Actin filamentous

Figure Bl.17.6. A protein complex (myosin SI decorated filamentous actin) embedded in a vitrified ice layer. Shown is a defociis series at (a) 580 mn, (b) 1130 mn, (c) 1700 mn and (d) 2600 mn underfocus. The pictures result from averagmg about 100 individual images from one electron micrograph the decorated filament length shown is 76.8 nm.

Figure Bl.17.12. Time-resolved visualization of the dissociation of myosin SI from filamentous actin (see also figure Bl.17.6). Shown are selected filament images before and after the release of a nucleotide analogue (AMPPNP) by photolysis (a) before flashing, (b) 20 ms, (c) 30 ms, (d) 80 ms and (e) 2 s after flashing. Note the change in obvious order (as shown by the diffraction insert in (a)) and the total dissociation of the complex in (e). The scale bar represents 35.4 mn. Picture with the courtesy of Academic Press.

Actin Filament Myosin Filament Actin Fll2uaent... 

Proteins of muscle filaments Actin Myosin Tropomyosin Troponin (Tpl,TpT,TpC) Actin Myosin Tropomyosin... 

Thin filaments Actin, tropomyosin, troponin Actin, tropomyosin Actin, tropomyosin... 

The NR2 subunit of the NMDA receptor binds to PDZ domains of PSD95 with its cytoplasmic C-terminus. PSD95 also binds a-actinin, which again binds to filamentous actin (F-actin), a main cytoskeletal protein in dendritic spines. In this manner PSD95 anchors the NMDA receptor to the cytoskeleton of the dendritic spine. 

Although many cellular proteins are potential targets for phosphorylation by PKC, the myristoylated alanine-rich protein kinase C substrate (MARCKS) appears to be a major in vivo substrate. MARCKS is an acidic filamentous actin cross-linking protein that is found in high concentrations at presynaptic junctions and that is directed to... 

With regard to microtubular ultrastructure, micro filaments (5-7 run in diameter) are composed of filamentous actin. The tubule-like structures are formed by a, P-tubulin heterodimers. The wall is composed of 13 parallel protofilaments. Various microtubule-associated proteins and motor proteins (kinesin and dynein) are bound to the wall. The microtubule is a polar structure, i.e., plus and minus ends. 

The cytoskeleton is the collective name for all structural filaments in the cell. The cytoskeletal filaments are involved in establishing cell shape, and providing mechanical strength, locomotion, intracellular transport of organelles and chromosome separation in mitosis and meiosis. The cytoskeleton is made up of three kinds of protein filaments actin filaments (also called microfilaments), intermediate filaments and microtubules. 

Abbreviations CNT, Carbonnanotube F-actin,Filamentous actin TNT, Tunneling nanotube... 

Fig. 16.2 Schematic representation of cellular and artificial membrane nanotubes. (A) Two cells are connected by a tunneling nanotube (arrowhead) containing a bundle of filamentous actin (red line). N (grey), nucleus M (purple), mitochondrium ER (green), endoplasmic reticulum G (blue), Golgi apparatus. (B) Lipid nanotube connecting two lipid vesicles formed by pulling a membrane tether. (C) Membrane tether pulled from the plasma membrane of a cell (see Color Plates)...

Contractile proteins which form the myofibrils are of two types myosin ( thick filaments each approximately 12 nm in diameter and 1.5 (im long) and actin ( thin filaments 6nm diameter and 1 (Am in length). These two proteins are found not only in muscle cells but widely throughout tissues being part of the cytoskeleton of all cell types. Filamentous actin (F-actin) is a polymer composed of two entwined chains each composed of globular actin (G-actin) monomers. Skeletal muscle F-actin has associated with it two accessory proteins, tropomyosin and troponin complex which are not found in smooth muscle, and which act to regulate the contraction cycle (Figure 7.1). 

Wymann, M. P., Kemen, P., Bengtsson, T., Andersson, T., Baggiolini, M., and Deranleau, D. A. (1990) Corresponding oscillations in neutrophil shape and filamentous actin content. J. Biol. Chem. 265, 619-622. 

Oda, A., Daley, J. F., Cabral, C., Kang, J., Smith, M., and Salzman, E. W. (1992) Heterogeneity in filamentous actin content among individual human blood platelets. Blood 79, 920-927. 

Actin, the most abundant protein in eukaryotic cells, is the protein component of the microfilaments (actin filaments). Actin occurs in two forms—a monomolecular form (C actin, globular actin) and a polymer (F actin, filamentous actin). G actin is an asymmetrical molecule with a mass of 42 kDa, consisting of two domains. As the ionic strength increases, G actin aggregates reversibly to form F actin, a helical homopolymer. G actin carries a firmly bound ATP molecule that is slowly hydrolyzed in F actin to form ADR Actin therefore also has enzyme properties (ATPase activity). 

The principal cytoskeletal proteins in non-muscle cells are actin, tubulin, and the components of intermediate filaments. Actin can exist either as monomers ( G-actin ) or polymerized into 70 A diameter double filament ( F-actin ). Polymerized actin usually is localized at the margins of the cells, linked by other proteins to the cell membrane. In contrast, tubulin forms hollow filaments, approximately 250 A in diameter, that are distributed within a cell in association, generally, with cell organelles. Stabilized microtubule structures are found in the flagella and cilia of eucaryotic cells however, in other instances - examples being the mitotic apparatus and the cytoskeletal elements arising in directed cell locomotion - the microtubules are temporal entities. Intermediate filaments, which are composed of keratin-like proteins, are approximately 100 A thick and form stable structural elements that impart rigidity, for example, to nerve axons and epithelial cells. 

Electron microscopy reveals several types of protein filaments crisscrossing the eukaryotic cell, forming an interlocking three-dimensional meshwork, the cytoskeleton. There are three general types of cytoplasmic filaments— actin filaments, microtubules, and intermediate filaments (Fig. 1-9)—differing in width (from about 6 to 22 nm), composition, and specific function. All types provide structure and organization to the cytoplasm and shape to the cell. Actin filaments and microtubules also help to produce the motion of organelles or of the whole cell. 

Cytoskeleton None Complex, with microtubules, intermediate filaments, actin filaments... 

An essential component of skeletal muscle (discussed further in Chapter 19) is filamentous actin (F-actin). It is composed of 375-residue globular subunits of a single type and with a highly conserved sequence.60 61 It is found not only in muscle but also in other cells where it is a component of the cytoskeleton. The actin microfilament has the geometry of a left-... 

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