Last modified 5th Jun '95 © Birkbeck College 1995
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This section describes an example of how the form of structural proteins
relates to motion: namely the contraction of skeletal muscle .
Other examples to investigate include:
Skeletal muscles (also called striated or striped muscle)
are only one type of muscle tissue occurring in vertebrates. They are generally
under voluntary control.
The other two are (i) cardiac (i.e. heart) muscle, which is specialized
but resembles skeletal muscle in many respects, and (ii) smooth muscle
which is generally controlled involuntarily by the autonomic nervous
- the beating of cilia and flagella, which are based on
- the movement of chromosomes, the precise coordination of which is
essential during cell division; this also involves
- cell movement and determination of cell shape- many aspects of
which involve interactions of membrane
proteins with the extracellular matrix (composed of glycoproteins,
fibrous proteins, polysaccharides)
The Structure of Skeletal Muscle
Muscles, Myofibres and Myofibrils
Skeletal muscle cells are highly specialized. They are called myofibres,
cylindrical in shape, 0.01 - 0.05 mm in diameter and 1 - 40 mm long. These
multinucleate cells consist of a bundle of myofibrils surrounded
by a plasma membrane. A muscle consists of a bundle of myofibres.
A myofibril consists of repeating identical units called sarcomeres.
Regular repeating formations of two types of protein filaments are the basis of
The structure of the sarcomere is described in the
- thin filaments- these consist mostly of
actin , with tropomyosin
and troponin; these have already been described in
an earlier chapter.
- thick filaments -composed of the protein myosin
Structure of Myosin
A single myosin molecule consists of two heavy chains and four light chains. It
is effectively a dimer of two heterotrimers, each of which consists of two
different light chains (approximately 20 kD in mass) and a single heavy chain
(230 kD). The latter has a globular head and a long alpha-helical tail. In fact
the two tails of the
complete molecule form a parallel coiled-coil, so that myosin consists of a long
(1500 Å) fibre, 20 Å thick, with a two-headed globular end.
There is a hinge region between each head and the tail section.
Here is a diagram.
A myosin molecule has three functions:
The two different structural domains are responsible for different functions,
as is revealed by treatment of myosin with proteases to give different
subunits. Cleavage of myosin with trypsin gives two products:
- binding to other myosin molecules to form filaments; this occurs
spontaneously in physiological conditions. At high ionic strengths, myosin
exists as individual molecules.
- binding to actin filaments
- it is an ATPase, i.e. it hydrolyzes ATP to give ADP and a phosphate
Treatment of HMM with the protease papain cleaves the two globular heads from
the tail section (S2). The two heads (termed S1) are not
surprisingly found to be the site of ATPase activity; they also bind to actin
- light meromyosin (LMM), an 850Å coiled-coil, i.e. a large
section of the myosin "tail". LMM aggregates to form filaments, but does
not bind to actin filaments, and does not hydrolyze ATP.
- heavy meromyosin (HMM), which consists of the globular heads and
a shorter section of tail. It does not aggregate to form filaments, but it
hydrolyzes ATP and binds to thin filaments.
Click here for the crystal structure (C-alpha atoms only) of a proteolytic
fragment of myosin from chicken muscle. This fragment consists of an entire
S1 head (843 residues) and two light chains.
This fragment (all
atoms) contains two light chains and 60 residues of the heavy chain.
Here is a diagram.
on to Page 2
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