Structure of astacin and implications for activation of astacins and zinc-ligation of collagenases.

Astacin, a digestive zinc-endopeptidase from the crayfish Astacus astacus L., is the prototype for the 'astacin family', which includes mammalian metallo-endopeptidases and developmentally regulated proteins of man, fruitfly, frog and sea urchin. Here we report the X-ray crystal structure of astacin, which reveals a deep active-site cleft, with the zinc at its bottom ligated by three histidines, a water molecule and a more remote tyrosine. The third histidine (His 102) forms part of a consensus sequence, shared not only by the members of the astacin family, but also by otherwise sequentially unrelated proteinases, such as vertebrate collagenases. It may therefore represent the elusive 'third' zinc ligand in these enzymes. The amino terminus of astacin is buried forming an internal salt-bridge with Glu 103, adjacent to His 102. Astacin pro-forms extended at the N terminus, as observed for some 'latent' mammalian astacin homologues, did not exhibit this 'active' conformation, indicating an activation mechanism reminiscent of trypsin-like serine proteinases.     

Chiral magnetic order at surfaces driven by inversion asymmetry

Chirality is a fascinating phenomenon that can manifest itself in subtle ways, for example in biochemistry (in the observed single-handedness of biomolecules) and in particle physics (in the charge-parity violation of electroweak interactions). In condensed matter, magnetic materials can also display single-handed, or homochiral, spin structures. This may be caused by the Dzyaloshinskii-Moriya interaction, which arises from spin-orbit scattering of electrons in an inversion-asymmetric crystal field. This effect is typically irrelevant in bulk metals as their crystals are inversion symmetric. However, low-dimensional systems lack structural inversion symmetry, so that homochiral spin structures may occur. Here we report the observation of magnetic order of a specific chirality in a single atomic layer of manganese on a tungsten (110) substrate. Spin-polarized scanning tunnelling microscopy reveals that adjacent spins are not perfectly antiferromagnetic but slightly canted, resulting in a spin spiral structure with a period of about 12 nm. We show by quantitative theory that this chiral order is caused by the Dzyaloshinskii-Moriya interaction and leads to a left-rotating spin cycloid. Our findings confirm the significance of this interaction for magnets in reduced dimensions. Chirality in nanoscale magnets may play a crucial role in spintronic devices, where the spin rather than the charge of an electron is used for data transmission and manipulation. For instance, a spin-polarized current flowing through chiral magnetic structures will exert a spin-torque on the magnetic structure, causing a variety of excitations or manipulations of the magnetization and giving rise to microwave emission, magnetization switching, or magnetic motors.     

Prioritizing global conservation efforts

One of the most pressing issues facing the global conservation community is how to distribute limited resources between regions identified as priorities for biodiversity conservation. Approaches such as biodiversity hotspots, endemic bird areas and ecoregions are used by international organizations to prioritize conservation efforts globally. Although identifying priority regions is an important first step in solving this problem, it does not indicate how limited resources should be allocated between regions. Here we formulate how to allocate optimally conservation resources between regions identified as priorities for conservation--the 'conservation resource allocation problem'. Stochastic dynamic programming is used to find the optimal schedule of resource allocation for small problems but is intractable for large problems owing to the "curse of dimensionality". We identify two easy-to-use and easy-to-interpret heuristics that closely approximate the optimal solution. We also show the importance of both correctly formulating the problem and using information on how investment returns change through time. Our conservation resource allocation approach can be applied at any spatial scale. We demonstrate the approach with an example of optimal resource allocation among five priority regions in Wallacea and Sundaland, the transition zone between Asia and Australasia.     

The staphylocoagulase family of zymogen activator and adhesion proteins

Staphylocoagulase (SC) secreted by Staphylococcus aureus is a potent non-proteolytic activator of the blood coagulation zymogen prothrombin and the prototype of a newly established zymogen activator and adhesion protein (ZAAP) family. The conformationally activated SC·prothrombin complex specifically cleaves fibrinogen to fibrin, which propagates the growth of bacteria-fibrin-platelet vegetations in acute bacterial endocarditis. Our recent 2.2 Å X-ray crystal structures of an active SC fragment [SC(1-325)] bound to the prothrombin zymogen catalytic domain, prethrombin 2, demonstrated that SC(1-325) represents a new type of non-proteolytic activator with a unique fold. The observed insertion of the SC(1-325) N-terminus into the Ile 16 cleft of prethrombin 2, which triggers the activating conformational change, provided the first unambiguous structural evidence for the molecular sexuality mechanism of non-proteolytic zymogen activation. Based on the SC(1-325) fold, a new family of bifunctional zymogen activator and adhesion proteins was identified that possess N-terminal domains homologous to SC(1-325) and C-terminal domains that mediate adhesion to plasma or extracellular matrix proteins. Further investigation of the ZAAP family may lead to new insights into the mechanisms of bacterial factors that hijack zymogens of the human blood coagulation and fibrinolytic systems to promote and disseminate endocarditis and other infectious diseases.Received 30 June 2004; received after revision 28 July 2004; accepted 4 August 2004     

Structural properties of matrix metalloproteinases

Matrix metalloproteinases (MMPs) are involved in extracellular matrix degradation. Their proteolytic activity must be precisely regulated by their endogenous protein inhibitors, the tissue inhibitors of metalloproteinases (TIMPs). Disruption of this balance results in serious diseases such as arthritis, tumour growth and metastasis. Knowledge of the tertiary structures of the proteins involved is crucial for understanding their functional properties and interference with associated dysfunctions. Within the last few years, several three-dimensional MMP and MMP-TIMP structures became available, showing the domain organization, polypeptide fold and main specificity determinants. Complexes of the catalytic MMP domains with various synthetic inhibitors enabled the structure-based design and improvement of high-affinity ligands, which might be elaborated into drugs. A multitude of reviews surveying work done on all aspects of MMPs have appeared in recent years, but none of them has focused on the three-dimensional structures. This review was written to close the gap. Received 18 November 1998; accepted 11 December 1998     

Interaction of glutamic-acid-rich proteins with the cGMP signalling pathway in rod photoreceptors.

The assembly of signalling molecules into macromolecular complexes (transducisomes) provides specificity, sensitivity and speed in intracellular signalling pathways. Rod photoreceptors in the eye contain an unusual set of glutamic-acid-rich proteins (GARPs) of unknown function. GARPs exist as two soluble forms, GARP1 and GARP2, and as a large cytoplasmic domain (GARP' part) of the beta-subunit of the cyclic GMP-gated channel. Here we identify GARPs as multivalent proteins that interact with the key players of cGMP signalling, phosphodiesterase and guanylate cyclase, and with a retina-specific ATP-binding cassette transporter (ABCR), through four, short, repetitive sequences. In electron micrographs, GARPs are restricted to the rim region and incisures of discs in close proximity to the guanylate cyclase and ABCR, whereas the phosphodiesterase is randomly distributed. GARP2, the most abundant splice form, associates more strongly with light-activated than with inactive phosphodiesterase, and GARP2 potently inhibits phosphodiesterase activity. Thus, the GARPs organize a dynamic protein complex near the disc rim that may control cGMP turnover and possibly other light-dependent processes. Because there are no similar GARPs in cones, we propose that GARPs may prevent unnecessary cGMP turnover during daylight, when rods are held in saturation by the relatively high light levels.     

A discoidin domain receptor 1 knock-out mouse as a novel model for osteoarthritis of the temporomandibular joint

Discoidin domain receptor 1 (DDR-1)-deficient mice exhibited a high incidence of osteoarthritis (OA) in the temporomandibular joint (TMJ) as early as 9 weeks of age. They showed typical histological signs of OA, including surface fissures, loss of proteoglycans, chondrocyte cluster formation, collagen type I upregulation, and atypical collagen fibril arrangements. Chondrocytes isolated from the TMJs of DDR-1-deficient mice maintained their osteoarthritic characteristics when placed in culture. They expressed high levels of runx-2 and collagen type I, as well as low levels of sox-9 and aggrecan. The expression of DDR-2, a key factor in OA, was increased. DDR-1-deficient chondrocytes from the TMJ were positively influenced towards chondrogenesis by a three-dimensional matrix combined with a runx-2 knockdown or stimulation with extracellular matrix components, such as nidogen-2. Therefore, the DDR-1 knock-out mouse can serve as a novel model for temporomandibular disorders, such as OA of the TMJ, and will help to develop new treatment options, particularly those involving tissue regeneration.     

An asymmetric shock wave in the 2006 outburst of the recurrent nova RS Ophiuchi

Nova outbursts take place in binary star systems comprising a white dwarf and either a low-mass Sun-like star or, as in the case of the recurrent nova RS Ophiuchi, a red giant. Although the cause of these outbursts is known to be thermonuclear explosion of matter transferred from the companion onto the surface of the white dwarf, models of the previous (1985) outburst of RS Ophiuchi failed to adequately fit the X-ray evolution and there was controversy over a single-epoch high-resolution radio image, which suggested that the remnant was bipolar rather than spherical as modelled. Here we report the detection of spatially resolved structure in RS Ophiuchi from two weeks after its 12 February 2006 outburst. We track an expanding shock wave as it sweeps through the red giant wind, producing a remnant similar to that of a type II supernova but evolving over months rather than millennia. As in supernova remnants, the radio emission is non-thermal (synchrotron emission), but asymmetries and multiple emission components clearly demonstrate that contrary to the assumptions of spherical symmetry in models of the 1985 explosion, the ejection is jet-like, collimated by the central binary whose orientation on the sky can be determined from these observations.     

裂解急冷油的热稳定性研究

在急冷循环过程中，裂解急冷油的粘度逐渐升高，往往给乙烯装置正常运行带来严重影响．以齐鲁石化公司烯烃厂乙烯装置急冷系统为对象进行了急冷油的热稳定性研究．认为急冷油粘度升高的内在原因是急冷油中的芳香分在急冷过程中发生缩合反应，不断生成胶质及沥青质．鉴于急冷油中芳香分的基本来源是裂解原料中的芳烃，因此应尽可能采用芳烃含量少的裂解原料．     

Crystal structures of the membrane-binding C2 domain of human coagulation factor V

Rapid and controlled clot formation is achieved through sequential activation of circulating serine proteinase precursors on phosphatidylserine-rich procoagulant membranes of activated platelets and endothelial cells. The homologous complexes Xase and prothrombinase, each consisting of an active proteinase and a non-enzymatic cofactor, perform critical steps within this coagulation cascade. The activated cofactors VIIIa and Va, highly specific for their cognate proteinases, are each derived from precursors with the same A1-A2-B-A3-C1-C2 architecture. Membrane binding is mediated by the C2 domains of both cofactors. Here we report two crystal structures of the C2 domain of human factor Va. The conserved beta-barrel framework provides a scaffold for three protruding loops, one of which adopts markedly different conformations in the two crystal forms. We propose a mechanism of calcium-independent, stereospecific binding of factors Va and VIIIa to phospholipid membranes, on the basis of (1) immersion of hydrophobic residues at the apices of these loops in the apolar membrane core; (2) specific interactions with phosphatidylserine head groups in the groove enclosed by these loops; and (3) favourable electrostatic contacts of basic side chains with negatively charged membrane phosphate groups.