Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

MICROTUBULES
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many structural features with motile cilia. Both motile and nonmotile cilia are
generated during interphase at plasma-membrane-associated structures called
basal bodies that firmly root them at the cell surface. At the core of each basal body
is a centriole, the same structure found embedded at the center of animal centrosomes,
with nine groups of fused triplet microtubules arranged in a cartwheel
(see Figure 16–48). Centrioles are multifunctional, contributing to assembly of the
mitotic spindle in dividing cells but migrating to the plasma membrane of interphase
cells to template the nucleation of the axoneme (Figure 16–66). Because
no protein translation occurs in cilia, construction of the axoneme requires intraflagellar
transport (IFT), a transport system discovered in the green algae Chlamydomonas.
Analogous to the axon, motors move cargoes in both anterograde and
retrograde directions, in this case driven by kinesin-2 and cytoplasmic dynein 2,
respectively.
Primary cilia are found on the surface of almost all cell types, where they sense
and respond to the exterior environment, functions best understood in the context
of smell and sight. In the nasal epithelium, cilia protruding from dendrites of
olfactory neurons are the site of both odorant reception and signal amplification.
Similarly, the rod and cone cells of the vertebrate retina possess a primary cilium
equipped with an expanded tip called the outer segment, which is specialized for
converting light into a neural signal (see Figure 15–38). Maintenance of the outer
segment requires continuous IFT-mediated transport of large quantities of lipids
and proteins into the cilium, at rates of up to 2000 molecules per minute. The links
between cilia function and the senses of sight and smell are underscored by Bardet-Biedl
syndrome, a set of disorders associated with defects in IFT, the cilium, or
the basal body. Patients with Bardet-Biedl syndrome cannot smell and suffer from
retinal degeneration. Other characteristics of this multifaceted disorder include
hearing loss, polycystic kidney disease, diabetes, obesity, and polydactyly, suggesting
that primary cilia have functions in many aspects of human physiology.
Summary
Microtubules are stiff polymers of tubulin molecules. They assemble by addition of
GTP-containing tubulin subunits to the free end of a microtubule, with one end (the
plus end) growing faster than the other. Hydrolysis of the bound GTP takes place
primary
cilium
+
primary
cilium
pericentriolar
material
plasma
membrane
basal body
–
mother
centriole
daughter
centriole
+
–
mother
centriole
mitotic spindle
(A)
daughter
centriole
mother
centriole
300 nm
daughter
centriole
(B)
centrosome
Figure 16–66 Primary cilia. (A) Electron micrograph and diagram of the basal body of a mouse neuron primary cilium. The axoneme of the primary
cilium (black arrow) is nucleated by the mother centriole at the basal body, which localizes at the plasma membrane near the cell surface.
(B) Centrioles function alternately as basal bodies and as the core of centrosomes. Before a cell enters the cell division cycle, the primary cilium is
shed or resorbed. The centrioles recruit pericentriolar material and duplicate during S phase, generating two centrosomes, each of which contains
a pair of centrioles. The centrosomes nucleate microtubules and localize to the poles of the mitotic spindle. Upon exit from mitosis, a primary cilium
again grows from the mother centriole. (A, courtesy of Josef Spacek.)