Last modified 28th April '95 © Birkbeck College 1995

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Larger Aggregations

The Cytoskeleton

A cell's shape, movement and internal organisation is dependen upon its cytoskeleton , a complex network of proteins arranged in filamentous structures. Two major types are microfilaments (also known as actin filaments ) and microtubules . Both of these structures are aggregations of large sumbers of globular protein subunits, which can polymerise and depolymerise rapidly. They can exhibit "treadmilling", whereby they polymerise at one end of the filament/tubule and depolymerise at the other. Intermediate filaments on the other hand are examples of fibrous proteins.

Actin Filaments

Actin has already been introduced in a previous chapter . The globular monomeric form is called G-actin, and has a bound calcium ion and a bound ATP molecule, which is hydrolyzed when the monomers polymerize to form F-actin . Electron micrographs indicate the parallel double-helical form of these filaments. In each turn of the helix involves 13.5 monomers, and is 360Å long. The diameter is approximately 70Å. The structure is indicated below.

Actin filament. Each sphere represents a globular actin monomer.

Actin filaments are the major component of thin filaments of muscle tissue. Each group of seven monomers in one strand of the helix has a bound molecule of tropomyosin , and one bound molecule of each of three troponin peptides: Tn-T, Tn-I and Tn-C :


Microtubules are composed of dimers of the protein tubulin. The heterodimer consists of one alpha-tubulin and one beta-tubulin subunit; the two are homologous. The dimers are aligned in rows called protofilaments; thirteen of these parallel protofilaments in a cylindrical arrangement form the microtubule. Each protofilament is staggered with respect to its neighbours, so that there is a slant to the side of the tubule. The diameter of the tubule is approximately 240Å.

A doublet microtubule consists of 23 protofilaments in a figure-of-eight arrangement. A ring of nine doublet microtubules, with two singlet microtubules at the centre, forms the basis of cilia and flagella .


Because the protein coat of a virus must enclose the nucleic acids which encode it, the coat must consist of a large number of one or a few kinds of protein subunit. Cylindrical or spherical arrangements are the most likely: in fact, all small viruses are rods, spheres, or a combination of the two. Cylindrical coats exhibit helical symmetry, while the spherical structures exhibit icosahedral symmetry. An extensive amount of material on the structure of viral protein coats is to be found in the Institute for Molecular Virology server at Wisconsin-Madison. In particular, there is a section on visualization of virus proteins . Examine the material on the various types of symmetry exhibited in a typical icosahedral virus (CCMV).


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J. Walshaw