Essential Cell Biology 5th edition

586 CHAPTER 17 Cytoskeleton
QUESTION 17–3
Dynamic instability causes
microtubules either to grow or to
shrink rapidly. Consider an individual
microtubule that is in its shrinking
phase.
A. What must happen at the end
of the microtubule in order for it to
stop shrinking and to start growing
again?
B. How would a change in the
tubulin concentration affect this
switch?
C. What would happen if only GDP,
but no GTP, were present in the
solution?
D. What would happen if the
solution contained an analog of GTP
that cannot be hydrolyzed?
Figure 17–19 Motor proteins move
along microtubules using their globular
heads. (A) Kinesins and cytoplasmic
dyneins are microtubule motor proteins
that generally move in opposite directions
along a microtubule. Most kinesins move
toward the plus end of a microtubule,
whereas dyneins move toward the minus
end (Movie 17.4). Each of these proteins
(drawn here roughly to scale) is a dimer
composed of two identical subunits. Each
dimer has two globular heads at one end,
which bind and hydrolyze ATP and interact
with microtubules, and a single tail at the
other end, which interacts with cargo,
either directly or indirectly through adaptor
proteins (see Figure 17–20). (B) Schematic
diagram of a kinesin motor protein “walking”
hand-over-hand along a filament. The two
heads use the energy of ATP binding and
hydrolysis to move in one direction along
the filament (ATP in red, ADP in pink). As
shown here, ATP hydrolysis and phosphate
release by the rear motor head loosens
its attachment to the microtubule. ADP
release and ATP binding by the front motor
head then cause a dramatic conformational
change that flips the rear motor head to the
front, thereby completing a single step. (See
also Figure 17–23B.)
others link microtubules to other cell components, including the other
types of cytoskeletal filaments (see Figure 17–9). Still others are motor
proteins that actively transport organelles, vesicles, and other macromolecules
along microtubules, as we discuss next.
Motor Proteins Drive Intracellular Transport
If a living cell is observed in a light microscope, its cytoplasm is seen
to be in continual motion. Mitochondria and the smaller membraneenclosed
organelles and vesicles travel in small, jerky steps—moving for
a short period, stopping, and then moving again. This saltatory movement
is much more sustained and directional than the continual, small,
Brownian movements caused by random thermal motions. Saltatory
movements can occur along either microtubules or actin filaments. In
both cases, the movements are driven by motor proteins, which use the
energy derived from repeated cycles of ATP hydrolysis to travel steadily
along the microtubule or actin filament in a single direction (see Figure
4–50). Because the motor proteins also attach to other cell components,
they can transport this cargo along the filaments.
The motor proteins that move along cytoplasmic microtubules, such as
those in the axon of a nerve cell, belong to two families: the kinesins
generally move toward the plus end of a microtubule (outward from
the cell body in Figure 17–17); the dyneins move toward the minus end
(toward the cell body in Figure 17–17). Kinesins and cytoplasmic dyneins
are generally dimers that have two globular ATP-binding heads and a
single tail (Figure 17–19A); members of a second class of dyneins, the
ciliary dyneins, have a different structure and will be discussed later. The
heads of kinesin and cytoplasmic dynein interact with microtubules in a
stereospecific manner, so that the motor protein will attach to a microtubule
in only one direction. The tail of a motor protein generally binds
stably to some cell component, such as a vesicle or an organelle, and
thereby determines the type of cargo that the motor protein can transport
(Figure 17–20). The globular heads of kinesin and dynein are enzymes
with ATP-hydrolyzing (ATPase) activity. This reaction provides the energy
for driving a directed series of conformational changes in the head that
enable the motor protein to move along the microtubule by a cycle of
binding, release, and rebinding to the microtubule (Figure 17–19B and
see Figure 4−50). For a discussion of the discovery and study of motor
proteins, see How We Know, pp. 588–589.
cytoplasmic
dynein
minus end
(A)
tail
globular
head
microtubule
10 nm
kinesin
plus end
ATP
(B)
ADP
P
ADP
ATP