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
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 .
Viruses
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).
References
- Amos, L.A. (1985) Structure of muscle filaments studied by electron
microscopy, Ann. Rev. Biophys. Biophys. Chem. 14, 291-313
- Cooke, R. (1986) Mechanism of muscle contraction, CRC Crit. Rev.
Biochem. 21 53-118
- Darnell, J., Lodish, H. and Baltimore, D. (1986) Molecular Cell Biology,
W.H. Freeman & Co., New York, Chapters 18,19
- Dustin, P. (1980) Microtubules, Sci. Am. 243(2), 67-76
- Phillips, G.N., Fillers, J.P. and Cohen, C. (1986) Tropomyosin crystal
structure and muscle regulation, J. Mol. Biol. 192, 111-131
- Pollard, T.D. and Cooper, J.A. (1986) Actin and actin-binding proteins. A
critical evaluation of mechanisms and functions, Ann. Rev. Biochem
55, 987-1035
- Timasheff, S.N. and Grisham, L.M. (1980) In vitro assembly of
cytoplasmic microtubules, Ann. Rev. Biochem. 49 565-591
- Voet, D. and Voet, J.G. (1990) Biochemistry, John Wiley & Sons,New York
pp. 1118-1139
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J. Walshaw
