Phosphorylation caldesmon role

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. 

The greatest challenge ahead, because it is the most difficult experimentally, is to determine the physiological role of caldesmon. In vitro experiments show how caldesmon might regulate smooth muscle contractility in concert with myosin phosphorylation, but they can never demonstrate that it does. For this we need new tools that can manipulate caldesmon within the intact cell. It is to be hoped that modern antisense RNA and transgenic techniques could provide the answer. 

To account for activation of arterial smooth muscle independently of LC20 phosphorylation, attention has been focused on the possible roles of the thin filament-associated regulatory proteins, caldesmon and calponin. Both proteins have been localized in the actomyosin domain of the smooth muscle cell and both have been shown to inhibit actin-activated myosin ATPase by interacting with F-actin, tropomyosin, and/or myosin (Clark et al., 1986 Takahashi et al.,... 

A2 or cPLA2), and proteins for which the function of phosphorylation is incompletely understood (caldesmon). In addition to cell growth and proliferation, the activation of MAPK is linked to physiological functions such as osmosensing in yeast, stretch sensing in cardiac tissue, and a potential role in smooth muscle contractile or cytoskeletal function. A complete description of the substrates phosphorylated by MAPK under physiological conditions in different cell types, and the responses controlled by those modifications, are areas of intense investigation. 

FIGURE 4 Schematic depicting the potential roles for MAPK in contractile versus proliferative (or cultured) smooth muscle. MAPK is activated in response to stimulation by growth factors, stretch, and pharmacological agents. This process can be inhibited by cAMP and cAMP-dependent protein kinase. Once activated, MAPK phos-phorylates a number of intracellular proteins (both cytoplasmic and nuclear) that result in an alteration of growth and proliferation in cultured cells. In contractile smooth muscle, MAPK phosphorylation of caldesmon may lead to alterations in muscle contractility or actin filament structure. 

A role for MAPK in the contractile responsiveness of smooth muscle may result from either of two possible mechanisms, both involving caldesmon phosphorylation. First, caldesmon phosphorylation by MAPK may lead directly to an alteration of actomyosin activity. Caldesmon may exert this effect alone, or in concert with other myofibrillar proteins such as cal-ponin. Second, phosphorylation of caldesmon may alter the dynamics of actin filament organization within the cell. Caldesmon phosphorylation may result in alterations of the cellular cytoskeleton that must occur during prolonged contractions. 

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