Calmodulin complexes

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. 

Calcium-dependent regulation involves the calcium-calmodulin complex that activates smooth muscle MLCK, a monomer of approximately 135 kDa. Dephosphorylation is initiated by MLCP. MLCP is a complex of three proteins a 110-130 kDa myosin phosphatase targeting and regulatory subunit (MYPT1), a 37 kDa catalytic subunit (PP-1C) and a 20 kDa subunit of unknown function. In most cases, calcium-independent regulation of smooth muscle tone is achieved by inhibition of MLCP activity at constant calcium level inducing an increase in phospho-rMLC and contraction (Fig. 1). 

Although in in vivo circumstances an intracellular free calcium increase apparently acts as the primary modulator of contraction, it can be bypassed in highly permeabilized smooth muscle preparations where the active subunit of MLCK can be introduced to phosphorylate myosin and induce contraction. The MLCK catalyzed phosphorylation of serine-19 is seen as the necessary event in the activation of smooth muscle myosin to form crossbridges. Thus, the rising phase of force during an isometric smooth muscle contraction follows an increase in the degree of phosphorylation of myosin, and that in turn follows the transient rise of (a) cytosolic free Ca, (b) Ca-calmodulin complexes, and (c) the active form of MLCK. The regulation of the intracellular calcium is discussed below. The dynam-... 

When MLCK is phosphorylated by cAMP activated protein kinase, it itself is harder to activate. Molecule for molecule, being phosphorylated does not diminish the effectiveness of MLCK in catalyzing the phosphorylation of myosin. However, phosphorylated MLCK has a much smaller affinity for the Ca-calmodulin complex, which activates it, than the uninhibited, nonphosphorylated form. Thus, phosphorylation of MLCK by protein kinase decreases the number of activated MLCK... 

Upon entering the smooth muscle cell, Ca++ ions bind with calmodulin, an intracellular protein with a chemical structure similar to that of troponin. The resulting Ca++-calmodulin complex binds to and activates myosin kinase. This activated enzyme then phosphorylates myosin. Crossbridge cycling in smooth muscle may take place only when myosin has been phosphorylated. 

The Ca2+-calmodulin complex may also activate nitric oxide synthase (NOS), which binds to a PDZ domain of PSD-95. Activated NOS produces NO from arginine NO, in turn, activates guanylate cyclase, the enzyme that catalyzes the conversion of GTP to the intracellular messenger cGMP, which activates protein kinase G (PKG). 

Ral has attracted much interest in recent years, not least because it was demonstrated to mediate part of Ras function as described above. In contrast to Rap, which rather inhibits Ras signaling, Ral is part of one of the essential Ras-activated pathways. Moreover, it has proved to be acting in parallel with the Raf pathway in cell transformation induced by oncogenic Ras [37, 77]. The case of Ral demonstrates the complexity - and the incomplete knowledge and understanding - of signal transduction. Ral can also be activated by Rap mediated by Rif [103] and, alternatively, by binding of a calcium/calmodulin complex to the Ral C-terminus which obviously does not affect the nucleotide state of Ral [111]. 

Yao, Y Squier, T.C. Variable conformation and dynamics of calmodulin complexed with peptides derived from the autoinhibitory domains of target proteins. Biochemistry 1996, 35, 6815-6827. 

Dolmetsch R, Pajvani U, Fife K, Spoils JM, Greenberg ME (2001) Signaling to the nucleus by an L-type caldiun channel-calmodulin complex through the MAP kinase pathway. Science 294 333-339... 

CaM kinase II <1, 3> ( phosphorylates and activates [26] <3> phosphorylation of Thr-311 results in 8-lOfold enzyme activation in the presence of 0.01 mM free Ca " and 0.002 mM calmodulin and in a 25fold increase in sensitivity to the Ca /calmodulin complex [26] <1,3> phosphorylation of isoenzyme B by calmodulin kinase II and protein kinase C added together results in a maximal 60-70fold activation, no effect on the sensitivity to the Ca2 /calmodulin complex, CaM kinase II alone activates 35-40fold in the presence of Ca " and calmodulin [28] <1> endogenous activator of isoform A [30]) [26, 28, 30]... 

In addition to JAKs, STAT1 is also directly phosphorylated by protein kinase C. This process is mediated by inositol triphosphate, Ca2+ release, formation of Ca2+-calmodulin complex and release of calcineurin. Calcineurin dephosphorylates NF-AT resulting in its translocation to the nucleus and subsequent activation of STAT1, in addition to other genes. 

A primary target of intracellular Ca2+ is calmodulin and the Ca2+/calmodulin complex in turn promotes the concomitant activation of the calmodulin-dependent... 

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