Myosin light chain phosphatase

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). 

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). 

In addition to the displacement of caldesmon, smooth muscle cell contraction requires kinase-induced phosphorylation of myosin. Smooth muscle has a unique type of myosin filament called p-light chains which are the target (substrate) for MLCK, but MLCK is only active when complexed with CaCM. Myosin light chain phosphatase reverses the PKA-mediated process and when cytosolic calcium ion concentration falls, CDM is released from CaCM and re-associates with the actin. The central role of calcium-calmodulin in smooth muscle contraction is shown in Figure 7.4. 

When the Ca ion concentration falls, the activity of the kinase falls, and a protein phosphatase now dephosphory-lates the light chain (smooth mnscle myosin light chain phosphatase) (Figure 22.12). 

Figure 6.10. Calcium-dependent signalling by adrenergic receptors. a p-Adrenergic receptors activate adenylate cyclase. cAMP activates protein kinase A (PKA). In heart muscle, PKA phospho-rylates several Ca transporters and charmels, so that the amount of Ca available for contraction is increased. PL Phospholam-ban SERCA SR/ER Ca transporter, b In smooth muscle, myosin activation in works by way of phosphorylation, which is performed by myosin light chain kinase (MLCK). Inactivation is accomplished by myosin light chain phosphatase (MLCP). c aj-Adrenergic receptors stimulate phospholipase C, which releases inositoltriphosphate (IP3). IP3 binds to a cognate ligand-gated Ca chaimel in the ER and releases Ca, which with calmodulin activates MLCK.

Plasma concentrations of NO metabolites were found to be markedly increased in septic shock shortly after the discoveiy that NO is a potent endogenous vasodilator. NO production is increased as a result of increased expression of the iNOS [64]. The vasodilating action of NO in shock is mediated by the activation of myosin light-chain phosphatase [65] and by the activation of potassium channels in the plasma membrane of vascular smooth-muscle cells [66]. Nitric oxide can activate KCa channels by two mechanisms direct nitrosylation of the channel and activation of a cGMP-dependent protein kinase [67,68]. 

MLCP Myosin light-chain phosphatase rDNA Recombinant DNA... 

Dephosphorylation of LC20 by myosin light chain phosphatase (MLCP) is described in Chapter 10, this volume. For dephosphorylation of distinct sites of LC20, we refer to Erdodi et al. (1989) and Barany et al. 

It has been established for several years that the major mechanism for regulation of contraction in smooth muscle is myosin phosphorylation (Hart-shorne, 1987). Phosphorylation of the two 20,000-dalton light chains of myosin (LC20) activates the actin-dependent ATPase activity of myosin and this initiates the contractile response. Dephosphorylated myosin is associated with relaxed muscle. In this scheme there are two key enzymes the myosin light chain kinase (MLCK) and the myosin light chain phosphatase (MLCP). Obviously a balance of these two activities determines the level of myosin phosphorylation. 

Sensitization by Inhibition of Myosin Light Chain Phosphatase Activity... 

Alteration of MLC phosphorylation can occur at four possible sites inhibition of Ca + binding to calmodulin, inhibition of calmodulin activation of MLCK, direct inhibition of MLCK catalytic activity, or activation of myosin light chain phosphatase (MLCP). Although inhibitors of MLCP, such as okadaic acid, are well characterized, there are no known specific activators of MLCP. 

As pointed out by Butler et al. (1994), the a-toxin-permeabilized preparation used by Kitazawa et al. might contain endogenous phosphatase inhibitors that have been lost in the Triton-skinned preparation. This could at least partially explain the observed difference in phosphatase rate, and is an intriguing possibility relating to the demonstration that receptor agonists are able to increase Ca2+ sensitivity in neuroeffector-coupled permeabilized preparations by a mechanism involving inhibited myosin light chain phosphatase (MLCP) activity (Kitazawa et al., 1991 Kubota et al.,... 

The dephosphorylation of the regulatory light chains of myosin is catalysed by myosin light chain phosphatase (MLCP). Compared to MLCK much less is known about MLCP. Based on their properties the protein phosphatases (PP) are categorised into two groups (Cohen 1989). Protein... 

Fig. 3. Schematic diagram of pathways the modulate Ca sensitivity by shifting the balance of the activities of myosin light chain kinase (MLCK) or myosin light chain phosphatase (MLCP) at constant [ Ca ]. A decrease or increase in the activities of either MLCK or MLC-P cheuiges the Ca -sensitivity of myosin light chain phosphorylation (MLC- and hence of force but does not alter the relation between force and myosin light chain phosphorylation (MLC- . For further details see text.

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