18 anization and Movement II英文.pdf

Chanpter 18 Cell Organization and Movement II: Microtubules and
Intermediate Filaments

Introduction

Newt lung cell in mitosis stained for centrosomes (magenta), microtubules (green), chromosomes (blue), keratin intermediate
filaments (red). [Courtesy of A. Khodjakor, from Nature 408:423–424 (2000).]
Three types of filaments make up the animal-cell cytoskeleton: microfilaments, microtubules, and
intermediate filaments. Why have these three distinct types of filaments evolved? It seems likely that
their physical properties are suited to different functions. In Chapter 17 we described how actin
filaments are often cross-linked into networks of bundles to form flexible and dynamic structures and
to serve as tracks for the many different classes of myosin motors. Microtubules are stiff tubes that
can exist as a single structure extending up to 20 μm in cells or as the bundled structures as seen in
cilia and flagella. A consequence of their tubular design is the ability of microtubules to generate
pulling and pushing forces without buckling, a property that allows single tubules to extend large
distances within a cell and bundles to slide past each other, as occurs in flagella and in the mitotic
spindle. Microtubules' ability to extend long distances in the cell, together with their intrinsic polarity,
is exploited by microtubule-dependent motors, which use microtubules as tracks for long-range
transport of organelles. Microtubules can be highly dynamic—being assembled and disassembled
from their ends—providing the cell with the flexibility to reorganize microtubule organization as
needed.
In contrast to microfilaments and microtubules, intermediate filaments have great tensile strength
and have evolved to withstand much larger stresses and strains. With properties akin to strong
molecular ropes, they are ideally suited to endow both cells and tissues with structural int