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The Environment Assembly sets the global environmental agenda in cooperation with UN institutions and Multilateral Environmental Agreements. The meetings of the Assembly are governed by its Rules of Procedure. The Assembly is led by a Bureau and its President. The Bureau is composed of ten Ministers of the Environment for a term of two years, and follows geographical rotations.

The Committee of Permanent Representatives is the inter-sessional intergovernmental body of the Assembly. Throughout the year, Member States engage in formal preparatory discussions under the framework of the Open-ended meetings of the Committee of Permanent Representatives. The Committee contributes to the preparation of the agenda of the UN Environment Assembly, provides advice to the Assembly on policy matters, prepares decisions for adoption by the UN Environment Assembly and oversees their implementation.

The outcomes of the most important preparatory meetings of the Committee of Permanent Representatives are available in the documents section. The mechanism underlying this network formation is based on the rapid lateral association of A 11 tetramers Fig. Accordingly, the assembly reaction was quenched after initiation of assembly from one second onward by addition of assembly buffer containing the strong fixative glutaraldehyde at various time points Herrmann et al.

After one minute, this background had disappeared, indicating the consumption of the ULF-assembly precursors. As measured in a quantitative study, the mean filament length grew from 55 nm at 5 sec to nm by 1 min, indicating that these filaments on average consist of three ULFs Kirmse et al. Note that, during longitudinal annealing of two ULFs, the length grows only by 43 nm because of the overlap of the two coil 2 segments in the annealing zone.

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Every further addition of an ULF yields a nm increase in length. To describe the kinetics of assembly, a suite of different types of complexes that can theoretically assemble from tetramers, such as 8-mers, mers, mers, and further multiples of 4, was investigated. The resulting association constants were applied to the experimentally derived mean length values and compared with a scenario in which tetramers predominantly formed eight-mers and multiples thereof, which eventually yielded the best approximation to the measured assembly data Kirmse et al. In a next step, the assembly process was modeled according to the experimentally determined filament length histograms at different time points and by considering geometrical constraints, as well as the diffusion behavior of rod-like aggregates Portet et al.

This way, it became clear how the measured IF assembly differs from the monomer-addition-type polymerization shown, for example, by actin and tubulin. With respect to the late stage of IF assembly, it was shown by TIRF microscopy of preassembled IFs that filaments of micrometer length and longer do longitudinally anneal, although it takes days for a significant number of elongation events to occur Winheim et al.

The segmental organization of IFs has also been visualized in IFs of cultured cells. In the case of human rhabdomyosarcoma-derived RD cells, which contain both vimentin and the muscle-specific desmin, individual IFs show alternating segments, being either rich in vimentin green or in desmin red indicating that vimentin and desmin segregate from each other up to the level of ULFs Fig.

Referring to this different assembly mechanism of IFs, it has been mooted by Douglas that the complex evolution in the assembly dynamics of the IFs, as proposed by Portet and colleagues Portet et al. The reason for this behavior might reside in the fact that IF proteins are polyelectrolytes with a negatively charged rod domain and a very basic head domain.

As a consequence, the IF is built as a complex network of ionic interactions formed by the acidic rod and the basic head domain, complemented by additional hydrophobic interactions of the head and the rod with neighboring subunits within the filament. In cells, additional mechanisms using various posttranslational modifications exist to remodel IF networks dynamically Snider and Omary An important organizational feature of IFs is the positioning of the protein tail domain within the filament.

Also, a direct demonstration of the tails protruding from the filament axis was shown by EM after glycerol spraying coupled with rotary metal shadowing of filaments assembled from the low-molecular-weight and high-molecular-weight forms of the neurofilament triplet proteins NF-L, NF-H formed in vitro. As documented in Figure 2 F, the long NF-H tails project radially, thereby giving the neurofilaments a millipede-like appearance see also Hisanaga and Hirokawa ; Heins and Aebi The tail domains are probably of general significance for the unusual resistance of IFs to mechanical stress displayed, for example, when filaments are stretched in bulk on an EM grid Fig.

Mechanical properties of intermediate filaments IFs. A A desmin IF network stretched on a glow-discharged carbon-coated copper grid followed by glutaraldehyde fixation and negative staining for electron microscopy.

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Reprinted from Kreplak et al. The upper left panel shows the filament before being pulled; the lower left image documents the breakage of the filament through the action of the AFM cantilever after being operated under high force; the white boxed area is shown enlarged on the right : arrows indicate the length of the stretched filament segments after pulling and breaking, as well as the length of the original filament segment, now appearing as a gap of nm.

C Rheological investigation of the effect of disease-causing point mutations in the human desmin tail domain symbols in left box on IF strain stiffening when compared with wild-type and tailless desmin symbols in right box. Note that tailless desmin shows no strain stiffening at all. D Inhibition of vimentin filament assembly in the presence of a peptide representing the last 58 amino acids of coil 2, as described in Strelkov et al.

Note that the presence of this peptide effectively prevents longitudinal annealing of unit-length filaments ULFs , but not their formation by lateral association of tetramers. In E , a selected x — y slice through a tomographic reconstruction stack is shown. In this view, filaments are either compact or on occasion partially unraveled, thereby exposing individual octameric protofibrils that show a twisted arrangement red arrows.

In F , an x — z slice of the same tomographic reconstruction stack is shown, yielding a prominent packing pattern of cross-sectioned IFs, with four protofibrils per filament white arrows. Scale bars, nm. Arrows point to partially unraveled IFs. E , F , Adapted from Goldie et al. Because of their exposure to the filament surface, the IF tails have important roles in the gelation—or network formation—of long filaments Beck et al. Instead of becoming stiffer, like wild-type desmin networks, when being mechanically stressed, tailless desmin filament networks actually become softer Fig.

As an important medical consequence, desmin tail mutations causing both cardiomyopathy and skeletal muscle disease show altered strain-stiffening behavior Fig. MPL measurements by STEM of tail-truncated vimentin variants made it clear that the first 30 amino acids of the tail domain are directly engaged in the packing of the coiled-coil domains within the filament core.

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Hence, tailless vimentin contains more tetramers per filament cross section than wild-type vimentin i. Despite their high resistance to mechanical stress, IFs are highly dynamic polymers both in vivo and in vitro. Hence, by simply challenging them with peptides e. When added during assembly, these peptides allow the formation of ULF-like structures but completely inhibit their longitudinal annealing Fig.

A molecular basis for such a drastic effect might lie in the fact that IFs are built from four protofibrillar strands, with appropriate space around and within the individual protofibrils. Hence, the dimeric head-to-tail overlaps within a protofibril can be rather freely accessed by the peptides. The impact of these peptides on cellular organization has also been shown directly in vivo, when, after its microinjection into serum-starved cells, the entire vimentin IF network retracted into the nuclear periphery and the cells showed extensive membrane ruffling Helfand et al.

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The subfilamentous structure of vimentin IFs is documented in fine detail by cryo-electron tomography of unfixed, unstained specimens. Accordingly, in selected x — y slices through tomographic reconstruction stacks, some filament segments appear flexible and partially unraveled, whereas others look stiffer and compact Fig. Similarly, in selected x — z slices of the same tomographic reconstruction stacks, a prominent packing pattern of cross-sectioned vimentin IFs with typically four protofibrils per filament is evident Fig.

When the same unfixed vimentin IF samples were adsorbed to carbon-coated grids, blotted to remove excess liquid, quick-frozen in liquid nitrogen, and unidirectionally metal shadowed, again a mixture of compact and partially unraveled filament segments was revealed by TEM Fig. Notably, as described above, brief incubation of keratin IFs with phosphate buffer on adsorption to an EM grid yielded their subfilamentous nature, too Aebi et al.

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Network formation and role of physiological effectors. Microspheres of different diameter were tracked in the absence left and presence right of 2 m m MgCl 2.

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The two images for every bead size exemplify the degree of restriction of diffusible space for the corresponding microspheres. The presence of Hsp27 impedes the bundling reaction, as the heat-shock protein affects the kinetics of filament formation by interacting with the K8 tail domain. Reprinted from Kayser et al. Recent studies aimed at addressing the question of how IFs behave in the context of other filament systems, such as actin filaments, have been performed Jensen et al.

The results show that different cytoskeletal systems influence each other in steric ways; however, a clear concept as to what this means has yet to be formulated. Nevertheless, it has become evident that it is the direct physical interaction between the three major cytoskeletal filament systems that comprises the primary determinants of the mechanical properties of cells and tissues Huber et al.

Research on IF proteins is coming of age. What started with peculiar diffraction patterns, whose beauty and simplicity was the subject of the Ph. For instance, the functional architecture of the epidermis is highly dependent on the proper keratin network organization.

Likewise, the ability of an eagle to generate feathers depends on the intricate interaction of epithelial and mesenchymal cell layers to organize two types of keratins into these complex biomaterials. Now, the role that IFs play in organogenesis and in human disease needs to be explained in molecular terms. Most important in this ongoing endeavor is to explore IF structure, function, and dynamics more systematically in the cellular and tissue context and to consequently complement the emerging findings with corresponding ex vivo and in vitro studies.

Both H. Additional Perspectives on The Cytoskeleton available at www. Previous Section Next Section. Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Previous Section. In Cell biology—A laboratory handbook , 3rd ed. Celis JE , pp. Elsevier , Amsterdam. Google Scholar. The fibrillar substructure of keratin filaments unraveled.

J Cell Biol 97 : — The nuclear lamina is a meshwork of intermediate-type filaments. Nature : — Some insight information about the properties of the wool fibre.