Essential Cell Biology 5th edition

592 CHAPTER 17 Cytoskeleton
Figure 17–28 The movement of dynein
causes the flagellum to bend. (A) If the
outer doublet microtubules and their
associated dynein molecules are freed from
other components of a sperm flagellum and
then exposed to ATP, the doublets slide
against each other, telescope-fashion, due
to the repetitive action of their associated
dyneins. (B) In an intact flagellum, however,
the doublets are tied to each other by
flexible protein links so that the action of
the system produces bending rather than
sliding.
plus end
+ATP
plus end
linking
proteins
+ATP
minus end
minus end
(A)
IN ISOLATED DOUBLET
MICROTUBULES: DYNEIN
PRODUCES
MICROTUBULE SLIDING
(B)
IN A NORMAL
FLAGELLUM: DYNEIN
CAUSES MICROTUBULE
BENDING
In humans, hereditary defects in ciliary dynein cause Kartagener’s syndrome.
Men with this disorder are infertile because their sperm are
nonmotile, and they have an increased susceptibility to bronchial infections
because the cilia that line their respiratory tract are paralyzed and
thus unable to clear bacteria and debris from the lungs.
QUESTION 17–4
Dynein arms in a cilium are arranged
so that, when activated, the heads
push their neighboring outer
doublet outward toward the tip of
the cilium. Consider a cross section
of a cilium (see Figure 17−27). Why
would no bending motion of the
cilium result if all dynein molecules
were active at the same time?
What pattern of dynein activity can
account for the bending of a cilium
in one direction?
Many animal cells that lack beating cilia contain a single, nonmotile primary
cilium. This appendage is much shorter than a beating cilium and
functions as an antenna for sensing certain extracellular signal molecules.
ECB5 e17.27/17.28
ACTIN FILAMENTS
Actin filaments, polymers of the protein actin, are present in most
eukaryotic cells and are essential for many of the cell’s movements,
especially those involving the cell surface. Without actin filaments, for
example, an animal cell could not crawl along a surface, engulf a large
particle by phagocytosis, or divide in two. Like microtubules, many actin
filaments are unstable, but by associating with other proteins they can
also form stable structures in cells, such as the contractile apparatus of
muscle cells. Actin filaments interact with a large number of actin-binding
proteins that enable the filaments to serve a variety of functions in cells.
Depending on which of these proteins they associate with, actin filaments
can form stiff and stable structures, such as the microvilli on the epithelial
cells lining the intestine (Figure 17–29A) or the small contractile bundles
that can contract and act like tiny muscles in most animal cells (Figure
17–29B and E). They can also form temporary structures, such as the
dynamic protrusions formed at the leading edge of a crawling cell (Figure
17–29C) or the contractile ring that pinches the cytoplasm in two when an
animal cell divides (Figure 17–29D). Actin-dependent movements usually
require actin’s association with a motor protein called myosin.
In this section, we see how the arrangements of actin filaments in a cell
depend on the types of actin-binding proteins present. Even though actin
filaments and microtubules are formed from unrelated types of subunit
proteins, we will see that the principles by which they assemble and disassemble,
control cell structure, and work with motor proteins to bring
about movement are strikingly similar.