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Folding
of the actin molecule is represented by ribbon
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tracing
of the a-carbon atoms. N and C correspond to the
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amino-
and carboxyl-terminals, respectively. The letters
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followed
by numbers represent amino acids in the
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polypeptide
chain. A hypothetical vertical line divides the
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actin
molecule into two domains "large", left side, and
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"small",
right side. ATP and Ca2+ are located between the
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two
domains. These two domains can be
subdivided
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further
into two subdomains each, the small domain being
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composed of
subdomains 1 and 2, and the 2 has
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significantly
less mass than the other three subdomains and
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this
is the reason of dividing actin into small and large
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domains).
The four subdomains are held together and
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stabilized
mainly by salt bridges and hydrogen bonds to
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the
phosphate groups of the bound ATP and to its
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associated
Ca2+ localized in the center of the molecule.
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