Guiqing Hu

Microstructure and formation mechanism of titanium dioxide nanotubes

Titanium dioxide nanotubes are synthesized using sol-gel method. Electron energy-loss spectroscopy has been used to investigate the chemical composition of the nanotubes. The results show that the atomic ratio of O/Ti is very close to two. Electron diffraction studies indicate that the nanotubes have the anatase structure. High-resolution transmission electron microscopy studies show that the nanotubes possess a layered structure with layer spacing of about 7.1 Angstrom. The tube axis is determined to be along [010] direction of the anatase phase. The formation mechanism of the nanotubes can be explained as 3D --> 2D --> 1D. The two-dimensional lamellar TiO2 is essential for the formation of the nanotubes

Recreation of the terminal events in physiological integrin activation

In vitro analysis confirms talin binding is sufficient for activation and extension of membrane-embedded integrin.

Structure of the Mycobacterium tuberculosis proteasome and mechanism of inhibition by a peptidyl boronate.

Mycobacterium tuberculosis (Mtb) has the remarkable ability to resist killing by human macrophages. The 750 kDa proteasome, not available in most eubacteria except Actinomycetes, appears to contribute to Mtbs resistance. The crystal structure of the Mtb proteasome at 3.0 Å resolution reveals a substrate‐binding pocket with composite features of the distinct β1, β2 and β5 substrate binding sites of eukaryotic proteasomes, accounting for the broad specificity of the Mtb proteasome towards oligopeptides described in the companion article [Lin et al. (2006), Mol Microbiol doi:10.1111/j.1365‐2958.2005.05035.x]. The substrate entrance at the end of the cylindrical proteasome appears open in the crystal structure due to partial disorder of the α‐subunit N‐terminal residues. However, cryo‐electron microscopy of the core particle reveals a closed end, compatible with the density observed in negative‐staining electron microscopy that depended on the presence of the N‐terminal octapetides of the α‐subunits in the companion article, suggesting that the Mtb proteasome has a gated structure. We determine for the first time the proteasomal inhibition mechanism of the dipeptidyl boronate N‐(4‐morpholine)carbonyl‐β‐(1‐naphthyl)‐l‐alanine‐l‐leucine boronic acid (MLN‐273), an analogue of the antimyeloma drug bortezomib. The structure improves prospects for designing Mtb‐specific proteasomal inhibitors as a novel approach to chemotherapy of tuberculosis.

Mycobacterium tuberculosis prcBA genes encode a gated proteasome with broad oligopeptide specificity

Genes predicted to be associated with the putative proteasome of Mycobacterium tuberculosis (Mtb) play a critical role in defence of the bacillus against nitrosative stress. However, proteasomes are uncommon in eubacteria and it remains to be established whether Mtbs prcBA genes in fact encode a proteasome. We found that coexpression of recombinant PrcB and PrcA in Escherichia coli over a prolonged period at 37°C allowed formation of an α7β7β7α7, 750 kDa cylindrical stack of four rings in which all 14 β‐subunits were proteolytically processed to expose the active site threonine. In contrast to another Actinomycete, Rhodococcus erythropolis, Mtbs β‐chain propeptide was not required for particle assembly. Peptidolytic activity of the 750 kDa particle towards a hydrophobic oligopeptide was nearly two orders of magnitude less than that of the Rhodococcus 20S proteasome, and unlike eukaryotic and archaeal proteasomes, activity of the Mtb 750 kDa particle could not be stimulated by SDS, Mg2+ or Ca2+. Electron microscopy revealed what appeared to be obstructed α‐rings in the Mtb 750 kDa particle. Deletion of the N‐terminal octapeptide from Mtbs α‐chain led to disappearance of the apparent obstruction and a marked increase of peptidolytic activity. Unlike proteasomes isolated from other Actinomycetes, the open‐gate Mtb mutant 750 kDa particle cleaved oligopeptides not only after hydrophobic residues but also after basic, acidic and small, neutral amino acids. Thus, Mtb encodes a broadly active, gated proteasome that may work in concert with an endogenous activator.

Structural Comparison of HIV-1 Envelope Spikes with and without the V1/V2 Loop

ABSTRACT We have used cryoelectron tomography of vitreous-ice-embedded HIV-1 virions to compare the envelope (Env) spikes of a wild-type strain with those of a mutant strain in which the V1/V2 loop has been deleted. Deletion of V1/V2 results in a spike with far more structural heterogeneity than is observed in the wild type, likely reflecting greatly enhanced gp120 protomer flexibility. A major difference between the two forms is a pronounced loss of mass from the “peak” of the native Env spike. The apparent loss of contact among three gp120 protomers likely accounts for the more open structure, heterogeneity in configuration, and previous observations that broadly neutralizing epitopes and reactive sites on other structural elements are more exposed in such constructs.

The growth morphologies of GaN layer on Si(111) substrate

The growth morphologies of metalorganic chemical vapor deposition (MOCVD) grown GaN layer on Si(111) substrate were studied using atomic force microscopy and transmission electron microscopy. It was found that the growth process of GaN/Si(111) consisted of two cycles of island growth and coalescence. These two cycles process differs markedly from that of one cycle process reported. The stress of evolving GaN layers on Si(111) was characterized by measuring the lattice constant c of GaN using X-ray diffraction (XRD) technique. It was proposed that the large tensile stress within the film during growth initiated this second island growth cycle, and the interaction between the GaN islands with high orientational fluctuation on the buffer layer induced this large tensile growth stress when coalescence occurred

Microstructure of GaN films grown on Si(111) substrates by metalorganic chemical vapor deposition

We report the transmission electron microscopy (TEM) study of the microstructure of wurtzitic GaN films grown on Si(I I I) substrates with AlN buffer layers by metalorganic chemical vapor deposition (MOCVD) method. An amorphous layer was formed at the interface between Si and AlN when thick GaN film was grown. We propose the amorphous layer was induced by the large stress at the interface when thick GaN was grown. The In0.1Ga0.9N/GaN multiple quantum well (MQW) reduced the dislocation density by obstructing the mixed and screw dislocations from passing through the MQW. But no evident reduction of the edge dislocations by the MQW was observed. It was found that dislocations located at the boundaries of grains slightly in-plane misoriented have screw component. Inversion domain is also observed

Polarity determination for GaN thin films by electron energy-loss spectroscopy

The intensity of the N K edge in electron energy-loss spectra from a GaN thin film shows a pronounced difference when the orientation of the film approaches the (0002) and (000-2) Bragg reflections, along the polar direction. This experimental result can be interpreted by the effect associated with interference between the Bloch waves of the incident electron in the GaN crystal. The theoretical calculations indicate that, at the Bragg condition of g=0002 along the Ga-N bond direction, the thickness-averaged electron current density on the N atom plane is much higher than that at g=000 (2) over bar, with a maximum as the specimen thickness is about 0.4xi(0002) (the two-beam extinction distance). The delocalization effect on the experimental spectra is also discussed

The Structure of a Full-length Membrane-embedded Integrin Bound to a Physiological Ligand

Background: Integrins undergo large conformational changes when ligand-bound. Results: Using nanodisc technology and EM, we observed Mn2+-activated αIIbβ3 integrin both alone and fibrin-bound. Conclusion: MnCl2-activated αIIbβ3 integrin alone has a compact conformation, becoming fully upright and open when fibrin-bound. Significance: The first structure of membrane-embedded integrins bound to physiological substrate reveals the importance of integrin extension in macromolecular ligand binding. Increased ligand binding to integrin (“activation”) underpins many biological processes, such as leukocyte trafficking, cell migration, host-pathogen interaction, and hemostasis. Integrins exist in several conformations, ranging from compact and bent to extended and open. However, the exact conformation of membrane-embedded, full-length integrin bound to its physiological macromolecular ligand is still unclear. Integrin αIIbβ3, the most abundant integrin in platelets, has been a prototype for integrin activation studies. Using negative stain electron microscopy and nanodisc-embedding to provide a membrane-like environment, we visualized the conformation of full-length αIIbβ3 in both a Mn2+-activated, ligand-free state and a Mn2+-activated, fibrin-bound state. Activated but ligand-free integrins exist mainly in the compact conformation, whereas fibrin-bound αIIbβ3 predominantly exists in a fully extended, headpiece open conformation. Our results show that membrane-embedded, full-length integrin adopts an extended and open conformation when bound to its physiological macromolecular ligand.

Formation mechanism of amorphous layer at the interface of Si(111) substrate and AlN buffer layer for GaN

chinese acad sci, inst phys, beijing lab electron microscopy, beijing 100080, peoples r china; chinese acad sci, inst semicond, lab semicond mat sci, beijing 100083, peoples r china