1,721,170 research outputs found
Targeting of proteins into and across the thylakoid membrane
No full textThe assembly of the photosynthetic apparatus utilizes component proteins that are synthesized by two genomes and then targeted into and across the thylakoid membrane. The emerging picture is one of a remarkably complex system of protein trafficking, in which at least four distinct pathways operate within the chloroplast - two for lumenal proteins and two for integral membrane proteins. Some of the pathways can be traced back to the prokaryotic ancestor of the chloroplast, whereas others appear to have arisen more recently - one in response to the transfer of genes to the plant nucleus and another, possibly, in response to the acquisition of new photosynthetic proteins. Remarkably, proteins in three of these pathways are synthesized with clearable signal-type peptides that are almost identical in overall structure, yet that execute entirely different functions. Recent studies have begun to reconcile the function of these targeting signals with the nature of the protein being targeted.</p
Dual signal peptides mediate the signal recognition particle/Sec- independent insertion of a thylakoid membrane polyprotein, psbY
No full textThe nuclear psbY gene (formerly ycf32) encodes two distinct single- spanning chloroplast thylakoid membrane proteins in Arabidopsis thaliana. After import into the chloroplast, the precursor protein is processed to a polyprotein in which each 'mature' protein is preceded by an additional hydrophobic region; we show that these regions function as signal peptides that are cleaved after insertion into the thylakoid membrane. Inhibition of the first or second signal cleavage reaction by enlargement of the -1 residues leads in each case to the accumulation of a thylakoid-integrated intermediate containing three hydrophobic regions after import into chloroplasts; a double mutant is converted to a protein containing all four hydrophobic regions. We propose that the overall insertion process involves (i) insertion as a double-loop structure, (ii) two cleavages by the thylakoidal processing peptidase on the lumenal face of the membrane, and (iii) cleavage by an unknown peptidase on the stromal face on the membrane between the first mature protein and the second signal peptide. We also show that this polyprotein can insert into the thylakoid membrane in the absence of stromal factors, nucleoside triphosphates, or a functional Sec apparatus; this effectively shows for the first time that a multispanning protein can insert posttranslationally without the aid of signal recognition particle, SecA, or the membrane-bound Sec machinery.</p
Sec/SRP-independent insertion of two thylakoid membrane proteins bearing cleavable signal peptides
No full textAbstractTwo imported thylakoid membrane proteins, PSII-X and PSII-W, are synthesised with cleavable N-terminal signal peptides that closely resemble those of Sec-dependent lumenal proteins. In this report we have reconstituted the insertion of pre-PSII-X and pre-PSII-W into isolated thylakoids. We show that insertion does not require either nucleoside triphosphates or stromal extracts, both of which are required for Sec- and signal recognition particle (SRP)-dependent targeting mechanisms. Insertion is furthermore unaffected by protease treatments that destroy the known protein translocation apparatus in the thylakoid membrane. We conclude that these membrane proteins are inserted by an unusual Sec/SRP-independent mechanism that probably resembles that used by CFoII, and we discuss possible parallels with the biogenesis of phage M13 procoat
An Arabidopsis cDNA encodes an apparent polyprotein of two non-identical thylakoid membrane proteins that are associated with photosystem II and homologous to algal ycf32 open reading frames
No full textAbstractWe have characterised an Arabidopsis thaliana cDNA homologous to the ycf32 open reading frames present in the Synechocystis genome and the plastid genomes of several eukaryotic algae. The predicted protein is also homologous to a novel protein reported to be associated with photosystem II. The protein is synthesised as a 23 kDa precursor with an N-terminal presequence that appears to be bipartite in structure, and the protein is targeted into the thylakoid membrane of pea chloroplasts. Although the Ycf32 presequence contains an apparent signal peptide, we find that this protein is not imported by either of the standard Sec- or ΔpH-dependent pathways. The mature protein is also unusual in two respects. First, there are two distinct, non-identical copies of typical single-span Ycf32 sequences in the Arabidopsis sequence, separated by an additional hydrophobic region. Secondly, the imported protein runs as a doublet of 6 kDa and 7 kDa polypeptides whereas the mature protein is predicted to be 14 kDa. We speculate that the protein undergoes further maturation once inserted into the thylakoid membrane to yield two separate Ycf32-like polypeptides
Characterisation of an Arabidopsis thaliana cDNA encoding a novel thylakoid lumen protein imported by the ΔpH-dependent pathway
No full textAn Arabidopsis thaliana (L.) Heynh. cDNA encoding a novel 16-kDa protein (P16) of the chloroplast thylakoid lumen has been characterised. The function of the protein is unknown but it shares some sequence similarity with alpha allophycocyanins. P16 is synthesised with a bipartite, lumen-targeting presequence, and import experiments demonstrated that this protein follows the ΔpH-dependent pathway. Analysis of the thylakoid transfer peptide revealed two unusual features. Firstly, the key targeting determinant is predicted to be a twin-arginine followed by a highly hydrophobic residue two residues later, rather than at the third position as in most transfer peptides. Secondly, the C-terminal domain of the transfer peptide contains multiple charged residues which may help to prevent mistargeting by the Sec-type protein translocase.</p
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
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