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Myosin phosphorylation is an important mechanism in regulating contractile activity of smooth muscle. The level of myosin phosphorylation depends on the balance of two enzymes, myosin light chain kinase and myosin phosphatase. Recently it has been discovered that myosin phosphatase can be regulated and this renewed interest in characterization of the phosphatase. It is suggested that the myosin phosphatase is composed of three subunits: a catalytic subunit of type 1 phosphatase (delta isoform; PP1c delta); and two non-catalytic subunits, large and small (M20). The large subunit is thought to be a targeting subunit and is termed myosin phosphatase target subunit (MYPT). There are several isoforms of MYPT and two genes have been identified on human chromosomes 1 and 12. A dominant feature of MYPT is a series of ankyrin repeats at the N-terminal end of the molecule and these may be involved in binding to the catalytic subunit and to substrate, phosphorylated myosin. In addition, at the N-terminal fringe of the ankyrin motifs is a consensus PP1c binding motif. The function of the M20 subunit is not established but is known to bind to the C-terminal end of MYPT. Various interactions between subunits that might be relevant for the regulation of phosphatase activity are discussed. |
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