We present the first results of a new global next-to-leading order analysis of spin-dependent parton distribution functions from the most recent world data on inclusive polarized deep-inelastic ...scattering, focusing in particular on the large-x and low-Q super(2) regions. By directly fitting polarization asymmetries we eliminate biases introduced by using polarized structure function data extracted under nonuniform assumptions for the unpolarized structure functions. For analysis of the large-x data we implement nuclear smearing corrections for deuterium and super(3)He nuclei, and systematically include target mass and higher twist corrections to the g sub(1) and g sub(2) structure functions at low Q super(2). We also explore the effects of Q super(2) and W super(2) cuts in the data sets, and the potential impact of future data on the behavior of the spin-dependent parton distributions at intermediate and large x.
Separate ion pathways in a Cl-/H+ exchanger Accardi, Alessio; Walden, Michael; Nguitragool, Wang ...
The Journal of general physiology,
12/2005, Volume:
126, Issue:
6
Journal Article
Peer reviewed
Open access
CLC-ec1 is a prokaryotic CLC-type Cl(-)/H+ exchange transporter. Little is known about the mechanism of H+ coupling to Cl-. A critical glutamate residue, E148, was previously shown to be required for ...Cl(-)/H+ exchange by mediating proton transfer between the protein and the extracellular solution. To test whether an analogous H+ acceptor exists near the intracellular side of the protein, we performed a mutagenesis scan of inward-facing carboxyl-bearing residues and identified E203 as the unique residue whose neutralization abolishes H+ coupling to Cl- transport. Glutamate at this position is strictly conserved in all known CLCs of the transporter subclass, while valine is always found here in CLC channels. The x-ray crystal structure of the E203Q mutant is similar to that of the wild-type protein. Cl- transport rate in E203Q is inhibited at neutral pH, and the double mutant, E148A/E203Q, shows maximal Cl- transport, independent of pH, as does the single mutant E148A. The results argue that substrate exchange by CLC-ec1 involves two separate but partially overlapping permeation pathways, one for Cl- and one for H+. These pathways are congruent from the protein's extracellular surface to E148, and they diverge beyond this point toward the intracellular side. This picture demands a transport mechanism fundamentally different from familiar alternating-access schemes.
Motivated by the wide range of kinematics covered by current and planned deep-inelastic scattering (DIS) facilities, we revisit the formalism, practical implementation, and numerical impact of target ...mass corrections (TMCs) for DIS on unpolarized nuclear targets. An important aspect is that we only use nuclear and later partonic degrees of freedom, carefully avoiding a picture of the nucleus in terms of nucleons. After establishing that formulae used for individual nucleon targets (p,n), derived in the Operator Product Expansion (OPE) formalism, are indeed applicable to nuclear targets, we rewrite expressions for nuclear TMCs in terms of re-scaled (or averaged) kinematic variables. As a consequence, we find a representation for nuclear TMCs that is approximately independent of the nuclear target. We go on to construct a single-parameter fit for all nuclear targets that is in good numerical agreement with full computations of TMCs. We discuss in detail qualitative and quantitative differences between nuclear TMCs built in the OPE and the parton model formalisms, as well as give numerical predictions for current and future facilities.
The Cl
−/H
+ exchange-transporter CLC-ec1 mediates stoichiometric transmembrane exchange of two Cl
− ions for one proton. A conserved tyrosine residue, Y445, coordinates one of the bound Cl
− ions ...visible in the structure of this protein and is located near the intersection of the Cl
− and H
+ pathways. Mutants of this tyrosine were scrutinized for effects on the coupled transport of Cl
− and H
+ determined electrophysiologically and on protein structure determined crystallographically. Despite the strong conservation of Y445 in the CLC family, substitution of F or W at this position preserves wild-type transport behavior. Substitution by A, E, or H, however, produces uncoupled proteins with robust Cl
− transport but greatly impaired movement of H
+. The obligatory 2 Cl
−/1 H
+ stoichiometry is thus lost in these mutants. The structures of all the mutants are essentially identical to wild-type, but apparent anion occupancy in the Cl
− binding region correlates with functional H
+ coupling. In particular, as determined by anomalous diffraction in crystals grown in Br
−, an electrophysiologically competent Cl
− analogue, the well-coupled transporters show strong Br
− electron density at the “inner” and “central” Cl
− binding sites. However, in the uncoupled mutants, Br
− density is absent at the central site, while still present at the inner site. An additional mutant, Y445L, is intermediate in both functional and structural features. This mutant clearly exchanges H
+ for Cl
−, but at a reduced H
+-to-Cl
− ratio; likewise, both the central and inner sites are occupied by Br
−, but the central site shows lower Br
− density than in wild-type (or in Y445F,W). The correlation between proton coupling and central-site occupancy argues that halide binding to the central transport site somehow facilitates movement of H
+, a synergism that is not readily understood in terms of alternating-site antiport schemes.
CLC-ec1 is an E. coli homologue of the CLC family of Cl- channels, which are widespread throughout eukaryotic organisms. The structure of this membrane protein is known, and its physiological role ...has been described, but our knowledge of its functional characteristics is severely limited by the absence of electrophysiological recordings. High-density reconstitution and incorporation of crystallization-quality CLC-ec1 in planar lipid bilayers failed to yield measurable CLC-ec1 currents due to porin contamination. A procedure developed to prepare the protein at a very high level of purity allowed us to measure macroscopic CLC-ec1 currents in lipid bilayers. The current is Cl- selective, and its pH dependence mimics that observed with a 36Cl- flux assay in reconstituted liposomes. The unitary conductance is estimated to be <0.2 pS. Surprisingly, the currents have a subnernstian reversal potential in a KCl gradient, indicating imperfect selectivity for anions over cations. Mutation of a conserved glutamate residue found in the selectivity filter eliminates the pH-dependence of both currents and 36Cl- flux and appears to trap CLC-ec1 in a constitutively active state. These effects correlate well with known characteristics of eukaryotic CLC channels. The E148A mutant displays nearly ideal Cl- selectivity.
PDF
