1,721,049 research outputs found
Passive water permeability of some wild type and mutagenized amino acid cotransporters of the SLC6/NSS family expressed in Xenopus laevis oocytes
No full textIn this paper passive water movement across the cell membrane mediated by wild type and mutagenized cotransporters was investigated. We evaluated water movement and, in parallel, amino acid uptake induced by some members of the SLC6/NSS family belonging to different kingdoms, namely the rat GABA transporter GAT1, the insect amino acid transporters KAAT1 and CAATCH1 and the bacterial leucine transporter LeuT, whose structure was recently solved. We also tested whether mutated proteins in which the solute translocation mechanism is altered or even abolished were able to induce water movement across cell membrane. The proteins of interest were expressed in Xenopus laevis oocytes and osmotic water permeabilities were estimated from the rate of cell volume change induced by an osmotic gradient in the absence of cotransported solutes. Under osmotic stress all the studied wild type amino acid cotransporters increased the water permeability of the membrane. The GABA transport inhibitor SKF 89976A inhibited both GABA transport and water movement induced by the expression of GAT1. Interestingly, the capacity of mutant proteins to induce water movement was not predictable on the basis of their substrate transport ability. In particular the GAT1 mutant Q291N, void of any transport activity, induced a water permeability similar to that induced by the wt protein. The KAAT1 mutant T339C, which showed a higher transport activity, induced a water permeability not significantly different from the wild type transporter. Interestingly, the bacterial leucine cotransporter LeuT, whose binding site for leucine and Na(+) is void of water, induced water movement through the plasma membrane
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Observing Xenopus laevis oocyte plasma membrane by Atomic Force Microscopy
No full textThis paper describes the use of Atomic Force Microscopy (AFM) to investigate the plasma membrane of Xenopus laevis oocyte. Different protocols of sample preparation to perform an AFM investigation of both external and intracellular sides of the oocyte plasma membrane are presented and discussed. Reproducible AFM images allowed visualization and dimensional characterization of protein complexes differently arranged on both sides of the oocyte plasma membrane. In particular, two different arrangements were visualized: (1) a heterogeneous and irregular distribution of the protein complexes and (2) in some cases a distribution of nanometer-sized membrane domains where protein complexes are densely packed and spatially arranged in an ordered hexagonal motif. In addition, a methodological approach based on the purification of oocyte plasma membrane by ultracentrifugation on sucrose gradient is also described in this work. The potential of AFM as a useful tool for the structural characterization of proteins in a native eukaryotic membrane was established and its relevance for describing the organization of protein complexes in native biological membranes was explored
The SLC6/NSS family members KAAT1 and CAATCH1 are weakly Cl--dependent
No full textKAAT1 and CAATCH1, amino acid transporters cloned from the intestine of the lepidoptera Manduca sexta (Castagna et al., 1998, PNAS, 95: 5395-5400; Feldman et al., 2000, J. Biol Chem., 275: 24518-24526), are members of the SLC6/NSS family, which groups neurotransmitter, aminergic transmitter, amino acid and osmolyte transporters coupled to Na+, K+ and Cl- gradients.
KAAT1 and CAATCH1 are able to transport different amino acids depending on the contransported cation (Na+ or K+) but their Cl--dependence is not completely defined yet.
The Cl- binding site of two members of the family, the serotonin transporter SERT (Forrest et al., 2007, PNAS, 104: 12761-12766) and the GABA transporter GAT1 (Zomot et al., 2007, Nature, 449: 726-730), has been recently modelled on the basis of their functional properties and by structural homology to the leucine transporter LeuT, a prokaryotic member of the family whose three-dimensional structure has been solved at atomic level (Yamashita et al., 2005, Nature, 437: 215-223). The analyses have highlighted the role of a serine residue, that in the Cl--independent LeuT corresponds to Glu 290, and of an asparagine (Asn 286) that also contributes to the coordination of Na+ in the Na1 binding site of LeuT.
With the aim to clarify the role exerted by chloride in SLC6/NSS transporters, the Cl--dependence of KAAT1 and CAATCH1 have been here investigated by the expression in Xenopus laevis oocytes and the measurement of induced amino acid uptakes.
Despite KAAT1 and CAATCH1 posses the same residue of serine (Ser342, KAAT1 numbering) present in strictly chloride dependent transporters, their transport activities resulted weakly Cl--dependent compared to GAT1. By analysis of the structure of Cl--dependent (SERT and GAT1) and Cl--independent (LeuT) transporters, we selected several residues present in the putative Cl- binding site of KAAT1 and investigated their involvement in chloride interaction
- …
