Essential Cell Biology 5th edition

580 CHAPTER 17 Cytoskeleton
Figure 17–10 Protein complexes bridge
the nucleus and cytoplasm through
the nuclear envelope. The cytoplasmic
cytoskeleton is connected across the
nuclear envelope to the nuclear lamina
or chromosomes through sets of linker
proteins of the SUN (orange) and KASH
(purple) families.
outer
nuclear
membrane
microtubule
nuclear
pore
motor
proteins
plectin
PERINUCLEAR
SPACE
actin
CYTOSOL
KASH-domain
proteins
SUN-domain
proteins
inner
nuclear
membrane
nuclear lamina
NUCLEUS
chromatin
(A)
(B)
(C)
10 µm
centrosome
NONDIVIDING CELL
poles of mitotic spindle
DIVIDING CELL
cilium
MICROTUBULES
Microtubules have a crucial ECB5 organizing m16.72/17.10 role in all eukaryotic cells. These
long and relatively stiff, hollow tubes of protein can rapidly disassemble
in one location and reassemble in another. In a typical animal cell,
microtubules grow out from a small structure near the center of the cell
called the centrosome (Figure 17–11A and B). Extending out toward the
cell periphery, they create a system of tracks within the cell, along which
vesicles, organelles, and other cell components can be transported.
These cytoplasmic microtubules are the part of the cytoskeleton mainly
responsible for transporting and positioning membrane-enclosed organelles
within the cell and for guiding the intracellular transport of various
cytosolic macromolecules.
When a cell enters mitosis, the cytoplasmic microtubules disassemble
and then reassemble into an intricate structure called the mitotic spindle.
As we discuss in Chapter 18, the mitotic spindle provides the machinery
that will segregate the chromosomes equally into the two daughter cells
just before a cell divides (Figure 17–11C). Microtubules can also form
stable structures, such as rhythmically beating cilia and flagella (Figure
17–11D). These hairlike structures extend from the surface of many
eukaryotic cells, which use them either to swim or to sweep fluid over
their surface. The core of a eukaryotic cilium or flagellum consists of a
highly organized and stable bundle of microtubules. (Bacterial flagella
have an entirely different structure and allow the cells to swim by a very
different mechanism.)
In this section, we first consider the structure and assembly of microtubules.
We then discuss their role in organizing the cytoplasm—an ability
that depends on their association with accessory proteins, especially the
motor proteins that propel organelles along cytoskeletal tracks. Finally,
we discuss the structure and function of cilia and flagella, in which microtubules
are stably associated with motor proteins that power the beating
of these mobile appendages.
(D)
basal body
CILIATED CELL
Figure 17–11 Microtubules usually grow out from an organizing
center. (A) Fluorescence micrograph of a cytoplasmic array of
microtubules in a cultured fibroblast. Unlike intermediate filaments,
microtubules (green) extend from organizing centers such as (B) a
centrosome, (C) the two poles of a mitotic spindle, or (D) the basal
body of a cilium. They can also grow from fragments of existing
microtubules (not shown). (A, reprinted with permission from Olympus
Corporation of the Americas Scientific Solutions Group.)