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The degree of
interaction is determined by the net level of
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phosphorylation
of the 20 kDa regulatory light chains of myosin II
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(rMLC).
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MLC is regulated
by MLC kinase (MLCK) and MLC phosphatase
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(MLCP or PP1M).
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The extent of
the rMLC phosphorylation and the amplitude of force
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production
depends on the balance of the activities of MLCK and
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MLCP.
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Under certain
conditions, force is also regulated independent of the
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changes in rMLC
phosphorylation levels perhaps by thin filament
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associated
proteins (caldesmon and calponin), which can be
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phosphorylated by
MAP kinase and/or other kinases.
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Thin filament
associated proteins might modulate the effect of rMLC
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phosphorylation,
which is alone sufficient to initiate and maintain
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contraction.
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MLCP is a trimer
comprising a 130 kD regulatory myosin binding
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subunit (MBS), a
37 kD catalytic subunit (PP1c), and a 20 kD protein
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of uncertain
function (M20).
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