Essential Cell Biology 5th edition

Intermediate Filaments
575
INTERMEDIATE FILAMENTS
Intermediate filaments have great strength, and their main function
is to enable cells to withstand the mechanical stress that occurs when
cells are stretched. The filaments are called “intermediate” because, in
the smooth muscle cells where they were first discovered, their diameter
(about 10 nm) is between that of the thinner actin filaments and
the thicker myosin filaments. Intermediate filaments are the toughest and
most durable of the cytoskeletal filaments: when cells are treated with
concentrated salt solutions and nonionic detergents, the intermediate
filaments survive, while most of the rest of the cytoskeleton is destroyed.
Intermediate filaments are found in the cytoplasm of most animal cells.
They typically form a network throughout the cytoplasm, surrounding
the nucleus and extending out to the cell periphery. There, they are
often anchored to the plasma membrane at cell–cell junctions called
desmosomes (discussed in Chapter 20), where the plasma membrane is
connected to that of another cell (Figure 17–3). Intermediate filaments
are also found within the nucleus of animal cells. There, they form a
meshwork called the nuclear lamina, which underlies and strengthens the
nuclear envelope. In this section, we see how the structure and assembly
of intermediate filaments makes them particularly suited to strengthening
cells and protecting them from tearing.
Intermediate Filaments Are Strong and Ropelike
An intermediate filament is like a rope in which many long strands are
twisted together to provide tensile strength—an ability to withstand tension
without breaking (Movie 17.1). The strands of this cable are made
of intermediate filament proteins, fibrous subunits each containing a central
elongated rod domain with distinct unstructured domains at either
bundles of
intermediate
filaments
desmosome
connecting
two cells
(A)
10 µm
(B)
5 µm
Figure 17–3 Intermediate filaments form a strong, durable network in the cytoplasm of the cell. (A) Immunofluorescence
micrograph of a sheet of epithelial cells in culture stained to show the lacelike network of intermediate keratin filaments (blue), which
surround the nuclei and extend through the cytoplasm of the cells. The filaments in each cell are indirectly connected to those of
neighboring cells through the desmosomes, establishing a continuous mechanical link from cell to cell throughout the epithelial sheet.
A second protein (red ) has been stained to show the locations of the cell boundaries. (B) Drawing from an electron micrograph of a
section of a skin cell, showing the bundles of intermediate filaments that traverse the cytoplasm and are inserted at desmosomes.
(A, from K.J. Green and C.A. Gaudry, Nat. Rev. Mol. Cell. Biol. 1:208–216, 2000. With permission from Macmillan Publishers Ltd; B, from
R.V. Krstić, Ultrastructure of the Mammalian Cell: An Atlas. Berlin: Springer, 1979. With permission from Springer-Verlag.)