Myosin light chain kinase MLCK

Myosin light-chain kinase (MLCK) —ia[Pg.467]

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... 

Of the several kinase activities which are important in smooth muscle, myosin light chain kinase, MLCK, is the one responsible for activation of the actin-myosin system to in vivo levels. MLCK is present in the other nonmuscle cell types which have the actin-myosin contractile system and all of these are probably activated in a manner similar to smooth muscle rather than by way of the Ca -troponin mechanism of striated muscle. MLCK from smooth muscle is about 130 kDa and is rather variable in shape. It is present in smooth muscle in 1-4 pM concentrations and binds with an equally high affinity to both myosin and actin. Thus, most MLCK molecules are bound to actin. Myosin light chain serine-19 is the primary target of smooth muscle myosin light chain kinase. 

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. 

The calcium mediated contraction of smooth muscle, which unlike striated muscle does not contain troponin, is quite different and requires a particular calcium-binding protein called calmodulin. Calmodulin (CM) is a widely distributed regulatory protein able to bind, with high affinity, four Ca2+ per protein molecule. The calcium—calmodulin (CaCM) complex associates with, and activates, regulatory proteins, usually enzymes, in many different cell types in smooth muscle the target regulatory proteins are caldesmon (CDM) and the enzyme myosin light chain kinase (MLCK). As described below, CaCM impacts on both actin and myosin filaments. 

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... 

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. 

An example of a specialized CaM kinase is myosin light chain kinase (MLCK), the primary fimction of which is to phosphorylate the light chain of myosin and thus to control the contraction of smooth musculature. 

Fig. 9. Helical net diagram (Crick, 1953) of a model calmodulin-binding peptide and the putative calmodulin-binding domains of two forms of myosin light-chain kinase (MLCK). The sequences are drawn together on a single helical net and are taken from (clockwise from left) the model peptide described by DeGrado et al. (1985), skeletal muscle MLCK peptide 342-359 (Edelman et al., 1985), and the N-terminal 18 residues of a peptide derived from smooth muscle MLCK (Lucas et al., 1986). The amino acids in the sequences are given in single letter codes. Positions that are hydrophobic in all three sequences are indicated by shading.

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 ... 

The regulatory light chains of myosin are known to be phosphorylated at different serine and threonine residues. Thr-18 and Ser-19 are substrates of the myosin light chain-kinase (MLCK) whereas Ser-1, Ser-2 und Ser-9 are phosphorylated by the protein kinase C (PKC) (Naka et al., 1988 Kawamoto et al., 1989 Moussavi et al., 1993). As the first slow phosphorylation by the MLCK is already present in the... 

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.

In smooth muscle, contraction is controlled by the phosphorylation state of the myosin regulatory light chain. The extent of this phosphorylation will depend on the regulatory states of both myosin light chain kinase (MLCK) and of... 

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