Actin microfilaments

Cytokinesis is the separation of daughter cells at the completion of the microtubule directed separation of duplicate chromosomes at mitosis or meiosis. This is usually accomplished by a purse string mechanism, whereby daughter cells become separated by the gradual constriction of a structure composed of actin microfilaments and myosins-II (Fujiwara and Pollard, 1976 Satterwhite and Pollard, 1992). 

Theriot, J.A., Mitchison, T.J. (1991). Actin microfilament dynamics in locomoting cells. Nature, 352, 126-131. 

Figure 48-5. Schematic representation of fibronectin interacting with an integrin fibronectin receptor situated in the exterior of the plasma membrane of a cell of the ECM and of various attachment proteins interacting indirectly or directly with an actin microfilament in the cytosol. For simplicity, the attachment proteins are represented as a complex.

Nevertheless, there are reports on enhancement of ocular drug absorption by bile salts [33], surfactants [200], and chelators [149], Newton et al. [35] demonstrated that Azone, an enhancer widely tested in transdermal drug delivery [201], increased the ocular absorption of cyclosporine, an immunosuppressant, by a factor of 3, thereby prolonging the survival of a corneal allograft. In 1986, Lee et al. [34] reported that 10 pg/mL cytochalasin B, an agent capable of condensing the actin microfilaments, increased the aqueous humor and iris-ciliary body concentrations of topically applied inulin (5 kDa) by about 70% and 700%, respectively, in the albino rabbit. 

This spectrin network further binds to actin microfilaments and to numerous other ligands. These associations are probably dynamic. For example, phosphorylation of ankyrin can alter its affinity for spectrin. The functions of the multiple protein-interaction domains of both spectrin and ankyrin have been as yet only partially defined (see Ch. 8). 

Actin microfilaments and the membrane cytoskeleton play critical roles in neuronal growth and secretion 129... 

Valderrama, F., Babia, T., Ayala, I., Kok, J. W., Renau-Piqueras, J. and Egea, G. Actin microfilaments are essential for the cytological positioning and morphology of the Golgi complex. Eur.. Cell Biol. 76 9-17,1998. 

One proposed mechanism for targeting of organelles to terminals might have general implications. The synapsin family of phosphoproteins [20], which is concentrated in the presynaptic terminal, may be involved in targeting synaptic vesicles. Dephosphorylated synapsin binds tightly to both synaptic vesicles and actin microfilaments (MFs), while phosphorylation releases both of them. 

Doris FP, Steer MW. Effects of fixatives and permeabilisation buffers on pollen tubes implications for localisation of actin microfilaments using phalloidin staining, Protoplasma, 1996 195 25-36. 

Both actin microfilaments and microtubules (see below) are involved in uptake as well as in the pathway to the lysosomes and the recycling pathway. Referring to clathrin-mediated uptake, Durrbach et al. showed actin participation in two steps of endocytosis the initial uptake mechanism and the delivery to lysosomes (116). 

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-... 

Tropomyosin is a two-stranded, o-helical coiled-coil molecule that aggregates head-to-tail with others to form long filamentous ropes. These lie in each of the two long period grooves of the actin microfilaments where, in vertebrate skeletal muscle, they play an important part in the Ca2+-mediated regulation of actin via troponin (a tropomyosin-associated protein). An important feature of tropomyosin is its 39.2-residue period— that is also quasi-halved (19.6 residues)—in the linear distribution of the acidic residues and, to a lesser extent, the apolar residues (McLachlan and Stewart, 1976 Parry, 1975). The number of residues in tropomyosin (284 residues), and the head-to-tail overlap (nine residues) that allows axial... 

Yang, Y., Dowling, J., Yu, Q.-C., Kouklis, P., Cleveland, D. W., and Fuchs, E. (1996). An essential cytoskeletal linker protein connecting actin microfilaments to intermediate filaments. Cell 86, 655-665. 

In addition, several annexins have been shown to interact directly or indirectly with actin microfilaments and microtubules, thus annexins have the potential to form a structural link between the membrane and the underlying cortical cytoskeleton (reviewed in Gerke et al. 2005 Hayes et al., 2004). As well as lateral inter-annexin interactions (either homotypic or heterotypic with other types of annexin)... 

Chailley, B. Nicolas, G. and Laine, M. C. (1989) Organization of actin microfilaments in the apical border of oviduct ciliated cells. Biology of the Cell 67 81-90... 

Eukaryotic cells have an internal scaffold, the cytoskeleton, that controls the shape and movement of the cell. The cytoskeleton is made up of actin microfilaments, intermediate filaments and microtubules. 

Together with actin microfilaments and microtubules, keratin filaments make up the cytoskeleton of vertebrate epithelial cells. Keratins belong to a family of intermediate filament proteins that form a-helical coiled-coil dimers that associate laterally and end to end to form 10 nm diameter filaments. Keratin and actin filaments and microtubules form an integrated cytoskeleton that preserves the shape and structural integrity of the ker-atinocyte as well as serves to transmit mechanical loads. Keratins account for about 30% of the total protein in basal cells. 

Microtubules and microfilaments are ubiquitous components of almost all cells (red blood cells excluded). Microfilaments are polymers of actin, which in the non-muscle cell can exist in two forms /3-actin and T-actin. Microfilaments have a diameter of about 6 nm. In certain structures such as the microvilli of intestinal mucosal cells, microfilaments are associated with myosin, so that movement is possible given an appropriate stimulus. Endocytosis and exocytosis events are apparently mediated via microfilaments. Microfilaments are disrupted by the drug cytochalasin. 

Disrupts actin cytoskeleton (cell adhesion inhibitor) [attractant feeding deterrent, antineoplastic, cytotoxic] Disrupts actin cytoskeleton blocks cell division by blocking actin microfilament formation [inhibits Glc transport, toxic]... 

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