334
10 Proteins of Muscle and the Cytoskeleton
muscle of the molluscs Pecten maximus and Sepia
officinalis [13]. Regulation of the smooth muscle
also probably involves calponin, a protein of 252
or 292 amino acids which belongs to the troponin
C family and inhibits actomyosin ATPase. Ca 2 +
binding has no effect on the inhibitory activity of
calponin, although this disappears after phosphorylation by Ca 2 + -calmodulin-dependent protein
kinase [277, 307].
The absence of an increase in the ATPase activity of a myosin preparation following the addition
of pure, troponin-free vertebrate actin indicates
the presence of M regulation. According to this
test, which was first performed on the adductor
muscle of the scallop, the muscles of molluscs,
nemertines, brachiopods and echinoderms are M
regulated, whereas the skeletal muscles of vertebrates and tunicates are A regulated; most arthropod muscles and the diagonally striated muscles
of nematodes, annelids and sipunculids are both
A and M regulated [238]. However, all three components of the troponin complex have been isolated from the striated adductor muscle of the
mussel Chlamys nipponensis akazara, and thus
the molluscs also appear to possess two different
regulation mechanisms (dual regulation) [207].
There are also indications of dual regulation in
the tunicates. Although a troponin-tropomyosin
complex is present in the smooth muscle of the
ascidian Halocynthia roretzi, and can combine
with the species' own or with mammalian actomyosin to give a calcium-regulated system, the
ATPase of ascidian actomyosin is clearly calcium
dependent without the troponin-tropomyosin
complex, even though this M regulation is of relatively little importance [273].
In many mollusc muscles, such as the smooth
shell adductor and the byssus retractor of the
mussel, there is a special contraction condition,
the catch, the molecular mechanism and regulation of which remain obscure despite many years
of intense effort [238]. The thick filaments in
these muscles have a special structure and contain
large amounts of the muscle protein paramyosin,
which is worthy of further discussion. The asynchronous flight muscles of the insects pose special
problems; these contain a functional tropomyosin-troponin complex but their contraction is
also modulated by a passive expansion. In the
muscle fibres of the water bug Lethocerus, the
large size of which makes them convenient subjects for investigation, Ca 2 + brings about a fourfold increase in ATPase activity, but a subsequent
expansion of 1 % brings a further threefold
increase. The molecular mechanism by which such
expansion influences the ATPase activity and tension development in insect muscle is not known.
Some of the many myofibrillar proteins of insect
muscle are unique to insects, and it may be that
one or other of them is involved in the special
regulatory characteristics of the muscles. The
thick filaments contain varying proportions of
paramyosin: it is higher in the case of insects with
low wing-beat frequencies (in Lethocerus it is
10 %) and lower in those with high beat frequencies (e.g. in flies it is only 2 %). There is also phosphorylation of the light myosin chains of insect
muscles, but the biological significance of this is
not completely clear [238]. In addition to actin
(43 kDa), the thin filaments in the flight muscles
of Drosophila and other insects also contain large
amounts of a similar protein, arthrin (55 kDa),
which is a combination of actin and ubiquitin [10].
10.2 Molecular Heterogeneity
of Skeletal Muscle Fibres
It is not surprising that different muscle types and
non-muscle cells exhibit differences in the function and molecular structure of their contractile
apparatus. However, in vertebrate skeletal
muscle there are, in fact, corresponding differences between single muscle fibres. Thus, in the
mammals one can distinguish between the slow
muscle fibres of class I (slow twitch) and the faster fibres of class II (fast twitch), which differ in
their innervation, the nature of their stimulitransmitting structures, the metabolic provision
of energy, and also the structural and functional
properties of their contractile apparatus. Each
skeletal muscle is made up of a mosaic of different types of fibre in varying proportions, according to the muscle function. All muscle fibres supplied by the same nerve fibre (motor units)
belong to the same class. Following a change in
muscle function, fibres can convert from one type
to the other; for example, during postnatal development of the rat m. soleus, type I changes to
type II. Persistent electrical excitation can effect
the conversion of type II to I [220]. The actinmyosin system of all fibre types, in fact of all
muscle types and non-muscle cells, is basically
very similar; functional differentiation is based
upon variety in the isoforms of protein components, i.e. the actins, myosins, tropomyosins, and
troponins. However, a more detailed examination
shows that the composition, for example, of the
tropomyosin-troponin system in different muscle
10 Proteins of Muscle and the Cytoskeleton
muscle of the molluscs Pecten maximus and Sepia
officinalis [13]. Regulation of the smooth muscle
also probably involves calponin, a protein of 252
or 292 amino acids which belongs to the troponin
C family and inhibits actomyosin ATPase. Ca 2 +
binding has no effect on the inhibitory activity of
calponin, although this disappears after phosphorylation by Ca 2 + -calmodulin-dependent protein
kinase [277, 307].
The absence of an increase in the ATPase activity of a myosin preparation following the addition
of pure, troponin-free vertebrate actin indicates
the presence of M regulation. According to this
test, which was first performed on the adductor
muscle of the scallop, the muscles of molluscs,
nemertines, brachiopods and echinoderms are M
regulated, whereas the skeletal muscles of vertebrates and tunicates are A regulated; most arthropod muscles and the diagonally striated muscles
of nematodes, annelids and sipunculids are both
A and M regulated [238]. However, all three components of the troponin complex have been isolated from the striated adductor muscle of the
mussel Chlamys nipponensis akazara, and thus
the molluscs also appear to possess two different
regulation mechanisms (dual regulation) [207].
There are also indications of dual regulation in
the tunicates. Although a troponin-tropomyosin
complex is present in the smooth muscle of the
ascidian Halocynthia roretzi, and can combine
with the species' own or with mammalian actomyosin to give a calcium-regulated system, the
ATPase of ascidian actomyosin is clearly calcium
dependent without the troponin-tropomyosin
complex, even though this M regulation is of relatively little importance [273].
In many mollusc muscles, such as the smooth
shell adductor and the byssus retractor of the
mussel, there is a special contraction condition,
the catch, the molecular mechanism and regulation of which remain obscure despite many years
of intense effort [238]. The thick filaments in
these muscles have a special structure and contain
large amounts of the muscle protein paramyosin,
which is worthy of further discussion. The asynchronous flight muscles of the insects pose special
problems; these contain a functional tropomyosin-troponin complex but their contraction is
also modulated by a passive expansion. In the
muscle fibres of the water bug Lethocerus, the
large size of which makes them convenient subjects for investigation, Ca 2 + brings about a fourfold increase in ATPase activity, but a subsequent
expansion of 1 % brings a further threefold
increase. The molecular mechanism by which such
expansion influences the ATPase activity and tension development in insect muscle is not known.
Some of the many myofibrillar proteins of insect
muscle are unique to insects, and it may be that
one or other of them is involved in the special
regulatory characteristics of the muscles. The
thick filaments contain varying proportions of
paramyosin: it is higher in the case of insects with
low wing-beat frequencies (in Lethocerus it is
10 %) and lower in those with high beat frequencies (e.g. in flies it is only 2 %). There is also phosphorylation of the light myosin chains of insect
muscles, but the biological significance of this is
not completely clear [238]. In addition to actin
(43 kDa), the thin filaments in the flight muscles
of Drosophila and other insects also contain large
amounts of a similar protein, arthrin (55 kDa),
which is a combination of actin and ubiquitin [10].
10.2 Molecular Heterogeneity
of Skeletal Muscle Fibres
It is not surprising that different muscle types and
non-muscle cells exhibit differences in the function and molecular structure of their contractile
apparatus. However, in vertebrate skeletal
muscle there are, in fact, corresponding differences between single muscle fibres. Thus, in the
mammals one can distinguish between the slow
muscle fibres of class I (slow twitch) and the faster fibres of class II (fast twitch), which differ in
their innervation, the nature of their stimulitransmitting structures, the metabolic provision
of energy, and also the structural and functional
properties of their contractile apparatus. Each
skeletal muscle is made up of a mosaic of different types of fibre in varying proportions, according to the muscle function. All muscle fibres supplied by the same nerve fibre (motor units)
belong to the same class. Following a change in
muscle function, fibres can convert from one type
to the other; for example, during postnatal development of the rat m. soleus, type I changes to
type II. Persistent electrical excitation can effect
the conversion of type II to I [220]. The actinmyosin system of all fibre types, in fact of all
muscle types and non-muscle cells, is basically
very similar; functional differentiation is based
upon variety in the isoforms of protein components, i.e. the actins, myosins, tropomyosins, and
troponins. However, a more detailed examination
shows that the composition, for example, of the
tropomyosin-troponin system in different muscle