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The
secondary structures of alpha- and beta-tubulin are basically
identical: each monomer is formed by a core of two
beta-sheets surrounded by alpha-helices. The monomer structure
is very compact, but can be divided into three functional
domains: the amino-terminal domain containing the
nucleotide-binding region, an intermediate domain containing the
Taxol-binding site, and the carboxy-terminal domain,
which probably constitutes the binding surface for motor
proteins.
GTP
and GDP molecules.
Each tubulin monomer binds a guanine nucleotide, which
is nonexchangeable when it is bound in the alpha subunit, or N
site, and exchangeable when bound in the beta subunit, or E
site. The α subunit is associated with a GTP molecule
(colored red) and the β subunit with a GDP molecule (colored
yellow). It also shows the antimitotic drug "Taxol" bound to
beta subunit. Taxol is used to stabilize microtubules for
isolation.
Beta Tubulin.
Overview of the monomer β subunit
bound to a GDP molecule (colored yellow) and Taxol (colored
green) bound to Taxol binding pocket located between H1, H6, H7
and S7. |