2,961 research outputs found
A “Push and Slide” Mechanism Allows Sequence-Insensitive Translocation of Secretory Proteins by the SecA ATPase
SummaryIn bacteria, most secretory proteins are translocated across the plasma membrane by the interplay of the SecA ATPase and the SecY channel. How SecA moves a broad range of polypeptide substrates is only poorly understood. Here we show that SecA moves polypeptides through the SecY channel by a “push and slide” mechanism. In its ATP-bound state, SecA interacts through a two-helix finger with a subset of amino acids in a substrate, pushing them into the channel. A polypeptide can also passively slide back and forth when SecA is in the predominant ADP-bound state or when SecA encounters a poorly interacting amino acid in its ATP-bound state. SecA performs multiple rounds of ATP hydrolysis before dissociating from SecY. The proposed push and slide mechanism is supported by a mathematical model and explains how SecA allows translocation of a wide range of polypeptides. This mechanism may also apply to hexameric polypeptide-translocating ATPases
Clustered yield model for SMT boards and MCM's, A
Includes bibliographical references.This paper describes a clustered yield model for complex surface mount technology (SMT) assemblies and multichip modules (MCM's). Based on yield modeling techniques that have been proven in the manufacturing of integrated circuits (IC's), this model uses the negative binomial distribution of defects to calculate board yield after test. Manufacturing data validates that this model accurately predicts the clustering of defects and the yield predictions are significantly better than traditional binomial models
Counterposition and negative phase velocity in uniformly moving dissipative materials
The Lorentz transformations of electric and magnetic fields were implemented
to study (i) the refraction of linearly polarized plane waves into a half-space
occupied by a uniformly moving material, and (ii) the traversal of linearly
polarized Gaussian beams through a uniformly moving slab. Motion was taken to
occur tangentially to the interface(s) and in the plane of incidence. The
moving materials were assumed to be isotropic, homogeneous, dissipative
dielectric materials from the perspective of a co-moving observer. Two
different moving materials were considered: from the perspective of a co-moving
observer, material A supports planewave propagation with only positive phase
velocity, whereas material B supports planewave propagation with both positive
and negative phase velocity, depending on the polarization state. For both
materials A and B, the sense of the phase velocity and whether or not
counterposition occurred, as perceived by a nonco-moving observer, could be
altered by varying the observer's velocity. Furthermore, the lateral position
of a beam upon propagating through a uniformly moving slab made of material A,
as perceived by a nonco-moving observer, could be controlled by varying the
observer's velocity. In particular, at certain velocities, the transmitted beam
emerged from the slab laterally displaced in the direction opposite to the
direction of incident beam. The transmittances of a uniformly moving slab made
of material B were very small and the energy density of the transmitted beam
was largely concentrated in the direction normal to the slab, regardless of the
observer's velocity
Coexistence of Ferroelectric Triclinic Phases and Origin of Large Piezoelectric Responses in Highly Strained BiFeO3 films
The structural evolution of the strain-driven morphotropic phase boundary
(MPB) in BiFeO3 films has been investigated using synchrotron x-ray
diffractometry in conjunction with scanning probe microscopy. Our results
demonstrate the existence of mixed-phase regions that are mainly made up of two
heavily tilted ferroelectric triclinic phases. Analysis of first-principles
computations suggests that these two triclinic phases originate from a phase
separation of a single monoclinic state accompanied by elastic matching between
the phase-separated states. These first-principle calculations further reveal
that the intrinsic piezoelectric response of these two low-symmetry triclinic
phases is not significantly large, which thus implies that the ease of phase
transition between these two energetically close triclinic phases is likely
responsible for the large piezoelectric response found in the BiFeO3 films near
its MPB. These findings not only enrich the understandings of the lattice and
domain structure of epitaxial BiFeO3 films but may also shed some light on the
origin of enhanced piezoelectric response near MPB.Comment: 19 pages, 3 figures and 1 tabl
On the Crepant Resolution Conjecture in the Local Case
In this paper we analyze four examples of birational transformations between
local Calabi-Yau 3-folds: two crepant resolutions, a crepant partial
resolution, and a flop. We study the effect of these transformations on
genus-zero Gromov-Witten invariants, proving the
Coates-Corti-Iritani-Tseng/Ruan form of the Crepant Resolution Conjecture in
each case. Our results suggest that this form of the Crepant Resolution
Conjecture may also hold for more general crepant birational transformations.
They also suggest that Ruan's original Crepant Resolution Conjecture should be
modified, by including appropriate "quantum corrections", and that there is no
straightforward generalization of either Ruan's original Conjecture or the
Cohomological Crepant Resolution Conjecture to the case of crepant partial
resolutions. Our methods are based on mirror symmetry for toric orbifolds.Comment: 27 pages. This is a substantially revised and shortened version of my
preprint "Wall-Crossings in Toric Gromov-Witten Theory II: Local Examples";
all results contained here are also proved there. To appear in Communications
in Mathematical Physic
Are autistic traits measured equivalently in individuals with and without an Autism Spectrum Disorder?:An invariance analysis of the Autism Spectrum Quotient Short Form
It is common to administer measures of autistic traits to those without autism spectrum disorders (ASDs) with, for example, the aim of understanding autistic personality characteristics in non-autistic individuals. Little research has examined the extent to which measures of autistic traits actually measure the same traits in the same way across those with and without an ASD. We addressed this question using a multi-group confirmatory factor invariance analysis of the Autism Quotient Short Form (AQ-S: Hoekstra et al. in J Autism Dev Disord 41(5):589-596, 2011) across those with (n = 148) and without (n = 168) ASD. Metric variance (equality of factor loadings), but not scalar invariance (equality of thresholds), held suggesting that the AQ-S measures the same latent traits in both groups, but with a bias in the manner in which trait levels are estimated. We, therefore, argue that the AQ-S can be used to investigate possible causes and consequences of autistic traits in both groups separately, but caution is due when combining or comparing levels of autistic traits across the two group
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Dual stresses of flooding and agricultural land use reduce earthworm populations more than the individual stressors
Global climate change is leading to a significant increase in flooding events in many countries. Current practices to prevent damage to downstream urban areas include allowing the flooding of upstream agricultural land. Earthworms are ecosystem engineers, but their abundances in arable land are already reduced due to pressure from farming practices. If flooding increases on agricultural land, it is important to understand how earthworms will respond to the dual stresses of flooding and agricultural land use. The earthworm populations under three land uses (pasture, field margin, and crops), across two UK fields, were sampled seasonally over an 18-month period in areas of the fields which flood frequently and areas which flood only rarely. Earthworm abundance in the crop and pasture soils and total earthworm biomass in the crop soils was significantly lower in the frequently flooded areas than in the rarely flooded areas. The relative percentage difference in the populations between the rarely and frequently flooded areas was greater in the crop soils (−59.18% abundance, −63.49% biomass) than the pasture soils (−13.39% abundance, −9.66% biomass). In the margin soils, earthworm abundance was significantly greater in the frequently flooded areas (+140.56%), likely due to higher soil organic matter content and lower bulk density resulting in soil conditions more amenable to earthworms. The findings of this study show that earthworm populations already stressed by the activities associated with arable land use are more susceptible to flooding than populations in pasture fields, suggesting that arable earthworm populations are likely to be increasingly at risk with increased flooding
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