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Search Results: 1 - 10 of 44868 matches for " Michael Overduin "
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Hinged Plakin Domains Provide Specialized Degrees of Articulation in Envoplakin, Periplakin and Desmoplakin
Caezar Al-Jassar, Pau Bernad?, Martyn Chidgey, Michael Overduin
PLOS ONE , 2013, DOI: 10.1371/journal.pone.0069767
Abstract: Envoplakin, periplakin and desmoplakin are cytoskeletal proteins that provide structural integrity within the skin and heart by resisting shear forces. Here we reveal the nature of unique hinges within their plakin domains that provides divergent degrees of flexibility between rigid long and short arms composed of spectrin repeats. The range of mobility of the two arms about the hinge is revealed by applying the ensemble optimization method to small-angle X-ray scattering data. Envoplakin and periplakin adopt ‘L’ shaped conformations exhibiting a ‘helicopter propeller’-like mobility about the hinge. By contrast desmoplakin exhibits essentially unrestricted mobility by ‘jack-knifing’ about the hinge. Thus the diversity of molecular jointing that can occur about plakin hinges includes ‘L’ shaped bends, ‘U’ turns and fully extended ‘I’ orientations between rigid blocks of spectrin repeats. This establishes specialised hinges in plakin domains as a key source of flexibility that may allow sweeping of cellular spaces during assembly of cellular structures and could impart adaptability, so preventing irreversible damage to desmosomes and the cell cytoskeleton upon exposure to mechanical stress.
Solar System Tests of Higher-Dimensional Gravity
Hongya Liu,James Overduin
Physics , 2000, DOI: 10.1086/309115
Abstract: The classical tests of general relativity - light deflection, time delay and perihelion shift - are applied, along with the geodetic precession test, to the five-dimensional extension of the theory known as Kaluza-Klein gravity, using an analogue of the four-dimensional Schwarzschild metric. The perihelion advance and geodetic precession calculations are generalized for the first time to situations in which the components of momentum and spin along the extra coordinate do not vanish. Existing data on light- bending around the Sun using long- baseline radio interferometry, ranging to Mars using the Viking lander, and the perihelion precession of Mercury all constrain a small parameter b associated with the extra part of the metric to be less than |b| < 0.07 in the solar system. An order-of-magnitude increase in sensitivity is possible from perihelion precession, if better limits on solar oblateness become available. Measurement of geodetic precession by the Gravity Probe B satellite will improve this significantly, probing values of b with an accuracy of one part in 10^4 or more.
Solar System Tests of the Equivalence Principle and Constraints on Higher-Dimensional Gravity
J. M. Overduin
Physics , 2000, DOI: 10.1103/PhysRevD.62.102001
Abstract: In most studies of equivalence principle violation by solar system bodies, it is assumed that the ratio of gravitational to inertial mass for a given body deviates from unity by a parameter Delta which is proportional to its gravitational self-energy. Here we inquire what experimental constraints can be set on Delta for various solar system objects when this assumption is relaxed. Extending an analysis originally due to Nordtvedt, we obtain upper limits on linearly independent combinations of Delta for two or more bodies from Kepler's third law, the position of Lagrange libration points, and the phenomenon of orbital polarization. Combining our results, we extract numerical upper bounds on Delta for the Sun, Moon, Earth and Jupiter, using observational data on their orbits as well as those of the Trojan asteroids. These are applied as a test case to the theory of higher-dimensional (Kaluza-Klein) gravity. The results are three to six orders of magnitude stronger than previous constraints on the theory, confirming earlier suggestions that extra dimensions play a negligible role in solar systemdynamics and reinforcing the value of equivalence principle tests as a probe of nonstandard gravitational theories.
How dominant is the vacuum?
James Overduin,Wolfgang Priester
Physics , 2001, DOI: 10.1007/s001140100219
Abstract: It would be hard to find a cosmologist today who does not believe that the vast bulk of the Universe (ninety-five percent or more) is hidden from our eyes. We review the evidence for this remarkable consensus, and for the latest proposal, that the mysterious dark matter consists of as many as {\em four separate ingredients}: baryons, massive neutrinos, ``exotic'' dark matter particles, and vacuum energy, also known as the cosmological constant (Lambda). Of these, only baryons fit within standard theoretical physics; the others, if their existence is confirmed, will mean rewriting textbooks. New experimental evidence has recently appeared for and against all four components, so that the subject is in a state of turmoil and excitement. The past three years in particular have seen the fourth (vacuum) component come into new prominence, largely at the expense of the third (exotic dark matter). We conclude our review by exploring the possibility that the energy density of the vacuum is in fact so dominant as to leave little room for significant amounts of exotic dark matter.
Key Amino Acid Residues of Ankyrin-Sensitive Phosphatidylethanolamine/Phosphatidylcholine-Lipid Binding Site of βI-Spectrin
Marcin Wolny, Micha? Grzybek, Ewa Bok, Anna Chorzalska, Marc Lenoir, Aleksander Czogalla, Klaudia Adamczyk, Adam Kolondra, Witold Diakowski, Michael Overduin, Aleksander F. Sikorski
PLOS ONE , 2011, DOI: 10.1371/journal.pone.0021538
Abstract: It was shown previously that an ankyrin-sensitive, phosphatidylethanolamine/phosphatidylcho?line(PE/PC) binding site maps to the N-terminal part of the ankyrin-binding domain of β-spectrin (ankBDn). Here we have identified the amino acid residues within this domain which are responsible for recognizing monolayers and bilayers composed of PE/PC mixtures. In vitro binding studies revealed that a quadruple mutant with substituted hydrophobic residues W1771, L1775, M1778 and W1779 not only failed to effectively bind PE/PC, but its residual PE/PC-binding activity was insensitive to inhibition with ankyrin. Structure prediction and analysis, supported by in vitro experiments, suggests that “opening” of the coiled-coil structure underlies the mechanism of this interaction. Experiments on red blood cells and HeLa cells supported the conclusions derived from the model and in vitro lipid-protein interaction results, and showed the potential physiological role of this binding. We postulate that direct interactions between spectrin ankBDn and PE-rich domains play an important role in stabilizing the structure of the spectrin-based membrane skeleton.
Symmetric Sensorimotor Somatotopy
Simon A. Overduin, Philip Servos
PLOS ONE , 2008, DOI: 10.1371/journal.pone.0001505
Abstract: Background Functional imaging has recently been used to investigate detailed somatosensory organization in human cortex. Such studies frequently assume that human cortical areas are only identifiable insofar as they resemble those measured invasively in monkeys. This is true despite the electrophysiological basis of the latter recordings, which are typically extracellular recordings of action potentials from a restricted sample of cells. Methodology/Principal Findings Using high-resolution functional magnetic resonance imaging in human subjects, we found a widely distributed cortical response in both primary somatosensory and motor cortex upon pneumatic stimulation of the hairless surface of the thumb, index and ring fingers. Though not organized in a discrete somatotopic fashion, the population activity in response to thumb and index finger stimulation indicated a disproportionate response to fingertip stimulation, and one that was modulated by stimulation direction. Furthermore, the activation was structured with a line of symmetry through the central sulcus reflecting inputs both to primary somatosensory cortex and, precentrally, to primary motor cortex. Conclusions/Significance In considering functional activation that is not somatotopically or anatomically restricted as in monkey electrophysiology studies, our methodology reveals finger-related activation that is not organized in a simple somatotopic manner but is nevertheless as structured as it is widespread. Our findings suggest a striking functional mirroring in cortical areas conventionally ascribed either an input or an output somatotopic function.
Dynamics of a Generalized Cosmological Scalar-Tensor Theory
T. Fukui,J. M. Overduin
Physics , 2002, DOI: 10.1142/S0218271802001901
Abstract: A generalized scalar-tensor theory is investigated whose cosmological term depends on both a scalar field and its time derivative. A correspondence with solutions of five-dimensional Space-Time-Matter theory is noted. Analytic solutions are found for the scale factor, scalar field and cosmological term. Models with free parameters of order unity are consistent with recent observational data and could be relevant to both the dark-matter and cosmological-"constant" problems.
Dark Matter and Background Light
Overduin, J. M.;Wesson, P. S.
High Energy Physics - Phenomenology , 2013, DOI: 10.1016/j.physrep.2004.07.006
Abstract: Progress in observational cosmology over the past five years has established that the Universe is dominated dynamically by dark matter and dark energy. Both these new and apparently independent forms of matter-energy have properties that are inconsistent with anything in the existing standard model of particle physics, and it appears that the latter must be extended. We review what is known about dark matter and energy from their impact on the light of the night sky. Most of the candidates that have been proposed so far are not perfectly black, but decay into or otherwise interact with photons in characteristic ways that can be accurately modelled and compared with observational data. We show how experimental limits on the intensity of cosmic background radiation in the microwave, infrared, optical, ultraviolet, x-ray and gamma-ray bands put strong limits on decaying vacuum energy, light axions, neutrinos, unstable weakly-interacting massive particles (WIMPs) and objects like black holes. Our conclusion is that the dark matter is most likely to be WIMPs if conventional cosmology holds; or higher-dimensional sources if spacetime needs to be extended.
Scaling Relations for the Cosmological “Constant” in Five-Dimensional Relativity
Paul S. Wesson,James M. Overduin
Advances in High Energy Physics , 2013, DOI: 10.1155/2013/214172
Abstract: When the cosmological “constant” is derived from modern five-dimensional relativity, exact solutions imply that for small systems it scales in proportion to the square of the mass. However, a duality transformation implies that for large systems it scales as the inverse square of the mass. 1. Introduction The cosmological “constant” as it appears in Einstein’s general relativity has several puzzling aspects, and it is a serious problem to understand why its value as inferred from cosmology is much smaller than its magnitude as implied by particle physics. However, it has been known for a long time that the cosmological “constant” appears more naturally when the world is taken to be five-dimensional [1], and recently there has been intense work on the modern versions of 5D relativity where the extra dimension is not compactified [2–4]. The purpose of the present paper is to draw together various results in the literature which indicate that there may be simple scaling relations between the values of the cosmological “constant” and the mass of the system concerned. Tentatively, we identify for small systems and for large, gravitationally-dominated systems. While these relations cannot be rigorously established with our present level of understanding, we believe that it is useful to point them out as guides for future research. The subjects which indicate possible relations are diverse and include the embedding of -dominated solutions of 4D general relativity in the so-called 5D canonical metric [5–8]; the embeddings which lead to variable values of [9–13]; the equations of motion for canonical and related metrics [14–20]; conformal transformations which affect and possibly [21, 22]; the vacuum and gauge fields associated with elementary particles [23, 24]; and the wave-particle duality connected with certain -dominated 5D metrics [25–27]. Most of our results are in Section 2. There we will reexamine the meaning of , reinterpret two classes of known solutions, and present a new class with interesting properties. Section 3 is a conclusion. To streamline the work, we will often absorb the speed of light , the gravitational constant , and the quantum of action , except in places where they are made explicit to aid in understanding. As usual, uppercase Latin letters run for time, space and the extra dimension. We label the last to avoid confusion. Lowercase Greek letters run . Other notation is standard. 2. The Cosmological “Constant” and Possible Scaling Relations In this section, we will examine certain subjects which involve the cosmological “constant” of a
A Scalar Field and the Einstein Vacuum in Modern Kaluza-Klein Theory
Paul S. Wesson,James M. Overduin
Physics , 2012,
Abstract: Five-dimensional relativity as an extension of general relativity has field equations that simplify considerably given the adoption of a new gauge. The result is a scalar field governed by the Klein-Gordon equation, in an empty spacetime with a cosmological constant governed by Einstein's equations. The main application is to the properties of massive particles in a vacuum-dominated universe.
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