section
2 1 .5
Nonmuscle Systems
481
F IG U R E 2 1 -1 7
Drawing of a microtubule, (a) Cross-sectional view of the 13
protofilaments, (b) Longitudinal view.
F IG U R E 2 1 -1 6
(a) A microtubule doublet consisting of an A subfiber with
13 protofilaments of tubulin and a B subfiber with 11 protofilaments.
(b) A cilium contains nine microtubule doublets and a central pair of
microtubules. The dynein arms provide for sliding of one doublet along
another during the beating of the cilium. The functions of radial spokes and
the projections from the central pair of microtubules have not been as well
defined.
joined to central singlets by radial spokes, which have a
relatively globular end, or head, that interacts with the
central singlet. The radial spokes are complex structures,
which seem to have 17 or more constituent proteins, at least
6
of which are in the head. They are arranged in pairs every
96 nm along the microtubules. The inner sheath comprises
primarily thin protein arms extending from the central mi-
crotubules at roughly 14-nm intervals, and the spoke heads
may interact with these.
Cilia grow from
basal bodies,
one of several types of
microtubule organizing centers in cells. Each basal body
contains nine fused triplets of microtubules that act as nu-
cleation centers for the growth of microtubules down the
axoneme. Each triplet contains one complete A micro-
tubule, that is continuous with the A microtubule of the
axonemal doublet, and an incomplete B microtubule that
is continuous with the B microtubule of the doublet. A sec-
ond incomplete microtubule, the C microtubule, is fused to
the B but does not extend beyond the basal body. The basal
body does not have central singlets. There are numerous
proteins beside the
a-
and
p
-tubulin of the microtubules in
the basal bodies, which are thought to be involved in con-
trolling polymerization, stabilizing the axoneme structure,
and securing the basal body to the cytoskeleton. These in-
clude a third type of tubulin,
y
-tubulin, which is believed
to form a ring that serves as the nucleus from which the
tubules grow.
Microtubules are constructed of protofilaments (Figure
21-17), the axis of each being parallel to the axis of the
microtubule. In cilia, the A microtubule and the central sin-
glets have 13 protofilaments, while the B microtubule has
10 protofilaments of its own and shares 4 with the A mi-
crotubule (Figure 21-16a). The protofilaments are chains
of heterodimers of a- and
p
-tubulin arranged end-to-end.
There are two models of protofilament arrangement.
In one, the heterodimers in adjacent protofilaments are
only slightly staggered, forming spiraling rows of
a-
and
p
-tubulin in the microtubule wall as in Figure 21-17.
In the other, the
a-
and ^-tubulin units are staggered
a half-unit apart, yielding a checkerboard pattern. The
strongest bonds in the structure are clearly along the axis
of the filament. In depolymerizing conditions, the disso-
ciating ends of the microtubules have a frayed appearance
due to the separation of protomers from one another.
The
tubulins
are globular proteins with a mean diame-
ter of 4 nm. They are about 450 amino acids long, (M.W.
50,000). The human genome contains 15-20 genes for
both the
a-
and /i-forms. Of these, probably five or six
of each are expressed in a tissue, developmental, or cell
cycle-dependent manner, and the rest are pseudogenes.
Tubulin synthesis is regulated at the level of transcription
and translation. Monomeric tubulin binds to ribosomes
during tubulin synthesis and causes degradation of tubu-
lin mRNA, thus providing a negative feedback on tubulin
