1,720,961 research outputs found
Conformation of a purified "spontaneously" inserting thylakoid membrane protein precursor in aqueous solvent and detergent micelles
No full textSubunit W of photosystem II (PsbW) is a single-span thylakoid membrane protein that is synthesized with a cleavable hydrophobic signal peptide and integrated into the thylakoid membrane by an apparently spontaneous mechanism. In this study, we have analyzed the secondary structure of the pre-protein at early stages of the insertion pathway, using purified recombinant pre-PsbW. We show that the protein remains soluble in Tris buffer after removal of detergent. Under these conditions pre-PsbW contains no detectable alpha -helix, whereas substantial alpha -helical structure is present in SDS micelles. In aqueous buffer, the tryptophan fluorescence emission characteristics are intermediate between those of solvent-exposed and hydrophobic environments, suggesting the formation of a partially folded structure. If denaturants are excluded from the purification protocol, pre-PsbW purifies instead as a 180-kDa oligomer with substantial alpha -helical structure. Mature-size PsbW was prepared by removal of the presequence, and we show that this protein also contains alpha -helix in detergent but in lower quantities than the pre-protein. We therefore propose that pre-PsbW contains alpha -helical structure in both the mature protein and the signal peptide in nonpolar environments. We propose that pre-PsbW acquires its alpha -helical structure only during the later, membrane-bound stages of the insertion pathway, after which it forms a "helical hairpin"-type loop intermediate in the thylakoid membrane
Insertion of psak into the thylakoid membrane in a 'horseshoe' conformation occurs in the absence of signal recognition particle, nucleoside triphosphates, or functional albino3
No full textThe photosystem I subunit PsaK spans the thylakoid membrane twice, with the N and C termini both located in the lumen. The insertion mechanism of a thylakoid membrane protein adopting this type of topology has not been studied before, and we have used in vitro assays to determine the requirements for PsaK insertion into thylakoids. PsaK inserts with high efficiency and we show that one transmembrane span (the C-terminal region) can insert independently of the other, indicating that a "hairpin"-type mechanism is not essential. Insertion of PsaK does not require stromal extract, indicating that signal recognition particle (SRP) is not involved. Removal of nucleoside triphosphates inhibits insertion only slightly, both in the presence and absence of stroma, suggesting a mild stimulatory effect of a factor in the translation system and again ruling out an involvement of SRP or its partner protein, FtsY. We, furthermore, find no evidence for the involvement of known membrane-bound translocation apparatus; proteolysis of thylakoids destroys the Sec and Tat translocons but does not block PsaK insertion, and antibodies against the Oxa1/YidC homolog, Alb3, block the SRP-dependent insertion of Lhcb1 but again have no effect on PsaK insertion. Because YidC is required for the efficient insertion of every membrane protein tested in Escherichia coli (whether SRP-dependent or -independent), PsaK is the first protein identified as being independent of YidC/Alb3-type factors in either thylakoids or bacteria. The data raise the possibility of a wholly spontaneous insertion pathway.</p
Distinct albino3-dependent and -independent pathways for thylakoid membrane protein insertion
Full text linkThe homologous proteins Oxa1, YidC and Alb3 mediate the insertion of membrane proteins
in mitochondria, bacteria and chloroplast thylakoids, respectively. Depletion of YidC in
Escherichia coli affects the integration of every membrane protein studied, and Alb3 has
previously been shown to be required for the insertion of a signal recognition particle (SRP)-
dependent protein, Lhcb1, in thylakoids. In this study we have analyzed the 'global' role of
Alb3 in the insertion of thylakoid membrane proteins. We show that insertion of two
chlorophyll-binding proteins, Lhcb4.1 and Lhcb5, is almost totally blocked by preincubation
of thylakoids with anti-Alb3 antibodies, indicating a requirement for Alb3 in the insertion
pathway. Insertion of the related PsbS protein, on the other hand, is unaffected by Alb3
antibodies, and insertion of a group of SRP-independent, signal peptide-bearing proteins,
PsbX, PsbW and PsbY is likewise completely unaffected. Proteinase K is furthermore able to
completely degrade Alb3 but this treatment does not affect the insertion of these proteins.
Among the thylakoid proteins studied here, Alb3 requirement correlates strictly with a
requirement for stromal factors and nucleoside triphosphates. However, the majority of
proteins tested do not require Alb3 or any other known form of translocation apparatus
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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