUL50 and pUL53 heterodimerize and form a core nuclear egress complex (NEC), which is anchored to the inner nuclear membrane and provides a scaffold for the assembly of a multimeric viral-cellular NEC. Here, we report the crystal structure of the pUL50-pUL53 heterodimer (amino acids 1–175 and 50–292, respectively) at 2.44 Å resolution. Both proteins adopt a globular fold with mixed α and β secondary structure elements. pUL53-specific features include a zinc-binding site and a hook-like N-terminal extension, the latter representing a hallmark element of the pUL50-pUL53 interaction. The hook-like extension (amino acids 59–87) embraces pUL50 and contributes 1510 Å2 to the total interface area (1880 Å2). The pUL50 structure overall resembles the recently published NMR structure of the murine cytomegalovirus homolog pM50 but reveals a considerable repositioning of the very C-terminal α-helix of pUL50 upon pUL53 binding. pUL53 shows structural resemblance with the GHKL domain of bacterial sensory histidine kinases. A close examination of the crystal structure indicates partial assembly of pUL50-pUL53 heterodimers to hexameric ring-like structures possibly providing additional scaffolding opportunities for NEC. In combination, the structural information on pUL50-pUL53 considerably improves our understanding of the mechanism of HCMV nuclear egress. It may also accelerate the validation of the NEC as a unique target for developing a novel type of antiviral drug and improved options of broad-spectrum antiherpesviral therapy.

Specific Residues of a Conserved Domain in the N Terminus of the Human Cytomegalovirus pUL50 Protein Determine Its Intranuclear Interaction with pUL53

Background: Interaction between the cytomegalovirus proteins pUL50 and pUL53 is essential for formation of a nuclear egress complex. Results: Mutations within a globular domain interfere with the function of pUL50. Conclusion: Residues Glu-56 and Tyr-57 of pUL50 are essential for binding to pUL53. Significance: Identification of the mode of important viral protein interactions promotes the development of novel antiviral strategies. Herpesviral capsids are assembled in the host cell nucleus and are subsequently translocated to the cytoplasm. During this process it has been demonstrated that the human cytomegalovirus proteins pUL50 and pUL53 interact and form, together with other viral and cellular proteins, the nuclear egress complex at the nuclear envelope. In this study we provide evidence that specific residues of a conserved N-terminal region of pUL50 determine its intranuclear interaction with pUL53. In silico evaluation and biophysical analyses suggested that the conserved region forms a regular secondary structure adopting a globular fold. Importantly, site-directed replacement of individual amino acids by alanine indicated a strong functional influence of specific residues inside this globular domain. In particular, mutation of the widely conserved residues Glu-56 or Tyr-57 led to a loss of interaction with pUL53. Consistent with the loss of binding properties, mutants E56A and Y57A showed a defective function in the recruitment of pUL53 to the nuclear envelope in expression plasmid-transfected and human cytomegalovirus-infected cells. In addition, in silico analysis suggested that residues 3–20 form an amphipathic α-helix that appears to be conserved among Herpesviridae. Point mutants revealed a structural role of this N-terminal α-helix for pUL50 stability rather than a direct role in the binding of pUL53. In contrast, the central part of the globular domain including Glu-56 and Tyr-57 is directly responsible for the functional interaction with pUL53 and thus determines formation of the basic nuclear egress complex.

Crystal structure of cytomegalovirus IE1 protein reveals targeting of TRIM family member PML via coiled-coil interactions.

PML nuclear bodies (PML-NBs) are enigmatic structures of the cell nucleus that act as key mediators of intrinsic immunity against viral pathogens. PML itself is a member of the E3-ligase TRIM family of proteins that regulates a variety of innate immune signaling pathways. Consequently, viruses have evolved effector proteins to modify PML-NBs; however, little is known concerning structure-function relationships of viral antagonists. The herpesvirus human cytomegalovirus (HCMV) expresses the abundant immediate-early protein IE1 that colocalizes with PML-NBs and induces their dispersal, which correlates with the antagonization of NB-mediated intrinsic immunity. Here, we delineate the molecular basis for this antagonization by presenting the first crystal structure for the evolutionary conserved primate cytomegalovirus IE1 proteins. We show that IE1 consists of a globular core (IE1CORE) flanked by intrinsically disordered regions. The 2.3 Å crystal structure of IE1CORE displays an all α-helical, femur-shaped fold, which lacks overall fold similarity with known protein structures, but shares secondary structure features recently observed in the coiled-coil domain of TRIM proteins. Yeast two-hybrid and coimmunoprecipitation experiments demonstrate that IE1CORE binds efficiently to the TRIM family member PML, and is able to induce PML deSUMOylation. Intriguingly, this results in the release of NB-associated proteins into the nucleoplasm, but not of PML itself. Importantly, we show that PML deSUMOylation by IE1CORE is sufficient to antagonize PML-NB-instituted intrinsic immunity. Moreover, co-immunoprecipitation experiments demonstrate that IE1CORE binds via the coiled-coil domain to PML and also interacts with TRIM5α We propose that IE1CORE sequesters PML and possibly other TRIM family members via structural mimicry using an extended binding surface formed by the coiled-coil region. This mode of interaction might render the antagonizing activity less susceptible to mutational escape.

Structures of viscotoxins A1 and B2 from European mistletoe solved using native data alone.

Crystals of the cytotoxic thionin proteins viscotoxins A1 and B2 extracted from mistletoe diffracted to high resolution (1.25 and 1.05 A, respectively) and are excellent candidates for testing crystallographic methods. Ab initio direct methods were only successful in solving the viscotoxin B2 structure, which with 861 unique non-H atoms is one of the largest unknown structures without an atom heavier than sulfur to be solved in this way, but sulfur-SAD phasing provided a convincing solution for viscotoxin A1. Both proteins form dimers in the crystal and viscotoxin B2 (net charge +4 per monomer), but not viscotoxin A1 (net charge +6), is coordinated by sulfate or phosphate anions. The viscotoxin A1 crystal has a higher solvent content than the viscotoxin B2 crystal (49% as opposed to 28%) with solvent channels along the crystallographic 4(3) axes.

Kettapeptin: Isolation, Structure Elucidation and Activity of a New Hexadepsipeptide Antibiotic from a Terrestrial Streptomyces sp.

The ethyl acetate extract of the Streptomyces sp. isolate GW99/1572 exhibited significant biological activity against Gram-positive bacteria and delivered kettapeptin (1), a new hexadepsipeptide antibiotic of the azinothricin type. The structure was elucidated by various 1D and 2D NMR techniques, mass spectrometry and by comparison of the NMR data with those of closely related antibiotics. The absolute configuration of the compound was derived by crystal structure analysis and by comparison with the optical rotation data of related compounds.

Sesquiterpene Lactones from Elephantopus scaber

The ethanolic and acetone extracts of the whole plant of Elephantopus scaber were found to contain ethyl hexadecanoate, ethyl-9,12-octadecadienoate, ethyl-(Z)-9-octadecenoate, ethyl octadecanoate, lupeol, stigmasterol, stigmasterol glucoside, deoxyelephantopin (1) and two new germacranolide sesquiterpene lactones named 17,19-dihydrodeoxyelephantopin (2) and iso-17,19- dihydrodeoxyelephantopin (3) whose stereostructures were determined by spectroscopic methods, comparison with reported data and single-crystal X-ray analysis.