Thick Filament Regulation

These observations raise a number of recurring issues regarding the role of phosphorylation (Murphy, 

Phosphorylation with Four Cross-bridge States 

The Hai and Murphy model proposes that crossbridge cycling is determined only by the ratio of MLCK to MLCP activity, and thus LC20 phosphorylation. The model predicts the behavior of swine carotid artery for which it was developed (Hai and Murphy, 1989a Paul, 

1990) and, as such, it meets the criteria listed in Table 1. However, this model is not incompatible with the coexistence of additional regulatory elements. 

If one accepts the evidence that force development and maintenance in smooth muscle are due to crossbridge cycling, then dephosphorylated cross-bridges must participate (Hai and Murphy, 1989b Butler et al., 

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Contractile proteins which form the myofibrils are of two types myosin ( thick filaments each approximately 12 nm in diameter and 1.5 (im long) and actin ( thin filaments 6nm diameter and 1 (Am in length). These two proteins are found not only in muscle cells but widely throughout tissues being part of the cytoskeleton of all cell types. Filamentous actin (F-actin) is a polymer composed of two entwined chains each composed of globular actin (G-actin) monomers. Skeletal muscle F-actin has associated with it two accessory proteins, tropomyosin and troponin complex which are not found in smooth muscle, and which act to regulate the contraction cycle (Figure 7.1). 

Whiting, A., Wardale.J., and Trinick, J. (1989). Does titin regulate the length of muscle thick filaments /. Mol. Biol. 205, 263-268. 

Skeletal muscle is composed of muscle cells, thin and thick filaments, and endomysium, a connective tissue containing fibroblasts that holds the fibers together. Mechanical loading regulates normal muscle metabolism in the absence of normal tensile loading muscle atrophy results. Interactions between thick (myosin) and thin (actin) filaments result in lines or bands containing one or more of the muscle fiber components. The H band represents overlap of only the thick filaments whereas the I band represents the area of overlap of thin and thick filaments. The z lines are the points at which the sarcomere repeats itself. 

Figure 8.12 Interaction between myosin and actin (thin and thick filaments) during muscle contraction. (Reproduced by permission from Adelstein RS, Eisenberg E. Regulation and kinetics of the actin-myosin-ATP interaction. Annu Rev Biochem 49 921-956, 1980.)... 

Question What regulates thin filament length and the position of the thick filaments in the sarcomere ... 

Smooth muscle myosin has distinctive characteristics that may form the basis for many of the unique functional properties exhibited by smooth muscle tissues. The following section will first review the molecular structure of smooth muscle myosin and the functional implications of its distinctive characteristics. This will be followed by a discussion of the regulation of the assembly of myosin into thick filaments, and the molecular organization of the thick filaments of smooth muscle tissues. 

As discussed above one important function of caldesmon appears to be in a thin filament associated regulatory system. In addition, caldesmon binds to myosin (Ikebe and Reardon 1988) and can cross-link and tether actin to myosin (Haeberle et al. 1992) and may possibly also have role in stabilising the thick filaments (Katayama et al. 1995). The regulation of these processes in the smooth muscle fibre is unknown. 

Titin Thick filament 2800-3000 kDa >3 Alternative splicing Regulating length and assembly of myofibrils... 

Contraction in amoeboid cells makes use of nonmuscle forms of myosin type II whieh form bipolar thiek filaments in the cytoplasm and in association with the actin filaments [23, 54]. In Dictyostelium, actin filaments form a eortieal shell directly under the plasma membrane, with random orientation of the filaments [132]. Activation of myosin contractile activity by phosphorylation of the myosin light ehain protein results in contraction of the cortical network [54, 110], It is unclear whether this contraetion is uniform throughout the eell or whether there is spatial regulation of the activity. In polarized motile Dictyostelium cells, myosin is concentrated at the rear of the cell [31], which could maintain the polarization, while a myosin heavy chain kinase (which phosphorylates the myosin heavy chain protein and inhibits thick filament formation), is localized at the front of the cell. 

Kuczmarsld, E.R. and Spudich, J.A. (1980). Regulation of myosin self-assembly phosphoiylation of Dictyostelium heavy chain inhibits thick filament formation. Proc. Natl. Acad. Sci. U.S.A. 77, 7292-7296. 

Caldesmon (CD) is a ubiquitous protein in smooth muscle cells. The smooth muscle isoform has a sequence-derived molecular mass of 89-93 kDa, but migrates on SDS gels at 120-150 kDa. Within the smooth muscle cell, CD is localized within the contractile apparatus (Furst et al., 1986) and when the contractile filaments are isolated it is found to be tightly bound to the thin filaments (Marston and Lehman, 1985). It is suggested that its in vivo function involves regulation of thin filament activity and possibly a role in the assembly and stabilization of thick and thin filaments. 

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