Myosin polarity

Warshaw D M, Hayes E, Gaffney D, Lauzon A-M, Wu J, Kennedy G, Trybus K, Lowey S and Berger C 1998 Myosin conformational states determined by single fluorophore polarization Proc. Natl Acad. Sc/. USA 95 8034-9... 

Fibronectin receptor is a two-chain glycoprotein of the integrin family that serves as a transmembrane linker by binding to talin on the cytoplasmic side and to fibronectin on the external side of the membrane. The pull exerted by stress fibers on attached structures may be produced by bipolar assemblies of nonmuscle myosin molecules producing a sliding of actin filaments of opposite polarity. 

Bundles of parallel actin filaments with uniform polarity. The microvilli of intestinal epithelial cells (enterocytes) are packed with actin filaments that are attached to the overlying plasma membrane through a complex composed of a 110-kD protein and calmodulin. The actin filaments are attached to each other through fimbrin (68 kD) and villin (95 kD). The actin bundles that emerge out of the roots of microvilli disperse horizontally to form a filamentous complex, the terminal web, in which several cytoskeletal proteins, spectrin (fodrin), myosin, actinin, and tropomyosin are present. Actin in the terminal web also forms a peripheral ring, which is associated with the plasma membrane on the lateral surfaces of the enterocyte (see Figure 5, p. 24). 

Just as myosins are able to move along microfilaments, there are motor proteins that move along microtubules. Microtubules, like microfilaments, are polar polymeric assemblies, but unlike actin-myosin interactions, microtubule-based motors exist that move along microtubules in either direction. A constant traffic of vesicles and organelles is visible in cultured cells especially using time-lapse photography. The larger part of this movement takes place on micrombules and is stimulated by phorbol ester (an activator of protein kinase C), and over-expression of N-J aj oncoprotein (Alexandrova et al., 1993). 

A basic structural property of protein filaments is polarity, that is, directionality. Almost all naturally occurring filaments are polar (e.g., F-actin, microtubules, TMV, and so on). The few exceptions are either bipolar, like myosin (Huxley, 1963 Squire, 1981), or nonpolar, like intermediate filaments (Herrmann and Aebi, 2004). One method of determining... 

Fig. 5-32 Components of a skeletal muscle myofibril (a) myosin. (b) thick filament, and (c) the contraction of a sarcomere. The polarities of the thin filaments are indicated by the arrows.

Smooth muscles derive their name from their appearance when viewed in polarized light microscopy in contrast to cardiac and skeletal muscles, which have striations (appearanee of parallel bands or lines), smooth muscle is unstriated. Striations result from the pattern of the myofilaments, actin and myosin, which line the myofibrils within each muscle cell. When many myofilaments align along the length of a muscle cell, light and dark regions create the striated appearance. This microscopic view of muscle reveals some hint of how muscles alter their shape to induce movement. Because muscle cells tend to be elongated, they are often called muscle fibers. Muscle cells are distinct from other cells in the body in shape, protein composition, and in the fact that they are multi-nucleated (have more than one nucleus per cell). 

Smooth muscle is unstriated with innervations from 2 both sympathetic (flight or fight) and parasympathetic (more relaxed) nerves of the autonomic nervous system. E. Smooth muscle appears unstriated under a polarized light microscope, because the myofilaments inside are less or-ganized. Smooth muscle fibers contain actin and myosin myofilaments which are more haphazardly arranged than they are in skeletal muscles. The sympathetic neurotransmitter, Ach, and parasympathetic neurotransmitter, norepinephrine, activate this type of muscle tissue. 

So far, it is not fully understood how the asymmetric PAR protein distribution is connected with the reorganization of the actomyosin cortex. Here, we have analyzed the polarity protein PAR-2 and a component of the actomyosin cortex, the non-muscle myosin NMY-2, in vivo with standard and scanning FCS inside the C3ffosol and on the cortex of C. elegans embryos. 

Without the atomic resolution afforded by x-ray crystallography, the cleft in an actin subunit and therefore the polarity of a filament are not detectable. However, the polarity of actin filaments can be demonstrated by electron microscopy in decoration experiments, which exploit the ability of myosin to bind specifically to actin filaments. In this type of experiment, an excess of myosin SI, the globular head domain of myosin, is mixed with actin filaments and binding is permitted to take place. Myosin attaches to the sides of a filament with a slight tilt. When all the actin subunits are bound by myosin, the filament appears coated ( decorated ) with arrowheads that all point toward one end of the filament (Figure 19-4). Because... 

A EXPERIMENTAL FIGURE 19-4 Decoration demonstrates the polarity of an actin filament. Myosin SI head domains bind to actin subunits in a particuiar orientation. When bound to aii the subunits in a fiiament, SI appears to spirai around the fiiament. This coating of myosin heads produces a series of arrowhead-iike decorations, most easiiy seen at the wide views of the fiiament. The poiarity in decoration defines a pointed (-) end and a barbed (-f) end the former corresponds to the top of the model in Figure 19-3c. [Courtesy of R. Craig.]... 

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