The Cytoskeleton

• establishing cell shape
• providing mechanical strength
• locomotion
• chromosome separation in mitosis and meiosis
• intracellular transport of organelles

• Actin filaments (also called microfilaments)
• Intermediate filaments and
• Microtubules

Actin Filaments

• form a band just beneath the plasma membrane that
  • provides mechanical strength to the cell
  • links transmembrane proteins (e.g., cell surface receptors) to cytoplasmic proteins
  • anchors the centrosomes at opposite poles of the cell during mitosis
  • pinches dividing animal cells apart during cytokinesis
• generate cytoplasmic streaming in some cells
• generate locomotion in cells such as white blood cells and the amoeba
• interact with myosin ("thick") filaments in skeletal muscle fibers to provide the force of muscular contraction

Intermediate Filaments

These cytoplasmic fibers average 10 nm in diameter (and thus are "intermediate" in size between actin filaments (8 nm) and microtubules (25 nm)(as well as of the thick filaments of skeletal muscle fibers).

There are several types of intermediate filament, each constructed from one or more proteins characteristic of it.

• keratins are found in epithelial cells and also form hair and nails;
• nuclear lamins form a meshwork that stabilizes the inner membrane of the nuclear envelope;
• neurofilaments strengthen the long axons of neurons;
• vimentins provide mechanical strength to muscle (and other) cells.

Despite their chemical diversity, intermediate filaments play similar roles in the cell: providing a supporting framework within the cell. For example, the nucleus in epithelial cells is held within the cell by a basketlike network of intermediate filaments made of keratins. (photo at right)

In the photo (courtesy of Mary Osborn), a fluorescent stain has been used to show the intermediate filaments of keratin in two epithelial cells. Note the basketlike arrangement of filaments around each nucleus.

Different kinds of epithelia use different keratins to build their intermediate filaments. Over 20 different kinds of keratins have been found, although each kind of epithelial cell may use no more than 2 of them. Up to 85% of the dry weight of squamous epithelial cells can consist of keratins.

Microtubules

• are straight, hollow cylinders whose wall is made up of a ring of 13 "protofilaments";
• have a diameter of about 25 nm;
• are variable in length but can grow 1000 times as long as they are wide;
• are built by the assembly of dimers of alpha tubulin and beta tubulin;
• are found in both animal and plant cells.

• grow at each end by the polymerization of tubulin dimers (powered by the hydrolysis of GTP), and
• shrink at each end by the release of tubulin dimers (depolymerization).

However, both processes always occur more rapidly at one end, called the plus end. The other, less active, end is the minus end.

Microtubules participate in a wide variety of cell activities. Most involve motion. The motion is provided by protein "motors" that use the energy of ATP to move along the microtubule.

Microtubule motors

• kinesins (most of these move toward the plus end of the microtubules) and
• dyneins (which move toward the minus end).

• The rapid transport of organelles, like vesicles and mitochondria, along the axons of neurons takes place along microtubules with their plus ends pointed toward the end of the axon. The motors are kinesins.

  Charcot-Marie-Tooth disease. One cause of this rare disorder is an inherited mutated gene for one of the kinesins. In these patients, axonal transport is defective (which probably accounts for their muscle weakness first occurring in muscles at the ends of the longer motor neurons).

• The migration of chromosomes in mitosis and meiosis takes place on microtubules that make up the spindle fibers. Both kinesins and dyneins are used as motors [Link].

In plant cells, microtubules are created at many sites scattered through the cell. In animal cells, the microtubules originate at the centrosome.

Cilia and Flagella

Cilia and flagella are built from arrays of microtubules. They are discussed on a separate page. Link to it.