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

942 Chapter 16: The Cytoskeleton
(A)
50 nm
(B)
100 nm
Molecules of axonemal dynein form bridges between the neighboring doublet
microtubules around the circumference of the axoneme (Figure 16–64). When
the motor domain of this dynein is activated, the dynein molecules attached to
one microtubule doublet (see Figure 16–59) attempt to walk along the adjacent
microtubule doublet, tending to force the adjacent doublets to slide relative to
one another, much as actin thin filaments slide during muscle contraction. However,
the presence of other links between the microtubule doublets prevents this
sliding, and the dynein force is instead converted into a bending motion (Figure
16–65).
In humans, hereditary defects in axonemal MBoC6 dynein m16.82/16.66 cause a condition called
primary ciliary dyskinesia or Kartagener’s syndrome. This syndrome is characterized
by inversion of the normal asymmetry of internal organs (sinus inversus) due
to disruption of fluid flow in the developing embryo, male sterility due to immotile
sperm, and a high susceptibility to lung infections due to paralyzed cilia being
unable to clear the respiratory tract of debris and bacteria.
Bacteria also swim using cell-surface structures called flagella, but these do
not contain microtubules or dynein and do not wave or beat. Instead, bacterial
flagella are long, rigid helical filaments, made up of repeating subunits of the protein
flagellin. The flagella rotate like propellers, driven by a special rotary motor
embedded in the bacterial cell wall. The use of the same name to denote these two
very different types of swimming apparatus is an unfortunate historical accident.
Primary Cilia Perform Important Signaling Functions in Animal Cells
Many cells possess a shorter, nonmotile counterpart of cilia and flagella called
the primary cilium. Primary cilia can be viewed as specialized cellular compartments
or organelles that perform a wide range of cellular functions, but share
Figure 16–64 Ciliary dynein. Ciliary
(axonemal) dynein is a large protein
assembly (nearly 2 million daltons)
composed of 9–12 polypeptide chains, the
largest of which is the heavy chain of more
than 500,000 daltons. (A) The heavy chains
form the major portion of the globular
head and stem domains, and many of the
smaller chains are clustered around the
base of the stem. There are two heads
in the outer dynein in metazoans (shown
here), but three heads in protozoa, each
formed from their own heavy chain. The
tail of the molecule binds tightly to an
A microtubule, while the large globular
heads have an ATP-dependent binding
site for a B microtubule (see Figure 16–63).
When the heads hydrolyze their bound ATP,
they move toward the minus end of the
B microtubule, thereby producing a sliding
force between the adjacent microtubule
doublets in a cilium or flagellum (see
Figure 16–59). (B) Freeze-etch electron
micrograph of a cilium showing the dynein
arms projecting at regular intervals from
the doublet microtubules. (B, courtesy of
John Heuser.)
+
+ +
+
+ +
linking
proteins
+
+
(A)
– –
+ATP
–
–
(B)
– –
bend
–
–
Figure 16–65 The bending of an
axoneme. (A) When axonemes are
exposed to the proteolytic enzyme trypsin,
the linkages holding neighboring doublet
microtubules together are broken. In this
case, the addition of ATP allows the motor
action of the dynein heads to slide one
pair of doublet microtubules against the
other pair. (B) In an intact axoneme (such
as in a sperm), flexible protein links prevent
the sliding of the doublet. The motor
action therefore causes a bending motion,
creating waves or beating motions.