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1.  Structure of the HIV-1 Full-Length Capsid in a Conformationally-Trapped Unassembled State Induced by Small-Molecule Binding 
Journal of molecular biology  2010;406(3):371-386.
The capsid protein (CA) plays crucial roles in HIV-infection and replication, essential to viral maturation. The absence of high-resolution structural data on unassembled CA hinders the development of antivirals effective in inhibiting assembly. Unlike enzymes that have targetable functional substrate binding sites, the CA does not have a known site that affects catalytic or other innate activity, which can be more readily targeted in drug development efforts. We report the crystal structure of the HIV-1 CA, revealing the domain organization in context of the wild-type full-length (FL) unassembled CA. The FL CA adopts an antiparallel dimer (APD) configuration, exhibiting a domain organization sterically incompatible with capsid assembly. A small compound, generated in-situ during crystallization, is bound tightly at a hinge-site (“H-site”), indicating that binding at this interdomain region stabilizes the ADP conformation. Electron microscopy studies on nascent crystals reveal both dimeric and hexameric lattices coexisting within a single condition, in agreement with the interconvertibility of oligomeric forms and supporting the feasibility of promoting assembly-incompetent dimeric states. Solution characterization in the presence of the H-site ligand shows predominantly unassembled dimeric CA, even under conditions that promote assembly. Our structure elucidation of the HIV-1 FL CA and characterization of a potential allosteric binding site provides 3D views of an assembly-defective conformation, a state targeted in and, thus, directly relevant to, inhibitor development. Based on our findings, we propose an unprecedented means of preventing CA assembly, by ‘conformationally-trapping’ CA in assembly-incompetent conformational states, induced by H-site binding.
PMCID: PMC3194004  PMID: 21146540
native full-length HIV-1 capsid crystal structure; assembly inhibitor; hinge- binding site; conformational-trapping; alternative dimer states
2.  Comparison of the Mechanism of Toxicity of Zinc Oxide and Cerium Oxide Nanoparticles Based on Dissolution and Oxidative Stress Properties 
ACS nano  2008;2(10):2121-2134.
Nanomaterials (NM) exhibit novel physicochemical properties that determine their interaction with biological substrates and processes. Three metal oxides nanoparticles that are currently being produced in high tonnage, TiO2, ZnO and CeO2, were synthesized by flame spray pyrolysis process and compared in a mechanistic study to elucidate the physicochemical characteristics that determine cellular uptake, subcellular localization, and toxic effects based on a test paradigm that was originally developed for oxidative stress and cytotoxicity in RAW 264.7 and BEAS-2B cell lines. ZnO induced toxicity in both cells, leading to the generation of reactive oxygen species (ROS), oxidant injury, excitation of inflammation and cell death. Using ICP-MS and fluorescent-labeled ZnO, it is found that ZnO dissolution could happen in culture medium and endosomes. Non-dissolved ZnO nanoparticles enter caveolae in BEAS-2B, but enter lysosomes in RAW 264.7 cells in which smaller particle remnants dissolve. In contrast, fluorescent-labeled CeO2 nanoparticles were taken up intact into caveolin-1 and LAMP-1 positive endosomal compartments, respectively, in BEAS-2B and RAW 264.7 cells, without inflammation or cytotoxicity. Instead, CeO2 suppressed ROS production and induced cellular resistance to an exogenous source of oxidative stress. Fluorescent-labeled TiO2 was processed by the same uptake pathways as CeO2 but did not elicit any adverse or protective effects. These results demonstrate that metal oxide nanoparticles induce a range of biological responses that vary from cytotoxic to cytoprotective and can only be properly understood by using a tiered test strategy such as we developed for oxidative stress and adapted to study other aspects of nanoparticle toxicity.
PMCID: PMC3959800  PMID: 19206459
Nanotoxicology; Nanoparticle; Reactive oxygen species; Oxidative stress; Dissolution; Nano-bio interface
3.  Effector Kinase Coupling Enables High-Throughput Screens for Direct HIV-1 Nef Antagonists with Anti-retroviral Activity 
Chemistry & biology  2013;20(1):82-91.
HIV-1 Nef, a critical AIDS progression factor, represents an important target protein for antiretroviral drug discovery. Because Nef lacks intrinsic enzymatic activity, we developed an assay that couples Nef to the activation of Hck, a Src-family member and Nef effector protein. Using this assay, we screened a large, diverse chemical library and identified small molecules that block Nef-dependent Hck activity with low micromolar potency. Of these, a diphenylpyrazolo compound demonstrated sub-micromolar potency in HIV-1 replication assays against a broad range of primary Nef variants. This compound binds directly to Nef via a pocket formed by the Nef dimerization interface and disrupts Nef dimerization in cells. Coupling of non-enzymatic viral accessory factors to host cell effector proteins amenable to high-throughput screening may represent a general strategy for the discovery of new antimicrobial agents.
PMCID: PMC3559019  PMID: 23352142
4.  First-in-human trial of a STAT3 decoy oligonucleotide in head and neck tumors: implications for cancer therapy 
Cancer discovery  2012;2(8):694-705.
Despite evidence implicating transcription factors, including STAT3, in oncogenesis, these proteins have been regarded as “undruggable”. We developed a decoy targeting STAT3 and performed a phase 0 trial. Expression levels of STAT3 target genes were decreased in the head and neck cancers following injection with the STAT3 decoy compared with tumors receiving saline control. Decoys have not been amenable to systemic administration due to instability. To overcome this barrier, we linked the oligonucleotide strands using hexa-ethyleneglycol spacers. This cyclic STAT3 decoy bound with high affinity to STAT3 protein, reduced cellular viability, and suppressed STAT3 target gene expression in cancer cells. Intravenous injection of the cyclic STAT3 decoy inhibited xenograft growth and downregulated STAT3 target genes in the tumors. These results provide the first demonstration of a successful strategy to inhibit tumor STAT3 signaling via systemic administration of a selective STAT3 inhibitor, thereby paving the way for broad clinical development.
PMCID: PMC3668699  PMID: 22719020
STAT3; decoy oligonucleotide; phase 0; head and neck cancer
5.  Interaction of haptoglobin with hemoglobin octamers based on the mutation αAsn78Cys or βGly83Cys 
Octameric hemoglobins have been developed by the introduction of surface cysteines in either the alpha or beta chain. Originally designed as a blood substitute, we report here the structure and ligand binding function; in addition the interaction with haptoglobin was studied. The recombinant Hbs (rHbs) with mutations alpha Asn78Cys or beta Gly83Cys spontaneously form octamers under conditions where the cysteines are oxidized. Oxygen binding curves and CO kinetic studies indicate a correct allosteric transition of the tetramers within the octamer. Crystallographic studies of the two rHbs show two disulfide bonds per octamer. Reducing agents may provoke dissociation to tetramers, but the octamers are stable when mixed with fresh human plasma, indicating that the reduction by plasma is slower than the oxidation by the dissolved oxygen, consistent with an enhanced stability. The octameric rHbs were also mixed with a solution of haptoglobin (Hp), which binds the dimers of Hb: there was little interaction for incubation times of 15 min; however, on longer timescales a complex was formed. Dynamic light scattering was used to follow the interaction of Hp with the alpha Asn78Cys octamer during 24 hours; a transition from a simple complex of 15 nm to a final size of 60 nm was observed. The results indicate a specific orientation of the αβ dimers may be of importance for the binding to haptoglobin.
PMCID: PMC3706102  PMID: 23847747
Octamers; Hemoglobin; Haptoglobin; Allosteric Transition; Crystallography
6.  HIV-1 Nef interaction influences the ATP-binding site of the Src-family kinase, Hck 
BMC Chemical Biology  2012;12:1.
Nef is an HIV-1 accessory protein essential for viral replication and AIDS progression. Nef interacts with a multitude of host cell signaling partners, including members of the Src kinase family. Nef preferentially activates Hck, a Src-family kinase (SFK) strongly expressed in macrophages and other HIV target cells, by binding to its regulatory SH3 domain. Recently, we identified a series of kinase inhibitors that preferentially inhibit Hck in the presence of Nef. These compounds also block Nef-dependent HIV replication, validating the Nef-SFK signaling pathway as an antiretroviral drug target. Our findings also suggested that by binding to the Hck SH3 domain, Nef indirectly affects the conformation of the kinase active site to favor inhibitor association.
To test this hypothesis, we engineered a "gatekeeper" mutant of Hck with enhanced sensitivity to the pyrazolopyrimidine tyrosine kinase inhibitor, NaPP1. We also modified the RT loop of the Hck SH3 domain to enhance interaction of the kinase with Nef. This modification stabilized Nef:Hck interaction in solution-based kinase assays, as a way to mimic the more stable association that likely occurs at cellular membranes. Introduction of the modified RT loop rendered Hck remarkably more sensitive to activation by Nef, and led to a significant decrease in the Km for ATP as well as enhanced inhibitor potency.
These observations suggest that stable interaction with Nef may induce Src-family kinase active site conformations amenable to selective inhibitor targeting.
PMCID: PMC3328272  PMID: 22420777
7.  The Crystal Structure of Non-Modified and Bipyridine-Modified PNA Duplexes 
Peptide nucleic acid (PNA) is a synthetic analogue of DNA that commonly has an N-aminoethlyl-glycine backbone. The crystal structure of two PNA duplexes, one containing eight standard nucleobase pairs (GGCATCGG)2 (pdb: 3MBS), and the other containing the same nucleobase pairs and a central pair of bipyridine ligands (pdb: 3MBU), has been solved with a resolution of 1.2 Å and 1.05 Å, respectively. The non-modified PNA duplex adopts a P-type helical structure s i m i l a r t o that of previously characterized PNAs. The atomic-level resolution of the structures allowed us to observe for the first time specific modes of interaction between the terminal lysines of the PNA and the backbone and nucleobases situated in the vicinity of the lysines, which are considered an important factor in the induction of a preferred handedness in PNA duplexes. These results support the notion that while PNA typically adopts a P-type helical structure, its flexibility is relatively high. For example, the base pair rise in the bipyridine-containing PNA is the largest measured to date in a PNA homoduplex. The two bipyridines are bulged out of the duplex and are aligned parallel to the minor groove of the PNA. In the case of the bipyridine-containing PNA, two bipyridines from adjacent PNA duplexes form a π-stacked pair that relates the duplexes within the crystal. The bulging out of the bipyridines causes bending of the PNA duplex, which is in contrast to the structure previously reported for biphenyl-modified DNA duplexes in solution, where the biphenyls are π-stacking with adjacent nucleobase pairs and adopt an intrahelical geometry [Johar et al., Chem. Eur. J., 2008, 14, 2080]. This difference shows that relatively small perturbations can significantly impact the relative position of nucleobase analogues in nucleic acid duplexes.
PMCID: PMC3194003  PMID: 20859960
PNA structure; X-ray crystallography; nucleic acids; bipyridine; nucleic acid bending
8.  Structural Characterizations of Glycerol Kinase: unraveling phosphorylation-induced long-range activation 
Biochemistry  2009;48(2):346-356.
Glycerol metabolism provides a central link between sugar and fatty acid catabolism. In most bacteria, glycerol kinase plays a crucial role in regulating channel/facilitator-independent uptake of glycerol into the cell. In the firmicute Enterococcus casseliflavus, this enzyme’s activity is enhanced by phosphorylation of the histidine residue (His232) located in its activation loop, approximately 25 Å away from its catalytic cleft. We reported earlier that some mutations of His232 altered enzyme activities; we present here the crystal structures of these mutant GlpK enzymes. The structure of a mutant enzyme with enhanced enzymatic activity, His232Arg, reveals that residues at the catalytic cleft are more optimally aligned to bind ATP and mediate phosphoryl transfer. Specifically, the position of Arg18 in His232Arg shifts by ~1 Å when compared to its position in WT, His232Ala, and His232Glu enzymes. This new conformation of Arg18 is more optimally positioned at the presumed γ-phosphate location of ATP, close to the glycerol substrate. In addition to structural changes exhibited at the active site, the conformational stability of the activation loop is decreased, as reflected by ~35% increase in B-factors (“thermal factors”) in a mutant enzyme displaying diminished activity, His232Glu. Correlating conformational changes to alteration of enzymatic activities in the mutant enzymes identifies distinct localized regions that can have profound effects on intramolecular signal transduction. Alterations of pairwise interactions across the dimer interface can communicate phosphorylation states over 25 Å from the activation loop to the catalytic cleft, positioning Arg18 to form favourable interactions at the β,γ-bridging position to ATP. This would offset loss of the hydrogen bonds at the γ-phosphate of ATP during phosphoryl transfer to glycerol, suggesting that appropriate alignment of the second substrate of glycerol kinase, the ATP molecule, may largely determine the rate of glycerol-3-phosphate production.
PMCID: PMC3158585  PMID: 19102629
9.  Crystal Structure of Chiral γ PNA with Complementary DNA Strand—Insights into the Stability and Specificity of Recognition and Conformational Preorganization 
Journal of the American Chemical Society  2010;132(31):10717-10727.
We have determined the structure of a PNA-DNA duplex to 1.7 Å resolution by multiple-wavelength anomalous diffraction on a zinc derivative. This structure represents the first high-resolution view of a hybrid duplex containing a contiguous chiral PNA strand with complete γ-backbone modification (“γPNA”). Unlike the achiral counterpart, which adopts a random-fold, this particular γPNA is already preorganized into a right-handed helix as a single strand. The new structure illustrates the unique characteristics of this modified PNA, possessing conformational flexibility while maintaining sufficient structural integrity to ultimately adopt the preferred P-helical conformation upon hybridization with DNA. The unusual structural adaptability found in the γPNA strand is crucial for enabling the accommodation of backbone modifications while constraining conformational states. In conjunction with NMR analysis characterizing the structures and substructures of the individual building blocks, these results provide unprecedented insights into how this new class of chiral γPNA is preorganized and stabilized, before and after hybridization with a complementary DNA strand. Such knowledge is crucial for the future design and development of PNA for applications in biology, biotechnology and medicine.
PMCID: PMC2929025  PMID: 20681704

Results 1-9 (9)