Actin filaments composition

Role of the Cytoskeleton in Cell Division Formation of the Mitotic Spindle, Mitosis, and Cytokinesis Drug Effects on Microtubules Mlcrofllaments Actin Filaments Structure and Composition... 

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. 

Kuroda et al. (1981) have discovered an interesting protein very similar to actin in molecular size and amino acid composition. They called it eu-actinin (42 kDa). eu-Actinin is located in Z lines and it interacts with F-actin to form bundles (Kuroda and Masaki, 1984). Unlike a-actinin, eu-actinin inhibits the onset of superprecipitation of actomyosin. Actin-binding protein (ABP, also called filamin 240 kDa X 2 Stossel and Hartwig, 1975), which causes gelation of actin filaments, has also been shown to exist in Z lines (Corner and Lazarides, 1981 Bechtel, 1979). 

The q toskeleton of eukaryotic cells is generally considered to be a meshwork of protein filaments that spans the space between the nucleus and the plasma membrane. In many cell types, the three-dimensional (3D) composite network of actin filaments, microtubules (MTs), and intermediate filaments (IPs) in the cytoplasm interfaces with two-dimensional networks composed largely of spectrins that line the plasma membrane and nuclear lamins that line the inner surface of the nuclear membrane. A few eukaryotic cell types contain an entirely different cytoskeleton that powers their locomotion and which is constmcted from the cationic major sperm protein instead of actin. The three cytoskeletal proteins, acdn, tubulin, and IF subunits, constitute a significant fraction of... 

Stehn JR, Schevzov G, O Neill GM, Guiming PW (2006) Specialisation of the Tropranyosin Composition of Actin Filaments Provides New Potential Targets for Chemotherapy. Cuir Cancer Drug Targets 6 245... 

Figure 4.11 (a) Diagrammatic representation of the myosin filament and its interaction with the actin filament during the ATP hydrolysis cycle. More detail of the steps involved is given in section S.l. (b) Outline of the composition of the myosin molecule indicating the SI head of each chain, which contains both the active site for the ATPase reaction and the actin binding site. The proteolytic cleavage sites, a and b, are involved in the production SI and the double headed heavy meromyosin (HMM) respectively. 

The fifth was a molecular biologist, who smiled sweetly and pointed out that all the others had missed the point. The frog jumps because of the biochemical properties of its muscles. The muscles are largely composed of two interdigitated filamentous proteins, actin and myosin, and they contract because the protein filaments slide past each other. This property of the actin and myosin is dependent on the amino acid composition of the two proteins, and hence on chemical, and thus on physical properties. In the last analysis, the molecular biologist insisted, following James Watson, we are all nothing but subatomic particles. 

Eukaryotic cells have an internal scaffolding called the cytoskeleton or cytomatrix that maintains their cellular morphology and enables them to migrate, undergo shape changes, and transport vesicles. Microfilaments, made of actin, intermediate filaments, which are composed of laminin and other proteins, and microtubules, formed from the protein tubulin, along with many different accessory proteins, comprise the cytoskeleton. Both the microfilaments and the microtubules can assemble and disassemble rapidly in the cell, whereas disassembly of intermediate filaments may require their destruction. Although much is known about the molecular composition of the cytoskeleton, the molecular events involved in most cell movements are still unknown. 

The amino acid composition of connectin is listed in Table IV. It is an acidic protein rather similar to actin. There are hardly any recognizable differences in amino acid composition between j8-connectin and a-con-nectin-rich samples. It is of some interest to note that C protein (MW 135 kDa) located on myosin filaments has an amino acid composition similar to connectin (Offer et al., 1973). Immunological tests showed that the two proteins are distinguishable. 

We have already noted that smooth muscle thin filaments contain both CD and CP. Analysis of isolated smooth muscle thin filaments has revealed, on average, similar amounts of CP and CD with a composition, on a molar basis of actin TM CD CP, of 7 0.9-l 0.5-0.6 0.45-0.7 (Marston, 1991 Nishida et al.,... 

From the perspective of the consilient mechanism, the assembly of filaments as required for muscle contraction and the necessary movement of components within the cell involves hydrophobic association/dissociation between composite subunits. The actin thin filament of... 

The simplistic picture of actin and MT polymerization is shown in Fig. 2. In fact, polymerization processes of both actin and MT are far more complex than the simplistic description considered above due to the following reasons. The asymmetric subunits bind with the filament at a specific orientation only. In addition, the on and off rates of subunits depend upon their state of hydrolysis e.g. ATP or GTP subunits have different binding/ unbinding rates compared to their ADP and GDP states respectively. Different permutations of nucleotide s shape and state of hydrolysis, give rise to at least eight rate constants essential for an improved description of Eq. 2. It has also been reported that the rate constants vary with the composition of the solvent and ambience temperature. Therefore, Eq. 2 will eventually need more terms to describe the polymerization process. 

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