The Cytoskeleton

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Cells contain elaborate arrays of protein fibers that serve such functions as: The cytoskeleton is made up of three kinds of protein filaments:

Actin Filaments

Monomers of the protein actin polymerize to form long, thin fibers. These are about 8 nm in diameter and, being the thinnest of the cytoskeletal filaments, are also called microfilaments. (In skeletal muscle fibers they are called "thin" filaments.) Some functions of actin filaments:

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.

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 Microtubules

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

There are two major groups of microtubule motors: Some examples:

Cilia and Flagella

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

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7 December 2013