Actin-myosin

Payment i, Hoiden H M, Whittacker M, Yohn C B, Lorenz M, Hoimes K C and Miiiigan R A 1993 Structure of the actin-myosin compiex and its impiications for muscie contraction Science 261 58-65... 

Rayment, 1., et al. Structure of the actin-myosin complex and its implications for muscle contraction. Science 261 58-65, 1996. 

Contractile and motile proteins Actin Myosin Tubulin Dyne in Kinesin... 

Troponin C Troponin I Troponin T Minor M protein 18 21 31 165 2 M line Ca binding Inhibits actin-myosin interaction Binds to tropomyosin Binds to myosin... 

FIGURE 17.23 The mechanism of skeletal muscle contraction. The free energy of ATP hydrolysis drives a conformational change in the myosin head, resulting in net movement of the myosin heads along the actin filament. Inset) A ribbon and space-filling representation of the actin—myosin interaction. (SI myosin image courtesy of Ivan Rayment and Hazel M. Holden, University of Wiseonsin, Madison.)... 

The ETa receptor activates G proteins of the Gq/n and G12/i3 family. The ETB receptor stimulates G proteins of the G and Gq/11 family. In endothelial cells, activation of the ETB receptor stimulates the release of NO and prostacyclin (PGI2) via pertussis toxin-sensitive G proteins. In smooth muscle cells, the activation of ETA receptors leads to an increase of intracellular calcium via pertussis toxin-insensitive G proteins of the Gq/11 family and to an activation of Rho proteins most likely via G proteins of the Gi2/i3 family. Increase of intracellular calcium results in a calmodulin-dependent activation of the myosin light chain kinase (MLCK, Fig. 2). MLCK phosphorylates the 20 kDa myosin light chain (MLC-20), which then stimulates actin-myosin interaction of vascular smooth muscle cells resulting in vasoconstriction. Since activated Rho... 

Cytokinesis (cell division) in animal cells involves the progressive formation in telophase of a furrow between the two daughter cells in the equator of the mitotic spindle. Immunofluorescent staining of the cortical cytoplasm at the site of the contraction ring reveals an abundance of actin as well as myosin, a-actinin, and filamin (Fishkind and Wang, 1995). Cytokinesis is highly sensitive to actin-myosin inhibitors such as cytochalasin and phalloidin. 

In an effort to understand how actin-actin interactions might be affected by the binding of the myosin head, and in order to gain more insight into the nature of the actin-myosin interface, we have investigated the nature of the kinetic actin-myosin intermediates involved in the process of S)-induced polymerization of G-actin. For this purpose, a variety of fluorescent probes (e.g., pyrene, NBD, AEDANS) have been covalently attached to the C-terminus of G-actin to probe the G-actin-S] interaction under conditions of tightest binding, i.e., in the absence of ATP. 

Chaussepied, P. Kasprzak, A.A. (1989). Isolation and characterization of the G-actin-myosin head complex. Namre 342,950-953. 

Momet, D., Bertrand, R., Pantel, P., Audemard, E.. Kassab, R. (1981). Structure of the actin-myosin interface. Nature 292, 301-310. 

Adelstein, R.S. Eisenberg, E. (1980). Regulation and kinetics of the actin-myosin-ATP interaction. Ann. Rev. Biochem. 49,921-956. 

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). 

Inside the typical smooth muscle cell, the cytoplasmic filaments course around the nuclei filling most of the cytoplasm between the nuclei and the plasma membrane. There are two filamentous systems in the smooth muscle cell which run lengthwise through the cell. The first is the more intensively studied actin-myosin sliding filament system. This is the system to which a consensus of investigators attribute most of the active mechanical properties of smooth muscle. It will be discussed in detail below. The second system is the intermediate filament system which to an unknown degree runs in parallel to the actin-myosin system and whose functional role has not yet been completely agreed upon. The intermediate filaments are so named because their diameters are intermediate between those of myosin and actin. These very stable filaments are functionally associated with various protein cytoarchitectural structures, microtubular systems, and desmosomes. Various proteins may participate in the formation of intermediate filaments, e.g., vimentin. 

The analytic validity of an abstract parallel elastic component rests on an assumption. On the basis of its presumed separate physical basis, it is ordinarily taken that the resistance to stretch present at rest is still there during activation. In short, it is in parallel with the filaments which generate active force. This assumption is especially attractive since the actin-myosin system has no demonstrable resistance to stretch in skeletal muscle. However, one should keep in mind, for example, that in smooth muscle cells there is an intracellular filament system which runs in parallel with the actin-myosin system, the intermediate filament system composed of an entirely different set of proteins, (vimentin, desmin, etc.), whose mechanical properties are essentially unknown. Moreover, as already mentioned, different smooth muscles have different extracellular volumes and different kinds of filaments between the cells. 

Of the several kinase activities which are important in smooth muscle, myosin light chain kinase, MLCK, is the one responsible for activation of the actin-myosin system to in vivo levels. MLCK is present in the other nonmuscle cell types which have the actin-myosin contractile system and all of these are probably activated in a manner similar to smooth muscle rather than by way of the Ca -troponin mechanism of striated muscle. MLCK from smooth muscle is about 130 kDa and is rather variable in shape. It is present in smooth muscle in 1-4 pM concentrations and binds with an equally high affinity to both myosin and actin. Thus, most MLCK molecules are bound to actin. Myosin light chain serine-19 is the primary target of smooth muscle myosin light chain kinase. 

Correlated with the wide distribution of the actin-myosin system found in smooth muscle, MLCK is also found in neural, epithelial, connective, and blood... 

For the purpose of discussion, crossbridge regulation can be split into three overlapping sets of reactions (a) the Ca-calmodulin cascade (MLCK activation), (b) the phosphorylation-dephosphorylation cycle (the Four State Model), and (c) actin-myosin cycle (chemomechanical transduction). 

Rayment, L, Holden, H.M., Whittaker, M., Yohn, C.B., Lorenz, M., Holmes, K.C., Milligan, R.A. (1993b). Structure of the actin-myosin complex and its implications for muscle contraction. Science 261, 58-65. 

In addition to its effects on enzymes and ion transport, Ca /calmodulin regulates the activity of many structural elements in cells. These include the actin-myosin complex of smooth muscle, which is under (3-adrenergic control, and various microfilament-medi-ated processes in noncontractile cells, including cell motility, cell conformation changes, mitosis, granule release, and endocytosis. 

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).

CTien contraction of muscle is stimulated (via events involving Ca +, troponin, tropomyosin, and actin, which are described below), actin becomes accessible and the S-1 head of myosin finds it, binds it, and forms the actin-myosin-ADP-P complex indicated. 

Formation of this complex promotes the release of Py which initiates the power stroke. This is followed by release of ADP and is accompanied by a large conformational change in the head of myosin in relation to its tail (Figure 49-7), pulling actin about 10 nm toward the center of the sarcomere. This is the power stroke. The myosin is now in a so-called low-energy state, indicated as actin-myosin. 

Another molecule of ATP binds to the S-1 head, forming an actin-myosin-ATP complex. 

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