1,721,379 research outputs found
Role of lipid phase separations and membrane hydration in phospholipid vesicle fusion.
No full textThe relationship between lipid phase separation and fusion of small unilamellar phosphatidylserine-containing vesicles was investigated. The kinetics of phase separation were monitored by following the increase of self-quenching of the fluorescent phospholipid analogue N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine, which occurs when the local concentration of the probe increases upon Ca2+-induced phase separation in phosphatidylserine (PS) bilayers [Hoekstra, D. (1982) Biochemistry 21, 1055-1061]. Fusion was determined by using the resonance energy transfer fusion assay [Struck, D. K., Hoekstra, D., & Pagano, R. E. (1981) Biochemistry 20, 4093-4099], which monitors the mixing of fluorescent lipid donor and acceptor molecules, resulting in an increase in energy transfer efficiency. The results show that in the presence of Ca2+, fusion proceeds much more rapidly (t 1/2 less than 5 s) than the process of phase separation (T 1/2 congruent to 1 min). Mg2+ also induced fusion, albeit at higher concentrations than Ca2+. Mg2+-induced phase separation were not detected, however. Subthreshold concentrations of Ca2+ (0.5 mM) or Mg2+ (2 mM) induced extensive fusion of PS-containing vesicles in poly(ethylene glycol) containing media. This effect did not appear to be a poly(ethylene glycol)-facilitated enhancement of cation binding to the bilayer, and consequently Ca2+-induced phase separation was not observed. The results suggest that macroscopic phase separation may facilitate but does not induced the fusion process and is therefore, not directly involved in the actual fusion mechanism. The fusion experiments performed in the presence of poly(ethylene glycol) suggest that the degree of bilayer dehydration and the creation of "point defects" in the bilayer without rigorous structural rearrangements in the membrane are dominant factors in the initial fusion events
Role of lipid phase separations and membrane hydration in phospholipid vesicle fusion.
No full textThe relationship between lipid phase separation and fusion of small unilamellar phosphatidylserine-containing vesicles was investigated. The kinetics of phase separation were monitored by following the increase of self-quenching of the fluorescent phospholipid analogue N-(7-nitro-2,1,3-benzoxadiazol-4-yl)phosphatidylethanolamine, which occurs when the local concentration of the probe increases upon Ca2+-induced phase separation in phosphatidylserine (PS) bilayers [Hoekstra, D. (1982) Biochemistry 21, 1055-1061]. Fusion was determined by using the resonance energy transfer fusion assay [Struck, D. K., Hoekstra, D., & Pagano, R. E. (1981) Biochemistry 20, 4093-4099], which monitors the mixing of fluorescent lipid donor and acceptor molecules, resulting in an increase in energy transfer efficiency. The results show that in the presence of Ca2+, fusion proceeds much more rapidly (t 1/2 less than 5 s) than the process of phase separation (T 1/2 congruent to 1 min). Mg2+ also induced fusion, albeit at higher concentrations than Ca2+. Mg2+-induced phase separation were not detected, however. Subthreshold concentrations of Ca2+ (0.5 mM) or Mg2+ (2 mM) induced extensive fusion of PS-containing vesicles in poly(ethylene glycol) containing media. This effect did not appear to be a poly(ethylene glycol)-facilitated enhancement of cation binding to the bilayer, and consequently Ca2+-induced phase separation was not observed. The results suggest that macroscopic phase separation may facilitate but does not induced the fusion process and is therefore, not directly involved in the actual fusion mechanism. The fusion experiments performed in the presence of poly(ethylene glycol) suggest that the degree of bilayer dehydration and the creation of "point defects" in the bilayer without rigorous structural rearrangements in the membrane are dominant factors in the initial fusion events
Transbilayer redistribution of phosphatidylethanolamine during fusion of phospholipid vesicles. Dependence on fusion rate, lipid phase separation, and formation of nonbilayer structures
No full textThe effect of membrane fusion on the transbilayer distribution of dioleoyl- and dipalmitoylphosphatidylethanolamine (DOPE and DPPE, respectively) in phosphatidylserine (PS) vesicles was investigated. A 7-fold increase in the external pool of DOPE, as determined by labeling of the vesicle surface with 2,4,6-trinitrobenzenesulfonic acid, was observed when multilamellar vesicles (MLV) consisting of PS and DOPE were incubated with small unilamellar vesicles (SUV) of PS in the presence of Ca2+. In contrast, no significant redistribution of DPPE was seen when similar experiments were performed by using PS bilayers that contained DPPE instead of DOPE. Redistribution of neither DOPE nor DPPE could be detected during SUV-SUV fusion. By using the resonance energy transfer fusion assay for mixing of membrane lipids [Struck, D.K., Hoekstra, D., & Pagano, R. E. (1981) Biochemistry 20, 4093-4099; Hoekstra, D. (1982) Biochemistry 21, 2833-2840], it was demonstrated that fusion between SUV and MLV had actually occurred. The results also suggested that (partial) fusion of internal bilayers within the multilamellar system must have occurred. Although Mg2+-induced fusion between SUV and MLV was also observed, no redistribution of DOPE was seen in this case. It is concluded that the observed translocation of DOPE during fusion was probably mediated via inverted micellar structures, which were formed when the lipid was converted to the hexagonal (HII) phase resulting from lipid phase separation between PS and DOPE. However, induction of the hexagonal phase per se, i.e., in the absence of fusion, did not cause substantial transbilayer redistribution of DOPE, suggesting that fusion was intimately involved in this phenomenon. It is suggested that fusion represents the "driving force" for transbilayer DOPE redistribution, requiring a (partial) overlap between the kinetics of phase separation and fusion
Transbilayer redistribution of phosphatidylethanolamine during fusion of phospholipid vesicles. Dependence on fusion rate, lipid phase separation, and formation of nonbilayer structures
No full textThe effect of membrane fusion on the transbilayer distribution of dioleoyl- and dipalmitoylphosphatidylethanolamine (DOPE and DPPE, respectively) in phosphatidylserine (PS) vesicles was investigated. A 7-fold increase in the external pool of DOPE, as determined by labeling of the vesicle surface with 2,4,6-trinitrobenzenesulfonic acid, was observed when multilamellar vesicles (MLV) consisting of PS and DOPE were incubated with small unilamellar vesicles (SUV) of PS in the presence of Ca2+. In contrast, no significant redistribution of DPPE was seen when similar experiments were performed by using PS bilayers that contained DPPE instead of DOPE. Redistribution of neither DOPE nor DPPE could be detected during SUV-SUV fusion. By using the resonance energy transfer fusion assay for mixing of membrane lipids [Struck, D.K., Hoekstra, D., & Pagano, R. E. (1981) Biochemistry 20, 4093-4099; Hoekstra, D. (1982) Biochemistry 21, 2833-2840], it was demonstrated that fusion between SUV and MLV had actually occurred. The results also suggested that (partial) fusion of internal bilayers within the multilamellar system must have occurred. Although Mg2+-induced fusion between SUV and MLV was also observed, no redistribution of DOPE was seen in this case. It is concluded that the observed translocation of DOPE during fusion was probably mediated via inverted micellar structures, which were formed when the lipid was converted to the hexagonal (HII) phase resulting from lipid phase separation between PS and DOPE. However, induction of the hexagonal phase per se, i.e., in the absence of fusion, did not cause substantial transbilayer redistribution of DOPE, suggesting that fusion was intimately involved in this phenomenon. It is suggested that fusion represents the "driving force" for transbilayer DOPE redistribution, requiring a (partial) overlap between the kinetics of phase separation and fusion
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
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