Crystal structure and mechanism of a calcium-gated potassium channel

Ion channels exhibit two essential biophysical properties; that is, selective ion conduction, and the ability to gate-open in response to an appropriate stimulus. Two general categories of ion channel gating are defined by the initiating stimulus: ligand binding (neurotransmitter- or second-messenger-gated channels) or membrane voltage (voltage-gated channels). Here we present the structural basis of ligand gating in a K(+) channel that opens in response to intracellular Ca(2+). We have cloned, expressed, analysed electrical properties, and determined the crystal structure of a K(+) channel (MthK) from Methanobacterium thermoautotrophicum in the Ca(2+)-bound, opened state. Eight RCK domains (regulators of K(+) conductance) form a gating ring at the intracellular membrane surface. The gating ring uses the free energy of Ca(2+) binding in a simple manner to perform mechanical work to open the pore.     

The open pore conformation of potassium channels

Living cells regulate the activity of their ion channels through a process known as gating. To open the pore, protein conformational changes must occur within a channel's membrane-spanning ion pathway. KcsA and MthK, closed and opened K(+) channels, respectively, reveal how such gating transitions occur. Pore-lining 'inner' helices contain a 'gating hinge' that bends by approximately 30 degrees. In a straight conformation four inner helices form a bundle, closing the pore near its intracellular surface. In a bent configuration the inner helices splay open creating a wide (12 A) entryway. Amino-acid sequence conservation suggests a common structural basis for gating in a wide range of K(+) channels, both ligand- and voltage-gated. The open conformation favours high conduction by compressing the membrane field to the selectivity filter, and also permits large organic cations and inactivation peptides to enter the pore from the intracellular solution.     

The Wnt/calcium pathway activates NF-AT and promotes ventral cell fate in Xenopus embryos

It is thought that inositol-1,4,5-trisphosphate (Ins(1,4,5)P(3))-Ca(2+) signalling has a function in dorsoventral axis formation in Xenopus embryos; however, the immediate target of free Ca(2+) is unclear. The secreted Wnt protein family comprises two functional groups, the canonical Wnt and Wnt/Ca(2+) pathways. The Wnt/Ca(2+) pathway interferes with the canonical Wnt pathway, but the underlying molecular mechanism is poorly understood. Here, we cloned the complementary DNA coding for the Xenopus homologue of nuclear factor of activated T cells (XNF-AT). A gain-of-function, calcineurin-independent active XNF-AT mutation (CA XNF-AT) inhibited anterior development of the primary axis, as well as Xwnt-8-induced ectopic dorsal axis development in embryos. A loss-of-function, dominant negative XNF-AT mutation (DN XNF-AT) induced ectopic dorsal axis formation and expression of the canonical Wnt signalling target molecules siamois and Xnr3 (ref. 4). Xwnt-5A induced translocation of XNF-AT from the cytosol to the nucleus. These data indicate that XNF-AT functions as a downstream target of the Wnt/Ca(2+) and Ins(1,4,5)P(3)-Ca(2+) pathways, and has an essential role in mediating ventral signals in the Xenopus embryo through suppression of the canonical Wnt pathway.     

Self-regeneration of a Pd-perovskite catalyst for automotive emissions control

Catalytic converters are widely used to reduce the amounts of nitrogen oxides, carbon monoxide and unburned hydrocarbons in automotive emissions. The catalysts are finely divided precious-metal particles dispersed on a solid support. During vehicle use, the converter is exposed to heat, which causes the metal particles to agglomerate and grow, and their overall surface area to decrease. As a result, catalyst activity deteriorates. The problem has been exacerbated in recent years by the trend to install catalytic converters closer to the engine, which ensures immediate activation of the catalyst on engine start-up, but also places demanding requirements on the catalyst's heat resistance. Conventional catalyst systems thus incorporate a sufficient excess of precious metal to guarantee continuous catalytic activity for vehicle use over 50,000 miles (80,000 km). Here we use X-ray diffraction and absorption to show that LaFe(0.57)Co(0.38)Pd(0.05)O(3), one of the perovskite-based catalysts investigated for catalytic converter applications since the early 1970s, retains its high metal dispersion owing to structural responses to the fluctuations in exhaust-gas composition that occur in state-of-the-art petrol engines. We find that as the catalyst is cycled between oxidative and reductive atmospheres typically encountered in exhaust gas, palladium (Pd) reversibly moves into and out of the perovskite lattice. This movement appears to suppress the growth of metallic Pd particles, and hence explains the retention of high catalyst activity during long-term use and ageing.     

Indium phosphide nanowires as building blocks for nanoscale electronic and optoelectronic devices

Nanowires and nanotubes carry charge and excitons efficiently, and are therefore potentially ideal building blocks for nanoscale electronics and optoelectronics. Carbon nanotubes have already been exploited in devices such as field-effect and single-electron transistors, but the practical utility of nanotube components for building electronic circuits is limited, as it is not yet possible to selectively grow semiconducting or metallic nanotubes. Here we report the assembly of functional nanoscale devices from indium phosphide nanowires, the electrical properties of which are controlled by selective doping. Gate-voltage-dependent transport measurements demonstrate that the nanowires can be predictably synthesized as either n- or p-type. These doped nanowires function as nanoscale field-effect transistors, and can be assembled into crossed-wire p-n junctions that exhibit rectifying behaviour. Significantly, the p-n junctions emit light strongly and are perhaps the smallest light-emitting diodes that have yet been made. Finally, we show that electric-field-directed assembly can be used to create highly integrated device arrays from nanowire building blocks.     

The lipid phosphatase SHIP2 controls insulin sensitivity

Insulin is the primary hormone involved in glucose homeostasis, and impairment of insulin action and/or secretion has a critical role in the pathogenesis of diabetes mellitus. Type-II SH2-domain-containing inositol 5-phosphatase, or 'SHIP2', is a member of the inositol polyphosphate 5-phosphatase family. In vitro studies have shown that SHIP2, in response to stimulation by numerous growth factors and insulin, is closely linked to signalling events mediated by both phosphoinositide-3-OH kinase and Ras/mitogen-activated protein kinase. Here we report the generation of mice lacking the SHIP2 gene. Loss of SHIP2 leads to increased sensitivity to insulin, which is characterized by severe neonatal hypoglycaemia, deregulated expression of the genes involved in gluconeogenesis, and perinatal death. Adult mice that are heterozygous for the SHIP2 mutation have increased glucose tolerance and insulin sensitivity associated with an increased recruitment of the GLUT4 glucose transporter and increased glycogen synthesis in skeletal muscles. Our results show that SHIP2 is a potent negative regulator of insulin signalling and insulin sensitivity in vivo.     

T-antigen regulated expression reduces apoptosis of Tag-transformed human myoblasts

The generation of human myogenic cell lines could potentially provide a valuable source for cell transplantation in myopathies. The dysregulation of proliferative-differentiative signals by viral oncogenes can result in the induction of apoptosis. Whether apoptosis occurred in myogenic cells expressing large T antigen (Tag) from SV40 upon differentiation was unknown. Human muscle satellite cells were transfected with two different constructs, containing either an origin-defective SV40 genome or Tag under vimentin promoter control. When differentiation was triggered, Tag expression reduced the formation of myotubes and dead cells showing apoptotic features were present. However, the cells expressing SV40 Tag under vimentin promoter control retained their capacity to form myotubes and expressed the myofibrillar proteins as myosin heavy chain and dystrophin when Tag expression was silent. Their apoptotic rate was similar to that of untransfected cells. The observation that apoptosis can be prevented by the down-regulation of Tag suggests that the programmed cell death induced in transformed cells can be reversed, and confirms the regulatory efficiency of the human vimentin promoter.     

Role of PHABULOSA and PHAVOLUTA in determining radial patterning in shoots

The upper side of the angiosperm leaf is specialized for efficient capture of sunlight whereas the lower side is specialized for gas exchange. In Arabidopsis, the establishment of polarity in the leaf probably requires the generation and perception of positional information along the radial (adaxial versus abaxial or central versus peripheral) dimension of the plant. This is because the future upper (adaxial) side of the leaf develops from cells closer to the centre of the shoot, whereas the future under (abaxial) side develops from cells located more peripherally. Here we implicate the Arabidopsis PHABULOSA and PHAVOLUTA genes in the perception of radial positional information in the leaf primordium. Dominant phabulosa (phb) and phavoluta (phv) mutations cause a dramatic transformation of abaxial leaf fates into adaxial leaf fates. They do so by altering the predicted sterol/lipid-binding domains of ATHB14 and ATHB9, proteins of previously unknown function that also contain DNA-binding motifs. This change probably renders the protein constitutively active, implicating this domain as a central regulator of protein function and the PHB and PHV proteins as receptors for an adaxializing signal.