Design, function and structure of a monomeric ClC transporter

Channels and transporters of the ClC family cause the transmembrane movement of inorganic anions in service of a variety of biological tasks, from the unusual-the generation of the kilowatt pulses with which electric fish stun their prey-to the quotidian-the acidification of endosomes, vacuoles and lysosomes. The homodimeric architecture of ClC proteins, initially inferred from single-molecule studies of an elasmobranch Cl(-) channel and later confirmed by crystal structures of bacterial Cl(-)/H(+) antiporters, is apparently universal. Moreover, the basic machinery that enables ion movement through these proteins-the aqueous pores for anion diffusion in the channels and the ion-coupling chambers that coordinate Cl(-) and H(+) antiport in the transporters-are contained wholly within each subunit of the homodimer. The near-normal function of a bacterial ClC transporter straitjacketed by covalent crosslinks across the dimer interface and the behaviour of a concatemeric human homologue argue that the transport cycle resides within each subunit and does not require rigid-body rearrangements between subunits. However, this evidence is only inferential, and because examples are known in which quaternary rearrangements of extramembrane ClC domains that contribute to dimerization modulate transport activity, we cannot declare as definitive a 'parallel-pathways' picture in which the homodimer consists of two single-subunit transporters operating independently. A strong prediction of such a view is that it should in principle be possible to obtain a monomeric ClC. Here we exploit the known structure of a ClC Cl(-)/H(+) exchanger, ClC-ec1 from Escherichia coli, to design mutants that destabilize the dimer interface while preserving both the structure and the transport function of individual subunits. The results demonstrate that the ClC subunit alone is the basic functional unit for transport and that cross-subunit interaction is not required for Cl(-)/H(+) exchange in ClC transporters.     

Nodal signalling in the epiblast patterns the early mouse embryo.

Shortly after implantation the mouse embryo comprises three tissue layers. The founder tissue of the embryo proper, the epiblast, forms a radially symmetric cup of epithelial cells that grows in close apposition to the extra-embryonic ectoderm and the visceral endoderm. This simple cylindrical structure exhibits a distinct molecular pattern along its proximal-distal axis. The anterior-posterior axis of the embryo is positioned later by coordinated cell movements that rotate the pre-existing proximal-distal axis. The transforming growth factor-beta family member Nodal is known to be required for formation of the anterior-posterior axis. Here we show that signals from the epiblast are responsible for the initiation of proximal-distal polarity. Nodal acts to promote posterior cell fates in the epiblast and to maintain molecular pattern in the adjacent extra-embryonic ectoderm. Both of these functions are independent of Smad2. Moreover, Nodal signals from the epiblast also pattern the visceral endoderm by activating the Smad2-dependent pathway required for specification of anterior identity in overlying epiblast cells. Our experiments show that proximal-distal and subsequent anterior-posterior polarity of the pregastrulation embryo result from reciprocal cell-cell interactions between the epiblast and the two extra-embryonic tissues.     

Generation time and temporal scaling of bird population dynamics

Theoretical studies have shown that variation in density regulation strongly influences population dynamics, yet our understanding of factors influencing the strength of density dependence in natural populations still is limited. Consequently, few general hypotheses have been advanced to explain the large differences between species in the magnitude of population fluctuations. One reason for this is that the detection of density regulation in population time series is complicated by time lags induced by the life history of species that make it difficult to separate the relative contributions of intrinsic and extrinsic factors to the population dynamics. Here we use population time series for 23 bird species to estimate parameters of a stochastic density-dependent age-structured model. We show that both the strength of total density dependence in the life history and the magnitude of environmental stochasticity, including transient fluctuations in age structure, increase with generation time. These results indicate that the relationships between demographic and life-history traits in birds translate into distinct population dynamical patterns that are apparent only on a scale of generations.     

MHC class I surface expression in embryo-derived cell lines inducible with peptide or interferon.

It has long been recognized that the absence of expression of products of the major histocompatibility complex (MHC) during early development might allow the fetus to escape recognition by maternal lymphocytes. In addition to the MHC class I heavy chain and beta 2-microglobulin, antigenic peptide is an essential structural component of the class I molecule. Indeed, there is evidence that MHC-linked genes encoding peptide transporter molecules and possibly components of a proteolytic complex are necessary for MHC class I assembly and stability at the cell surface. Here we demonstrate that embryonic cells in general show a defect in MHC class I assembly. Surface expression was rescued in the presence of an appropriate antigenic peptide, or by treatment with interferon. Consistent with this, HAM1 messenger RNA was not constitutively expressed, but was inducible by interferon, and during differentiation in vitro. Thus, tolerance of the fetal allograft may in part be controlled at the level of peptide-dependent MHC class I assembly.     

Proton Exchange Membrane (PEM) Material Synthetic Design for Fuel Cells

1 Results Hydrocarbon PEM materials are being widely studied as replacements for Nafion-type perfluorinated polymeric materials to reduce cost and improve performance such as operating temperature and methanol crossover in the DMFC application. Among some of the important property considerations required are thermal and chemical stability, low dimensional swelling, low methanol permeability in the case of DMFC and high proton conductivity. Careful structural design can reduce the effect of swelling as...     

Crystal structure of the phosphotyrosine recognition domain SH2 of v-src complexed with tyrosine-phosphorylated peptides.

Three-dimensional structures of complexes of the SH2 domain of the v-src oncogene product with two phosphotyrosyl peptides have been determined by X-ray crystallography at resolutions of 1.5 and 2.0 A, respectively. A central antiparallel beta-sheet in the structure is flanked by two alpha-helices, with peptide binding mediated by the sheet, intervening loops and one of the helices. The specific recognition of phosphotyrosine involves amino-aromatic interactions between lysine and arginine side chains and the ring system in addition to hydrogen-bonding interactions with the phosphate.