Myosin light chains regulation

Ca2+ entry, Ca2+-uptake into the SR by SERCA, Ca2+ extrusion from the cell and dephosphorylation of the myosin light chains. The t ype 1 phosphatase, myosin light chain phosphatase (MLCP) dephosphorylates myosin. As with MLCK its activity is physiologically regulated, e.g. its activity is decreased following phosphorylation via Rho associated kinase (Somlyo Somlyo 2000). In the uterus we have found a small but significant reduction of force, but not Ca2+ when Rho-associated kinase is inhibited (Kupittayanant et al 2001b). 

As mentioned above, the junctional SR is connected to sheets of perpendicular SR (Fig. 4), which extend from the PM through a peripheral cytoplasmic region with lower myofilament density into the myoplasm. It is proposed that during the active state of wave-like [Ca2+]j oscillations, Ca2+ taken up by the junctional SR is released by these perpendicular sheets near the calmodulin, which is tethered to the myosin light chain kinase (MLCK) of the thin filaments (M. Walsh, personal communication, 2001). This process would enhance the specificity and efficiency of Ca2+ regulation of contraction. 

Mills, J.C., Stone, N.L., Erhardt, J., and Pittman, R.N., 1998, Apoptotic membraneblebbing is regulated by myosin light chain phosphorylation./. Cell Biol. 140 627-636. 

These messengers also play a role in regulating contraction of myometrium, which consists of smooth muscle fibres. Contraction is controlled by increases in the concentration of cytosolic Ca ions. Prostaglandins activate Ca ion channels in the plasma membrane of the fibres oxytocin activates release of Ca from intracellular stores. The increase in concentration of Ca ions leads to activation of myosin light-chain kinase which leads to crossbridge cycling and contraction (as described in Chapter 22 Figure 22.12). 

Figure 22.12 Regulation of actin-myosin interaction in smooth muscle via the light-chain kinase and phosphatase and effect on blood pressure. ions bind to calmodulin and the complex stimulates the conversion of inactive myosin light chain kinase (MLCK) to active MLCK which then phosphorylates the light chain. This results in activation of the cross-bridge cycle. The overall effect is vasoconstriction of the arteriole, which increases blood pressure.

The best characterized substrate of Ca Vcalmodulin is the Ca /calmodulin-depen-dent protein kinase (CaM kinase). CaM kinase has an important function in neuronal signal transduction. The mechanism of Ca Vcalmodulin activation of CaM kinase is described in more detail in Section 7.4, together with regulation of protein kinases. Another substrate of Ca Vcalmodulin is myosin light chain kinase (MLCK), involved in contraction of smooth musculature. 

Verin, A.D. Gilbert-McClain, L.I. Patterson, C.E. Garcia, J.G.N. Biochemical regulation of the nonmuscle myosin light chain kinase isoform in bovine endothelium. Am. J. Respir. Cell Mol. Biol., 19,(1998)... 

Fujita, K. Ye, L.-H. Sato, M. Okagaki, T. Nagamachi, Y. Kohama, K. Myosin light chain kinase from skeletal muscle regulates an ATP-dependent in-... 

A remarkable experiment205) has been performed on the function of myosin light chains in calcium regulation. It shows that rabbit DTNB (5,5-dithiobis-2-nitro-... 

Vascular tone is regulated by the cytosolic calcium level, the interaction of calcium and calmodulin with myosin light-chain kinase, and subsequent myosin light-chain phosphorylation, which promotes the interaction of myosin with actin and finally leads to contraction. 

Myosin light chain 18.5 4.4 30 i 4 Muscle Myosin regulation 47... 

Smooth muscles are not regulated by the troponin system. Rather it is the phosphorylation of the myosin light chains which appears to be the event which activates smooth muscle contraction. 

Fig. 4. The temporal sequence of events when a resting strip of tracheal smooth muscle is activated by carbacholamine addition at 10 min. There is a transient rise in [Ca2+]c (—) followed by a transient increase in the content (—) of phosphorylated myosin light chains (MLC-P) which lead in turn to the initiation of force development (—). Increased force is sustained even though the content of MLC-P declines. Preceding the sustained phase of force maintenance, there is an increase in the phosphorylation of desmin (D-P), synemin (S-P), caldesmon (CD-P) and a number of low molecular weight cytosolic proteins (X-P). These remain phosphorylated throughout the sustained phase of the response during which there is a sustained increase in Ca2+ cycling across the plasma membrane which regulates the activity of the membrane-associated protein kinase C.

Myosin light chain kinase is a Ser/Thr-type protein kinase involved in regulation of smooth muscle. The enzyme is also found in smooth muscle and platelets. This assay uses a synthetic substrate that is not radioactively labeled. 

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