Three-dimensional structures actin filament

The above processes describe how the growth and depolymerisation of actin filaments thin filaments) is controlled. However, actin filaments are assembled into filamentous networks, and these three-dimensional structures are themselves controlled and also stabilised by a number of proteins ... 

A well-defined actin cytoskeleton is unique to eukaryotes prokaryotes lack such structures. How did filamentous actin evolve Comparison of the three-dimensional structure of G-actin with other proteins revealed remarkable similarity to several other proteins, including sugar kinases such as hexokinase (Figure 34.16 see also Section 16.1.1). 

Second, as a complex with G-actin, profilin is postulated to assist in the addition of monomers to the (-f) end of an actin filament. This hypothesis is consistent with the three-dimensional structure of the profilin-actin complex in which profilin is bound to the part of an actin monomer opposite the ATP-binding end, thereby leaving it free to associate with the (+) end of a filament (see Figure 19-3). After the complex binds transiently to the filament, the profilin dissociates from actin. 

If the cross-links or polymers are severed, then some elastic energy is released and the system will adopt a new (larger) equilibrium volume where greater distortion is conferred upon a meshwork that has fewer cross-links. Thus, upon increases in Ca2+, gelsolin activity leads to an increase in the volume of the actin-filament network. The additional influence of myosin on such a meshwork is similar to that proposed in the Stossel model. Thus, three-dimensional Ca2+ gradients (between a localised region of the cell surface and an external structure) can result in complex shape changes. 

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. 

Vibert P, Craig R, Lehman W (1993) Three-dimensional reconstruction of caldesmon-containing smooth muscle thin filaments. J Cell Biol 123 313-321 Vibert PJ, Haselgrove JC, Lowy J, Poulsen FR (1972) Structural changes in actin-con-taining filaments of muscle. J Mol Biol 71 757-767 Vorotnikov AV, Gusev NB, Hua S, Collins JH, Redwood CS, Marston SB (1993) Identification of casein kinase II as a major endogeneous caldesmon kinase in sheep aorta smooth muscle. FEBS Lett 334 18-22... 

On the side adjacent to the endothelium the cytoplasm contains a dense meshwork of fine filaments, whereas on the outer surface numerous pi-nocytotic vesicles are observed (Weibel 1974). Pulmonary pericytes influenced the actin distribution of pulmonary microvascular endothelial cells (Shepro and Morel 1993). Pericytes enhanced the formation of a distinct dense peripheral band at microvascular endothelial cell periphery and promoted close apposition of adjacent endothelial cells. Transforming growth factor-P appears to mediate this pericyte-endothehal interaction. The role played by TGF-P is supported by the finding that this agonist modulates extracellular matrix organisation and the formation of tubelike structures with apparent tight junctions in three-dimensional cultures of rat epididymal fat pad microvascular en-dothehal cells (Merwin et al. 1990). 

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