Essential Cell Biology 5th edition

A:44 Answers
tubulin subunits in adjacent protofilaments forming the
ringlike cross section). Thus, to initiate a microtubule from
scratch, enough tubulin dimers have to come together, and
remain bound to one another for long enough, for other
tubulin molecules to add to them. Only when a number
of tubulin dimers have already assembled will the binding
of the next subunit be favored. The formation of these
initial “nucleating sites” is therefore rare and does not
occur spontaneously at cellular concentrations of tubulin.
Centrosomes contain preassembled rings of γ-tubulin (in
which the γ-tubulin subunits are held together in much
tighter side-to-side interactions than αβ-tubulin can form)
to which αβ-tubulin dimers can bind. The binding conditions
of αβ-tubulin dimers resemble those of adding to the end
of an assembled microtubule. The γ-tubulin rings in the
centrosome can therefore be thought of as permanently
preassembled nucleation sites.
ANSWER 17–3
A. The microtubule is shrinking because it has lost its
GTP cap; that is the tubulin subunits at its end are all
in their GDP-bound form. GTP-loaded tubulin subunits
from solution will still add to this end, but they will be
short-lived—either because they hydrolyze their GTP
or because they fall off as the microtubule rim around
them disassembles. If, however, sufficient GTP-loaded
subunits are added quickly enough to cover up the GDPcontaining
tubulin subunits at the microtubule end, a
new GTP cap can form and regrowth is favored.
B. The rate of addition of GTP-tubulin will be greater at
higher tubulin concentrations. The frequency with which
shrinking microtubules switch to the growing mode will
therefore increase with increasing tubulin concentration.
The consequence of this regulation is that the system is
self-balancing: the more microtubules shrink (resulting
in a higher concentration of free tubulin), the more
frequently microtubules will start to grow again.
Conversely, the more microtubules grow, the lower the
concentration of free tubulin will become and the rate of
GTP-tubulin addition will slow down; at some point, GTP
hydrolysis will catch up with new GTP-tubulin addition,
the GTP cap will be destroyed, and the microtubule will
switch to the shrinking mode.
C. If only GDP were present, microtubules would continue
to shrink and eventually disappear, because tubulin
dimers with GDP have very low affinity for each other
and will not add stably to microtubules.
D. If GTP is present but cannot be hydrolyzed, microtubules
will continue to grow until all free tubulin subunits have
been used up.
ANSWER 17–4 If all the dynein arms were equally active,
there could be no significant relative motion of one
microtubule to the other as required for bending. (Think of a
circle of nine weightlifters, each trying to lift his neighbor off
the ground: if they all succeeded, the group would levitate!).
Thus, a few ciliary dynein molecules must be activated
selectively on one side of the cilium. As they move their
neighboring microtubules toward the tip of the cilium, the
cilium bends away from the side containing the activated
dyneins.
ANSWER 17–5 Any actin-binding protein that stabilizes
complexes of two or more actin monomers without blocking
the ends required for filament growth will facilitate the
initiation of a new filament (nucleation).
ANSWER 17–6 Only fluorescent actin molecules assembled
into filaments are visible, because unpolymerized actin
molecules diffuse so rapidly that they produce a dim,
uniform background. Since, in your experiment, so few
actin molecules are labeled (1:10,000), there should be
at most one labeled actin monomer per filament (see
Figure 17−30). The lamellipodium as a whole has many
actin filaments, some of which overlap, and it therefore
shows a random, speckled pattern of actin molecules, each
marking a different filament. This technique (called “speckle
fluorescence”) can be used to follow the movement of
polymerized actin in a migrating cell. If you watch this
pattern with time, you will see that individual fluorescent
spots move steadily back from the leading edge toward
the interior of the cell, a movement that occurs whether
or not the cell is actually migrating. Rearward movement
takes place because actin monomers are added to filaments
at the plus end and are lost from the minus end (where
they are depolymerized) (see Figure 17−35B). In effect,
actin monomers “move through” the actin filaments, a
phenomenon termed “treadmilling.” Treadmilling has been
demonstrated to occur in isolated actin filaments in solution
and also in dynamic microtubules, such as those within a
mitotic spindle.
ANSWER 17–7 Cells contain actin-binding proteins that
bundle and cross-link actin filaments (see Figure 17−32). The
filaments extending the lamellipodia and filopodia are firmly
anchored in the filamentous meshwork of the cell cortex,
thus providing the mechanical anchorage required for the
growing rodlike filaments to deform the cell membrane.
ANSWER 17–8 Although the subunits are indeed held
together by noncovalent bonds that are individually weak,
there are a very large number of them, distributed among
a very large number of filaments. As a result, the stress a
human being exerts by lifting a heavy object is dispersed
over so many subunits that their interaction strength is not
exceeded. By analogy, a single thread of silk is not nearly
strong enough to hold a human, but a rope woven of such
fibers is.
ANSWER 17–9 Both filaments are composed of subunits
in the form of protein dimers that are held together by
coiled-coil interactions. Moreover, in both cases, the dimers
polymerize through their coiled-coil domains into filaments.
Whereas intermediate filament dimers assemble head-tohead,
however, and thereby create a filament that has no
polarity, all myosin molecules in the same half of the myosin
filament are oriented with their heads pointing in the same
direction. This polarity is necessary for them to be able to
develop a contractile force in muscle.
ANSWER 17–10
A. Successive actin molecules in an actin filament are
identical in position and conformation. After a first
protein (such as troponin) has bound to the actin
filament, there would be no way in which a second
protein could recognize every seventh monomer in a
naked actin filament. Tropomyosin, however, binds along
the length of an actin filament, spanning precisely seven
monomers, and thus provides a molecular “ruler” that
measures the length of seven actin monomers. Troponin