Regulation of Smooth Muscle Actomyosin Function
It is widely accepted that the smooth muscle contractile apparatus is primarily regulated by reversible phosphorylation and dephosphorylation of the 20 kDa light chain of myosin (Hartshorne, 1987). The enzyme which is responsible for the phosphorylation is the Ca2+/calmodulin dependent myosin light chain kinase (MLCK) and this is the key enzyme to confer Ca2+ sensitivity to the smooth muscle contractile apparatus (Hartshorne, 1987). Another critical component of the phosphorylation hypothesis is the dephosphorylation of the myosin light chain which is catalyzed by a protein phosphatase. Several protein phosphatases have been purified from the soluble fraction of smooth muscle cells. Pato and collaborators (Pato and Adelstein, 1983a, 1983b; Pato and Kerc, 1985) reported four protein phosphatases from gizzard smooth muscle soluble fraction and three of which (SMP I, II and IV) have been purified. Among them, SMP III and IV can dephosphorylate intact myosin. The purified SMP-IV has a molecular weight of 150 kDa and is composed of two subunits of MW 58 kDa and 40 kDa. SMP-IV preferentially dephosphory-lates the α-subunit of Phosphorylase kinase (a known substrate of Type 1 phosphatases) but is insensitive to Inhibitor-2 which is known to inhibit Type 1 phosphatases specifically. Phosphatases have also been purified from the soluble fraction of aortic smooth muscle (Werth et al., 1982; DiSalvo and Gifford 1983; Erdodi et al., 1989). Erdodi et al. (1989) recently purified two types of phosphatases from dog aortic smooth muscle which dephosphorylate native actomyosin. A 260 kDa phosphatase was similar in its properties to the Type 2A phosphatases. On the other hand, a 150 kDa phosphatase dephosphorylated the α-subunit of Phosphorylase kinase although it was not inhibited by Inhibitor 1 or Inhibitor 2, which are known to be specific inhibitors of Type 1 protein phosphatase. These reports suggest that smooth muscle phosphatases purified from the soluble fraction, which dephosphorylate myosin, may be different from both Type 1 and Type 2 phosphatases. However, it is still obscure whether or not these phosphatases are responsible for the regulation of contraction in vivo. Furthermore, it is not known if phosphatase activity is physiologically regulated, and this is one of the most important areas to be elucidated.
KeywordsLight Chain ATPase Activity Myosin Light Chain Kinase Regulatory Light Chain Smooth Muscle Myosin
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