Nonmuscle systems

Actin is present in all eukaryotic cells where it has structural and mobility functions. Most movement associated with microfilaments requires myosin. The myosin-to-actin ratio is much lower in nonmuscle cells, and myosin bundles are much smaller (10-20 molecules rather than about 500), but the interaction between myosin and actin in nonmuscle cells is generally similar to that in muscle. As in smooth muscle, myosin aggregation and activation of the actin-myosin interaction are regulated primarily by light chain phosphorylation. Myosins involved in transporting organelles along actin filaments are often activated by Ca-CaM. 

The rate-limiting step in actin polymerization appears to be nucleation, the formation of an actin cluster large enough (typically three or four G-actins) for the rate of monomer association to exceed the rate of dissociation. Once filaments of this size form, they continue to grow, and the concentration of G-actin monomers decreases until it is in equilibrium with F-actin. The concentration of monomeric actin at equilibrium is called the critical concentration, Cc. In vitro, Cc is 0.1 mM. The value in vivo is variable, depending in part on the concentration of ATP. In the presence of ADP, instead of ATP, both ends of the filament grow at the characteristic slow rate. ATP speeds up the rate of polymerization and lowers the effective Cc. 

If nascent actin filaments anchored to the cytoskeleton (by binding proteins such as those listed in Table 21-6) 

Filopodia. lamellipodia. stress libers, microvilli, acrosomal process 

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Interest in the biochemical mechanisms of smooth muscle developed alongside the study of contraction in nonmuscle systems that expanded in the 1960s. It became apparent that the contracto-regulatory system of smooth muscle exhibited more basic similarities to nonmuscle systems than to those of striated tissue. A further stimulus was provided by reports (Perrie et al, 1972,1973) of the phosphorylation of the light chain of striated muscle myosin, which was catalyzed by a specific myosin light chain kinase (Pires et al., 1974 Pires and Perry, 1977). A phosphatase involved in the de-phosphorylation of the phosphorylated light chain... 

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. 

Myosin II is in the same subfamily as the myosins in muscle thick filaments and it forms large, two-headed myosins with two light chains per heavy chain. Although myosin II is abundantly expressed in brain, little is known about its function in the nervous system. In other nonmuscle cells, myosin II has been implicated in many types of cellular contractility and may serve a similar function in developing neurons. However, myosin II remains abundant in the mature nervous system, where examples of cell contractility are less common. 

Compounds 338 and 339 have been shown to disrupt microfilament organization and exert profound effects on the morphology of nonmuscle cells without affecting the organization of the microtubular system [256]. Compound 338 was found to affect the polymerization of pure actin in a manner consistent with the formation of a 1 1 complex with G-actin. This phenomenon affected different components of the actin-based cytoskeleton [256]. Comparison to cytochalasin showed 338 to be an order... 

Kinetic studies have shown that filament initiation is more difficult than subsequent elongation (Cross et al., 1991). In a system where assembly-disassembly might play a large role, for example, in nonmuscle vertebrate cells, this property predicts that the rate at which monomers become available for polymerization could alter both the number and length of myosin filaments that are formed. Thus control of kinase activity, which controls the number of assembly competent extended monomers, could be a factor in determining subsequent polymerization. 

In the case of C02, system was broken down into muscle and nonmuscle tissue compartments with unsteady lumped analysis being applicable in each. For 02, only one tissue compartment was employed. 

Sobue K, Sellers JR (1991) Caldesmon, a novel regulatory protein in smooth muscle and nonmuscle actomyosin systems. J Biol Chem 266 12115-12118... 

Takahashi M, Kawamoto S, Adelstein RS (1992) Evidence for inserted sequences in the head region of nonmuscle myosin specific to the nervous system. J Biol Chem 267 17864-17871... 

As would be expected, most effort has been most focused on the detection of nonmuscle tissue that is related to the enforcement of human health and food safety. To enable effective enforcement and removal of SRM, there has been a need for the development of robust CNS (central nervous system)-specific detection methods that are applicable to a wide range of processed meat products. In the case of non-DNA-based techniques, methods for the detection of CNS tissue in meat include direct tissue examination (Bauer et al., 1996), histological preparation and microscopic examination (USDA-FSIS, 1998), and the analysis of cholesterol content (LiickeretaL, 1999 Schmidt et al., 1999). 

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