structure of the ATP-bound protomer and ADP-Pi-bound protomer are not altogether the same.
7.6 Microtubule
7.6.1 Structure of Microtubule
Microtubule is a hollow cylinder and is much thicker than actin filament (outer
diameter: ~25 nm as compared with ~7 nm for an actin filament). A microtubule is
consisted of equal number of alpha and beta tubulins: the two types of tubulins are
structurally very similar [57] and form a heterodimer that behaves as a unit in the
process of binding to and dissociation from the microtubule. The approximate
dimension of the heterodimer is 8 nm  4.6 nm  6.5 nm [58]. As shown in
Fig. 7.16a, in a microtubule, the heterodimers make head-to-tail association with
Seam
1.5
Tubulin dimer
a
b
Proto-filament
β-tubulin
Microtubule
25 nm
13 proto-filaments
13 th
proto-filament
GTP (in
E-site)
α-tubulin
GTP (in
N-site)
Fig. 7.16 The schematic
representation of a structure
of a tubulin dimer and a
microtubule. Panel a, left, a
tubulin dimer (alpha-tubulin
is in gray and the betatubulin is in white); right,
the dimers form a protofilament and 13 protofilaments are aligned in
parallel to form a hollow
cylinder. Panel b, The rise of
the 3-start helix (the shaded
parallelogram) is 1.5 times l,
which is the size of the
heterodimer. As a result, the
lateral alpha-to-alpha or
beta-to-beta contact between
adjacent proto-filaments is
disrupted at the contact
between the first and the
13th proto-filament, and the
alpha-to-beta contact is
formed. This type of contact
formed between the 1st and
the 13th filaments is
called seam
116
7 Moving Life
7.6 Microtubule
7.6.1 Structure of Microtubule
Microtubule is a hollow cylinder and is much thicker than actin filament (outer
diameter: ~25 nm as compared with ~7 nm for an actin filament). A microtubule is
consisted of equal number of alpha and beta tubulins: the two types of tubulins are
structurally very similar [57] and form a heterodimer that behaves as a unit in the
process of binding to and dissociation from the microtubule. The approximate
dimension of the heterodimer is 8 nm  4.6 nm  6.5 nm [58]. As shown in
Fig. 7.16a, in a microtubule, the heterodimers make head-to-tail association with
Seam
1.5
Tubulin dimer
a
b
Proto-filament
β-tubulin
Microtubule
25 nm
13 proto-filaments
13 th
proto-filament
GTP (in
E-site)
α-tubulin
GTP (in
N-site)
Fig. 7.16 The schematic
representation of a structure
of a tubulin dimer and a
microtubule. Panel a, left, a
tubulin dimer (alpha-tubulin
is in gray and the betatubulin is in white); right,
the dimers form a protofilament and 13 protofilaments are aligned in
parallel to form a hollow
cylinder. Panel b, The rise of
the 3-start helix (the shaded
parallelogram) is 1.5 times l,
which is the size of the
heterodimer. As a result, the
lateral alpha-to-alpha or
beta-to-beta contact between
adjacent proto-filaments is
disrupted at the contact
between the first and the
13th proto-filament, and the
alpha-to-beta contact is
formed. This type of contact
formed between the 1st and
the 13th filaments is
called seam
116
7 Moving Life