Myosin assembly

Myosin-II phosphorylation is also an important mechanism for regulating myosin assembly in nonmuscle and smooth muscle cells (Kom and Hammer, 1988). For example, myosin-II ixomAcanthamoeba is more soluble when the heavy chain is phosphorylated compared to the unphosphorylated species. Similarly, phosphorylation of the light chains of vertebrate smooth muscle and nonmuscle myosin-II affects filament formation by these myosins. These myosins undergo a... 

Rico, M. Egelhoff, T.T. Myosin heavy chain kinase B participates in the regulation of myosin assembly into the cytoskeleton. J. Cell. Biochem., 88, 521-532 (2003)... 

Janiesch PC, Kim J, Mouysset J et al. (2007) The ubiquitin-selective chaperone CDC-48/p97 links myosin assembly to human myopathy. Nat Cell Biol 9(4), 379-390. 

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. 

Myosin Subftagment-I Interacts With Two G-Actin Molecules Oligomers of G-Actin and S] Are the Second Intennediates in F-Actin-Si Assembly Conclusion... 

In a previous section we mentioned the significance of myosin filament structure. In nematodes two forms of myosin-II, myosin A and B, are required for proper filament stmcture (Epstein, 1988). The two forms of myosin are expressed at the proper time to allow for correct filament assembly. An accessory protein called paramyosin is also required for correct filament assembly. In vertebrate cardiac muscle, there are also two isoforms of myosin-II a-myosin and p-myosin. The proper ratio of these two proteins is of utmost importance for proper muscle activity. The incorrect synthesis of a- and P-myosins results in a severe cardiac disorder known as hypertrophic cardiomyopathy. Genetic transmission of the disease occurs in about 55% of families. The inherited condition is called familial hypertrophic cardiomyopathy (FHC), and this condition is a leading cause of sudden death in young athletes. 

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). 

Kolega, J., Taylor, D.L. (1993). Gradients in the concentration and assembly of myosin II in living fibroblasts during locomotion and fiber transport. Mol. Biol. Cell 4, 819-836. 

Organization into macromolecular structures. There are no apparent templates necessary for the assembly of muscle filaments. The association of the component proteins in vitro is spontaneous, stable, and relatively quick. Filaments will form in vitro from the myosins or actins from all three kinds of muscle. Yet in vitro smooth muscle myosin filaments are found to be stable only in solutions somewhat different from in vivo conditions. The organizing principles which govern the assembly of myosin filaments in smooth muscle are not well understood. It is clear, however, a filament is a sturdy structure and that individual myosin molecules go in and out of filaments whose structure remains in a functional steady-state. As described above, the crossbridges sticking out of one side of a smooth muscle myosin filament are all oriented and presumably all pull on the actin filament in one direction along the filament axis, while on the other side the crossbridges all point and pull in the opposite direction. The complement of minor proteins involved in the structure of the smooth muscle myosin filament is unknown, albeit not the same as that of skeletal muscle since C-protein and M-protein are absent. 

The superstructure of smooth muscle actin filaments is differentiated from those of striated muscle by the absence of the troponins and the lateral organization by association of the filaments with dense bodies instead of with the Z-line. How these differences are encoded is again not at all clear. However, the myofibrillar structure and the alignment of the alternating actin and myosin filaments is apparently due primarily to dense bodies and the actin-actinin macrostructures. As the bent dumbbell shaped actins assemble into filaments they are all oriented in the same direction. The S-1 fragments of myosin will bind to actin filaments in vitro and in... 

Myosin may be extracted via high-ionic-strength buffers and purified. Synthetic thick filaments of myosin spontaneously assemble upon lowering the ionic strength of its solution, exhibiting the morphological characteristics of native thick filaments. This process initiates with myosin monomers assembled into parallel dimers. The dimers assemble into antiparallel tetramers, the tetramers into octamers, and the octamers into minifilaments... 

Figure 49-3. Schematic representation of the thin fiiament, showing the spatiai configuration of its three major protein components actin, myosin, and tropomyosin. The upper panei shows individual molecules of G-actin. The middle panel shows actin monomers assembled into F-actin. Individual molecules of tropomyosin (two strands wound around one another) and of troponin (made up of its three subunits) are also shown. The lower panel shows the assembled thin filament, consisting of F-actin, tropomyosin, and the three subunits of troponin (TpC, Tpl, andTpT).

The process of activation of neutrophils is essentially similar. They are activated, via specific receptors, by interaction with bacteria, binding of chemotactic factors, or antibody-antigen complexes. The resultant rise in intracellular Ca affects many processes in neutrophils, such as assembly of micrombules and the actin-myosin system. These processes are respectively involved in secretion of contents of granules and in motility, which enables neutrophils to seek out the invaders. The activated neutrophils are now ready to destroy the invaders by mechanisms that include production of active derivatives of oxygen. 

Of the seven mapped genes underlying Usher I, five have been identified. These included cadherin-23 and myosin-7a, both described above, harmonin and SANS, both scaffold proteins, and protocadherin-15, another member of the cadherin superfamily. Biochemical evidence and phenotypic similarity suggests that these proteins may assemble into a complex [29], although conclusive evidence for such a complex is lacking. 

Assembly of the actin network merely by interaction with these binding proteins can itself account for pseudopodia formation and propulsive movement. However, there is some evidence to suggest that F-actin-myosin interactions are required for vectorial movement hence it has been demonstrated that pseudopodia contain filament networks comprising actin and myosin. Myosin plays a role in the contractile movement of neutrophils in a... 

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