Single-crystal gallium nitride nanotubes

Since the discovery of carbon nanotubes in 1991 (ref. 1), there have been significant research efforts to synthesize nanometre-scale tubular forms of various solids. The formation of tubular nanostructure generally requires a layered or anisotropic crystal structure. There are reports of nanotubes made from silica, alumina, silicon and metals that do not have a layered crystal structure; they are synthesized by using carbon nanotubes and porous membranes as templates, or by thin-film rolling. These nanotubes, however, are either amorphous, polycrystalline or exist only in ultrahigh vacuum. The growth of single-crystal semiconductor hollow nanotubes would be advantageous in potential nanoscale electronics, optoelectronics and biochemical-sensing applications. Here we report an 'epitaxial casting' approach for the synthesis of single-crystal GaN nanotubes with inner diameters of 30-200 nm and wall thicknesses of 5-50 nm. Hexagonal ZnO nanowires were used as templates for the epitaxial overgrowth of thin GaN layers in a chemical vapour deposition system. The ZnO nanowire templates were subsequently removed by thermal reduction and evaporation, resulting in ordered arrays of GaN nanotubes on the substrates. This templating process should be applicable to many other semiconductor systems.     

Functional genomics in the rice blast fungus to unravel the fungal pathogenicity

A rapidly growing number of successful genome sequencing projects in plant pathogenic fungi greatly increase the demands for tools and methodologies to study fungal pathogenicity at genomic scale. Magnaporthe oryzae is an economically important plant pathogenic fungus whose genome is fully sequenced. Recently we have reported the development and application of functional genomics platform technologies in M. oryzae. This model approach would have many practical ramifications in design and implementation of upcoming functional genomics studies of filamentous fungi aimed at understanding fungal pathogenicity.     

A terahertz metamaterial with unnaturally high refractive index

Controlling the electromagnetic properties of materials, going beyond the limit that is attainable with naturally existing substances, has become a reality with the advent of metamaterials. The range of various structured artificial 'atoms' has promised a vast variety of otherwise unexpected physical phenomena, among which the experimental realization of a negative refractive index has been one of the main foci thus far. Expanding the refractive index into a high positive regime will complete the spectrum of achievable refractive index and provide more design flexibility for transformation optics. Naturally existing transparent materials possess small positive indices of refraction, except for a few semiconductors and insulators, such as lead sulphide or strontium titanate, that exhibit a rather high peak refractive index at mid- and far-infrared frequencies. Previous approaches using metamaterials were not successful in realizing broadband high refractive indices. A broadband high-refractive-index metamaterial structure was theoretically investigated only recently, but the proposed structure does not lend itself to easy implementation. Here we demonstrate that a broadband, extremely high index of refraction can be realized from large-area, free-standing, flexible terahertz metamaterials composed of strongly coupled unit cells. By drastically increasing the effective permittivity through strong capacitive coupling and decreasing the diamagnetic response with a thin metallic structure in the unit cell, a peak refractive index of 38.6 along with a low-frequency quasi-static value of over 20 were experimentally realized for a single-layer terahertz metamaterial, while maintaining low losses. As a natural extension of these single-layer metamaterials, we fabricated quasi-three-dimensional high-refractive-index metamaterials, and obtained a maximum bulk refractive index of 33.2 along with a value of around 8 at the quasi-static limit.     

Drosophila Crumbs is a positional cue in photoreceptor adherens junctions and rhabdomeres

Drosophila Crumbs (Crb) is required for apical-basal polarity and is an apical determinant in embryonic epithelia. Here, we describe properties of Crb that control the position and integrity of the photoreceptor adherens junction and photosensitive organ, or rhabdomere. In contrast to normal photoreceptor adherens junctions and rhabdomeres, which span the depth of the retina, adherens junctions and rhabdomeres of Crb-deficient photoreceptors initially accumulate at the top of the retina and fail to maintain their integrity as they stretch to the retinal floor. We show that Crb controls localization of the adherens junction through its intracellular domain containing a putative binding site for a protein 4.1 superfamily protein (FERM). Although loss of Crb or overexpression of the FERM binding domain causes mislocalization of adherens junctions, they do not result in a significant loss of photoreceptor polarity. Mutations in CRB1, a human homologue of crb, are associated with photoreceptor degeneration in retinitis pigmentosa 12 (RP12) and Leber congenital amaurosis (LCA). The intracellular domain of CRB1 behaves similarly to its Drosophila counterpart when overexpressed in the fly eye. Our studies may provide clues for mechanisms of photoreceptor degeneration in RP12 and LCA.     

The origin of the anomalous superconducting properties of MgB(2)

Magnesium diboride differs from ordinary metallic superconductors in several important ways, including the failure of conventional models to predict accurately its unusually high transition temperature, the effects of isotope substitution on the critical transition temperature, and its anomalous specific heat. A detailed examination of the energy associated with the formation of charge-carrying pairs, referred to as the 'superconducting energy gap', should clarify why MgB(2) is different. Some early experimental studies have indicated that MgB(2) has multiple gaps, but past theoretical studies have not explained from first principles the origin of these gaps and their effects. Here we report an ab initio calculation of the superconducting gaps in MgB(2) and their effects on measurable quantities. An important feature is that the electronic states dominated by orbitals in the boron plane couple strongly to specific phonon modes, making pair formation favourable. This explains the high transition temperature, the anomalous structure in the specific heat, and the existence of multiple gaps in this material. Our analysis suggests comparable or higher transition temperatures may result in layered materials based on B, C and N with partially filled planar orbitals.     

Drosophila TCTP is essential for growth and proliferation through regulation of dRheb GTPase

Cellular growth and proliferation are coordinated during organogenesis. Misregulation of these processes leads to pathological conditions such as cancer. Tuberous sclerosis (TSC) is a benign tumour syndrome caused by mutations in either TSC1 or TSC2 tumour suppressor genes. Studies in Drosophila and other organisms have identified TSC signalling as a conserved pathway for growth control. Activation of the TSC pathway is mediated by Rheb (Ras homologue enriched in brain), a Ras superfamily GTPase. Rheb is a direct target of TSC2 and is negatively regulated by its GTPase-activating protein activity. However, molecules required for positive regulation of Rheb have not been identified. Here we show that a conserved protein, translationally controlled tumour protein (TCTP), is an essential new component of the TSC-Rheb pathway. Reducing Drosophila TCTP (dTCTP) levels reduces cell size, cell number and organ size, which mimics Drosophila Rheb (dRheb) mutant phenotypes. dTCTP is genetically epistatic to Tsc1 and dRheb, but acts upstream of dS6k, a downstream target of dRheb. dTCTP directly associates with dRheb and displays guanine nucleotide exchange activity with it in vivo and in vitro. Human TCTP (hTCTP) shows similar biochemical properties compared to dTCTP and can rescue dTCTP mutant phenotypes, suggesting that the function of TCTP in the TSC pathway is evolutionarily conserved. Our studies identify TCTP as a direct regulator of Rheb and a potential therapeutic target for TSC disease.     

Torsional Models of Two - ply Yarn

In this paper,the vector equation of a generaliseddoubly wound helix was derived.Treloar's pliedyarn geometry could be obtained as a special case ofthe generalised doubly wound helix.The shortest fi-bre length around the surface of a helical tube(formed by fibre helices)was determined by apply-ing variational principles.The fibre length as calcu-lated by using Treloar's geometry was compared in-directly with the shortest possible fibre length at dif-ferent levels of yarn deformation when some of Tre-loar's rigid geometrical constraints were relaxed.Anew idea based on non-concentric circles was intro-duced to approximate the fibre helix movement in atwo-ply yarn.A torsional model of two-ply yarnwas developed and the theoretical predictions werecompared with some preliminary experimental re-sults.     

Residues of the variable region of the T-cell-receptor beta-chain that interact with S. aureus toxin superantigens

The alpha beta T-cell antigen receptor (TCR) recognizes antigenic peptides in the context of self major histocompatibility complex (MHC) molecules. The specificity of recognition of MHC plus antigen is generally determined by a combination of the variable elements of alpha- and beta-chains of the TCR. Several types of antigen, however, have been identified that, when bound to MHC molecules, stimulate T cells bearing particular variable-region beta-chain (V beta) elements irrespective of the other variable components of the TCR. These have been termed 'superantigens', and here we are concerned with one type of superantigen, the toxins produced by Staphylococcus aureus. T cells have been found that bear closely related members of the same V beta family but respond differently to S. aureus toxins; in particular, cells bearing the human V beta 13.2 element respond to toxin SEC2, whereas cells bearing human V beta 13.1 do not. We have now defined the residues of the V beta element responsible for this difference, and find that they reside in a region thought to lie on the side of the TCR molecule, away from the conventional antigen/MHC-binding site. The evolutionary conservation of this site may be due to its having an important role in some function of the TCR other than the binding of conventional antigen plus MHC.     

Isolation of human epidermal stem cells by adherence and the reconstruction of skin equivalents

The isolation of human epidermal stem cells is critical for their clinical applications. In the present study, we isolated three populations of epidermal keratinocytes according to their ability to adhere to collagen type IV: i.e., rapidly adhering (RA), slowly adhering (SA), and non-adhering (NA) cells. The aim of this study was to characterize RA cells and to investigate the possibility of using these cells for epidermis reconstruction. To identify RA cells, flow cytometric analysis was performed using anti-₆ integrin and anti-CD71 antibodies. RA cells express high levels of ₆ integrin and low levels of CD71, which are considered as markers of an epidermal stem cell nature. Furthermore, electron microscopy showed that RA cells are small and have a high nuclear to cytoplasmic ratio, whereas SA and NA cells have well-developed cellular organelles and abundant tonofilaments. Western blot analysis showed that RA cells are slow cycling and express p63, a putative epidermal stem cell marker, whereas SA and NA cells express c-Myc, which is known to regulate stem cell fate. To compare epidermal regenerative abilities, skin equivalents (SEs) were made using RA, SA, and NA cells. The epidermis constructed from RA cells was well formed compared to those formed from SA or NA cells. In addition, only SEs with RA cells expressed ₆ integrin and ₁ integrin at the basal layer. These results indicate that RA cells represent epidermal stem cells and are predominately comprised of stem cells. Therefore, the isolation of RA cells using a simple technique offers a potential route to their clinical application, because they are easily isolated and provide a high yield of epidermal stem cells.Received 2 July 2004; received after revision 20 August 2004; accepted 10 September 2004