Myosin light chain kinase, phosphorylation

Myosin light chain kinase can be phosphorylated on several residues (Adelstein et al., 1978). Myosin light chain kinase phosphorylation at "site A" decreases the Ca2+ sensitivity of myosin light chain kinase (Stull et al., 1990). Both cAMP-dependent protein kinase and Ca2+-calmodulin-dependent protein kinase II phosphorylate myosin light chain kinase on site A in vitro. Myosin light chain kinase phosphorylation on site A depends primarily on [Ca +Jj regardless of the stimuli (Stull et al., 1990) Van Riper et al.,... 

Activated myosin light chain kinase phosphorylates the 20-kDa light chain of myosin on serine 19. This phosphorylation is associated with an increase in the actin-activated myosin ATPase activity (Ikebe et al.,... 

Just I, Selzer J, Wilm M, von Eichel-Streiber C, Mann M, Aktories K (1995) Glucosylation of Rho proteins by Clostridium difficile toxin B. Nature 375 500-503 Kamm KE, Stull JT (1985) The function of myosin and myosin light chain kinase phosphorylation in smooth muscle. Ann Rev Pharmacol Toxicol 25 593-620 Kamm KE, Stull JT (1986) Activation of smooth muscle contraction relation between myosin phosphorylation and stifftiess. Science 232 80-82 Kanamori M, Naka M, Asano M, Hidaka H (1981) Effects of N-(6-aminohexyl)-5-chloro-l-naphtalene ulfonamide and other calmodulin antagonists (calmodulin interacting scents) on calcium-induced contraction of rabbit aortic strips. J Pharmacol Exp Ther 217 494-499... 

Miller, JR, Silver, PJ, Stull, JT (1983) The role of myosin light chain kinase phosphorylation in hetfl-adrenergic rela ration of tracheal smooth muscle. Mol Pharmacol 24 235-242... 

Stull JT, Hsu L-C, Tansey MG, Kamm KE (1990) Myosin light chain kinase phosphorylation in tracheal smooth muscle. J Biol Chem 265 16683-16690 Stull JT, Krueger JK, Kamm KE, Gao Z-H, Zhi G, Padre R (1996) Myosin light chain kinase. In Barany M (ed) Biochemistry of smooth muscle contraction. Academic press pp pp 119-130... 

Van Riper D, Weaver BA, Stull JT, Rembold CM (1995) Myosin light chain kinase phosphorylation in swine carotid artery contraction and relaxation. Am J Physiol 268 H2466-H2475... 

In the presence of calcium, the primary contractile protein, myosin, is phosphorylated by the myosin light-chain kinase initiating the subsequent actin-activation of the myosin adenosine triphosphate activity and resulting in muscle contraction. Removal of calcium inactivates the kinase and allows the myosin light chain to dephosphorylate myosin which results in muscle relaxation. Therefore the general biochemical mechanism for the muscle contractile process is dependent on the avaUabUity of a sufficient intraceUular calcium concentration. 

Smooth muscle contractions are subject to the actions of hormones and related agents. As shown in Figure 17.32, binding of the hormone epinephrine to smooth muscle receptors activates an intracellular adenylyl cyclase reaction that produces cyclic AMP (cAMP). The cAMP serves to activate a protein kinase that phosphorylates the myosin light chain kinase. The phosphorylated MLCK has a lower affinity for the Ca -calmodulin complex and thus is physiologically inactive. Reversal of this inactivation occurs via myosin light chain kinase phosphatase. 

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... 

Smooth muscle myosin contains two myosin light chains. Phosphorylation of the regulatory light chain by myosin light chain kinase is a mandatory step to induce contraction. 

Myosin Light Chain Kinase Is Activated by Calmodulin-4Ca + Then Phosphorylates the Light Chains... 

The calmodulin-4Ca +-activated light chain kinase phosphorylates the hght chains, which then ceases to inhibit the myosin-F-actin intetaction. The contraction cycle then begins. 

Effect of protein-bound Ca TpC 4Ca antagonizes Tpl inhibition of F-actin-myosin interaction (allows F-actin activation of ATPase) Calmodulin 4Ca activates myosin light chain kinase that phosphorylates myosin p-light chain. The phosphorylated p-light chain no longer inhibits F-actin-myosin interaction (allows F-actin activation of ATPase). 

Protein phosphorylation Calmodulin kinase I ATI Elongation factor-2 kinase Phosphorylase kinase Myosin Light Chain kinase... 

The remaining three types of CaMK are phosphorylase kinase, myosin light-chain kinase and CaMKIII. These each appear to phosphorylate fewer substrate proteins, and in some cases only one protein, under physiological conditions, and each may therefore mediate relatively fewer actions of Ca2+ in the nervous system. 

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.

Many of the biochemical and molecular events that are responsible for uterine smooth muscle contraction are the same as those that control other smooth muscle tissues (Fig. 62.1). Once uterine smooth muscle sensitivity has been augmented, actin and myosin must interact for contraction to occur. This interaction depends on the phosphorylation of the contractile proteins by the enzyme myosin light chain kinase (MLCK). This enzyme requires Ca++ and is active only when associated with calmodulin. Activation of the entire muscle contraction... 

---