Myosin light chain phosphorylation

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.

A Ca2+-CaM-dependent PK with a very specific protein substrate is myosin light chain kinase (MLCK) that phosphorylates myosin light chains (MLCs) associated with the head ... 

At a high concentration of calcium ion, calcium binds to calmodulin. In turn, calmodulin binds to and activates a protein kinase that phosphorylates myosin light chains. At low calcium concentration, the light chains are dephosphorylated by a calcium-independent phosphatase. 

Figure 19-3 Mechanism of nitrovasodilators Nitric oxide (NO) formed in smooth muscle from nitrovasodilators or from endothelial cells (EDRF) activates guanylate cyclase (GC ). GC activates cGMP-dependent protein kinases that phosphorylate myosin light-chain kinase (MLCK), causing its inactivation and subsequent muscle relaxation (see also Fig. 19-2)...

As shown in Figure III-5-2, smooth muscle contraction is triggered by the influx of Ca2+ through voltage-regulated membrane channels. Ca2+ combines with calmodulin, and the complex activates myosin LC kinase, the active form of which (MLCK ) phosphorylates myosin light chains, enabling the interaction between myosin and actin. 

Pharmacomechanical mechanisms for relaxation include (1) G kinase-dependent increases in the activity of sarcoplasmic reticulum Ca pumps (SERCA). Ca pumps on the plasma membrane may also be stimulated (not shown). This increase in Ca sequestration and extrusion decreases [Ca +Jj and induces relaxation as shown in the foregoing. (2) Some agents appear to decrease 1,4,5-1P3 formation and may relax smooth muscle by decrease Ca + release (not shown) (3) Finally, increases in [cAMP] activate cAMP-dependent protein kinase (A kinase), which could phosphorylate myosin light chain kinase and decrease its Ca sensitivity (this mechanism has not been demonstrated in intact smooth muscle). 

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.,... 

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. 

The regulatory light chains from vertebrate forms of myosin-II undergo reversible phosphorylation by a calmodulin dependent enzyme called myosin light chain... 

If MLCK activates contraction by increasing myosin phosphorylation, then an increase in the activity of myosin light chain phosphatase, MLCP, by decreasing the fraction of myosin which is phosphorylated, should lead to relaxation from the active (contractile) state. Cyclic adenosine monophosphate (AMP) is a strong inhibitor of smooth muscle contraction and it has been suggested that activation of MLCP could result from its phosphorylation via cAMP activated protein kinase (see Figure 5). 

Phosphorylation of Myosin Light Chains Initiates Contraction of Smooth Muscle... 

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

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... 

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