1,721,108 research outputs found
Direct Observation of Subunit Exchange along Mature Vimentin Intermediate Filaments
No full textAbstractActin filaments, microtubules, and intermediate filaments (IFs) are central elements of the metazoan cytoskeleton. At the molecular level, the assembly mechanism for actin filaments and microtubules is fundamentally different from that of IFs. The former two types of filaments assemble from globular proteins. By contrast, IFs assemble from tetrameric complexes of extended, half-staggered, and antiparallel oriented coiled-coils. These tetramers laterally associate into unit-length filaments; subsequent longitudinal annealing of unit-length filaments yields mature IFs. In vitro, IFs form open structures without a fixed number of tetramers per cross-section along the filament. Therefore, a central question for the structural biology of IFs is whether individual subunits can dissociate from assembled filaments and rebind at other sites. Using the fluorescently labeled IF-protein vimentin for assembly, we directly observe and quantitatively determine subunit exchange events between filaments as well as with soluble vimentin pools. Thereby we demonstrate that the cross-sectional polymorphism of donor and acceptor filaments plays an important role. We propose that in segments of donor filaments with more than the standard 32 molecules per cross-section, subunits are not as tightly bound and are predisposed to be released from the filament
Ion type and valency differentially drive vimentin tetramers into intermediate filaments or higher order assemblies
No full textVimentin intermediate filaments, together with actin filaments and microtubules, constitute the cytoskeleton
in cells of mesenchymal origin. The mechanical properties of the filaments themselves are encoded in their
molecular architecture and depend on their ionic environment. It is thus of great interest to disentangle the
influence of both the ion type and their concentration on vimentin assembly. We combine small angle X-ray
scattering and fluorescence microscopy and show that vimentin in the presence of the monovalent ions,
K+ and Na+, assembles into ‘‘standard filaments’’ with a radius of about 6 nm and 32 monomers per crosssection.
In contrast, di- and multivalent ions, independent of type and valency, lead to the formation of
thicker filaments associating over time into higher order structures. Hence, our results may indeed be of
relevance for living cells, as local ion concentrations in the cytoplasm during certain physiological activities
may differ considerably from average intracellular concentrations
Nanomechanics of vimentin intermediate filament networks
No full textIntermediate filaments are one of three classes of fibrous proteins in the cytoskeleton of eukaryotes, the others being actin filaments and microtubules. The dense filamentous networks and bundles provide important mechanical stability for the cell. Here we directly measure both the structure and mechanical properties of an in vitro model system for intermediate filaments reconstituted from purified vimentin protein at 1 mg mL(-1). We show that the mesh size is on the order of 1 mm, a value that is preserved upon addition of divalent ions. These ions act as effective cross-linkers, further stiffening the network
New roles for desmin in the maintenance of muscle homeostasis
No full textDesmin is the primary intermediate filament (IF) of cardiac, skeletal, and smooth muscle. By linking the contractile myofibrils to the sarcolemma and cellular organelles, desmin IF contributes to muscle structural and cellular integrity, force transmission, and mitochondrial homeostasis. Mutations in desmin cause myofibril misalignment, mitochondrial dysfunction, and impaired mechanical integrity leading to cardiac and skeletal myopathies in humans, often characterized by the accumulation of protein aggregates. Recent evidence indicates that desmin filaments also regulate proteostasis and cell size. In skeletal muscle, changes in desmin filament dynamics can facilitate catabolic events as an adaptive response to a changing environment. In addition, post-translational modifications of desmin and its misfolding in the heart have emerged as key determinants of homeostasis and disease. In this review, we provide an overview of the structural and cellular roles of desmin and propose new models for its novel functions in preserving the homeostasis of striated muscles
Competitive Counterion Binding Regulates the Aggregation Onset of Vimentin Intermediate Filaments
No full textAggregation is a commonly observed phenomenon when mixing charged biopolymers and counterions. However, it remains a challenge to find a general description for the involved mechanisms. Here, we investigate vimentin intermediate filaments, which are overall highly negatively charged, as a prime example of such biological polyelectrolytes. We experimentally determine the onset cation concentration for aggregation, and set it in relation to the calculated relative cation concentrations in the vicinity of the filament surface. Our analysis shows that to achieve aggregation, about 50% of the monovalent cations originally present have to be replaced by di- or multivalent cations. We are thus able to predict the bulk cation concentration, which mediates aggregation of highly negatively charged vimentin filaments
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
Assembly of Simple Epithelial Keratin Filaments: Deciphering the Ion Dependence in Filament Organization
No full textThe intermediate filament proteins keratin K8 and K18 constitute an essential part of the cytoskeleton in simple epithelial cell layers, structurally enforcing their mechanical resistance. K8/K18 heterodimers form extended filaments and higher-order structures including bundles and networks that bind to cell junctions. We study the assembly of these proteins in the presence of monovalent or divalent ions by small-angle X-ray scattering. We find that both ion species cause an increase of the filament diameter when their concentration is increased; albeit, much higher values are needed for the monovalent compared to the divalent ions for the same effect. Bundling occurs also for monovalent ions and at comparatively low concentrations of divalent ions, very different from vimentin intermediate filaments, a fibroblast-specific cytoskeleton component. We explain these differences by variations in charge and hydrophobicity patterns of the proteins. These differences may reflect the respective physiological situation in stationary cell layers versus single migrating fibroblasts
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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