Myosin assembly control

L. S., Iwadate, Y., Nagasaki, A., et al. (2005) Multiple myosin 11 heavy chain kinases roles in filament assembly control and proper cytokinesis in Dictyostelium. Mol. Biol. Cell 6, 4256 266. 

An increase in Ca2+ (e.g., from 10 8M to 10 5M) acts as the trigger. In striated muscle, Ca2+ is released from the endoplasmic reticulum into the cytoplasm following stimulation of the muscle cell via its attached motor nerve. The Ca2+ interacts with the troponin complex, causing a movement of tropomyosin to expose the myosin binding sites on the thin filaments (see Example 5.21). In smooth muscle, the released Ca2+ indirectly activates myosin light chain kinase which phosphorylates the light chains of myosin. Hence, the control is at the level of the thick filament. In some nonmuscle cells, the control by Ca2+ is at the level of the assembly of myosin into filaments. 

Liang W, Warrick HM, Spudich JA. 1999. A structural model for phosphorylation control of Dictyostelium myosin II thick filament assembly. J Cell Biol 147 1039-1047. 

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. 

Rho Controls organization of actin cytoskeleton and gene expression (F-actin bundling, myosin filament assembly) Rho (A-E), Cdc42, Rac (1-3) Anchored to plasma membrane by lipids, and translocates to cytosol... 

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