Last modified 5th Jun '95 © Birkbeck College 1995

Muscle Fibres

Introduction

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:

the beating of cilia and flagella, which are based on microtubules
the movement of chromosomes, the precise coordination of which is essential during cell division; this also involves microtubules
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)

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 system .

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.

Myofibrils

A myofibril consists of repeating identical units called sarcomeres. Regular repeating formations of two types of protein filaments are the basis of the sarcomere:

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

The structure of the sarcomere is described in the -->

next page.

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.

myosin icon Here is a diagram.

A myosin molecule has three functions:

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 (Pi).

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:

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.

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 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.
thick filament icon Here is a diagram.

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J. Walshaw