fibres is rather a continuum, which is only crudely
reflected by classification into types. In addition, it
should not be forgotten that each skeletal muscle
fibre possesses many nuclei which may exhibit
different patterns of gene expression [242, 258].
There are also characteristic differences between
the fibre types in the enzymes of energy-providing
metabolism, and these are the basis of a more
exact classification, e.g. slow-twitch oxidative
(SO), or fast-twitch glycolytic (FG) and fast-twitch
oxidative glycolytic (FOG), or in the acid stability
of the myosin ATPase (subclasses IIA and lIB)
[220]. The heterogeneity of invertebrate muscle
fibres is less marked. Thus, for example, in the
mussel Spisula solidissima the adductor, foot
muscle, mantle and heart all contain the same
forms of actin, myosin and tropomyosin. On the
other hand, the outer muscle cylinder of the earthworm Lumbricus terrestris has two different types
of fibre which differ in the light myosin chain and
in succinate dehydrogenase activity [55]. The clawclosing muscles of the lobster Homarus americanus contain three types of muscle fibre, which
show qualitative and quantitative differences in
their protein spectra, and are distributed characteristically between the functionally different cutter and crusher claws. Altogether, two paramyosins, three troponin-Ts, five troponin-Is, two
troponin-Cs, three myosin-aLCs and one myosin-~LC can be isolated from these muscles [201].
The existence of many isoforms of almost all
muscle proteins is partly due to the expression of
different genes; multi gene families are known in
particular for the actins, the heavy and light
chains of the myosins, and the tropomyosins. A
further mechanism for the production of protein
heterogeneity is particularly characteristic of the
muscle proteins, namely, the variable splicing of
the primary transcript of one gene. Alternative
splicing has in fact been shown for almost all
muscle proteins: actins, heavy myosin chains,
light myosin chains, tropomyosins, troponin T,
troponin I and calsequestrin. Finally, muscle proteins may be post-translationally modified in different ways. Phosphorylation, for example, is
found in light and heavy myosin chains, troponin I and T, and caldesmon, although the functional meaning is in many instances by no means
clear. Methylation is also very common amongst
the muscle proteins, Monomethyl-Iysine and -histidine, trimethyl-Iysine and histidine have been
demonstrated in myosins [175]; dimethyl- and
trimethyl-Iysine are found in calmodulins and
other calcium-binding proteins [139, 243], as well
as in the actin-binding protein profilin [4].
10.3 Myosins
335
10.3 Myosins
Two fundamentally different types of myosin may
be distinguished. The myosins of type II are
soluble at a salt concentration of 600 mm01ll
and at physiological salt concentrations of
100-250 mm01ll they aggregate to form filaments. The molecules of myosin II have a rodshaped tail of about 150 nm and two globular
heads of about 20 x 7 nm. They are hexamers of
460-500 kDa, consisting of two heavy chains
(HCs) of about 200 kDa and four light chains
(LCs) of 15-27 kDa. The C-terminal ends of both
HCs are twisted into an a-helical coiled-coil and
form the tail region. The head region is made up
of the N-terminal half of each HC together with
one each of the regulatory and essential LCs, and
carries binding sites for actin and adenine nucleotide (Fig. 10.1). The myosin molecule can be
cleaved in various ways by partial proteolysis: the
lighter fragment, LMM, corresponds to the Cterminal two-thirds of the tail, and the heavier
HMM to the rest of the tail and the two heads.
The subfragment designated S-l consists ofthe Nterminal half of an HC, folded into a head, together with the two associated LCs; S-2 is the tail
section between the LMMlHMM border and the
heads. The thick filaments of vertebrate skeletal
muscle each contain several hundred myosin
molecules arranged in a bipolar fashion, such that
the heads of both filament halves point to their
respective ends. There are further proteins in
the thick filaments, e.g. the C protein, found
in several muscle-type-specific isoforms of
135-150 kDa, and the similar X protein. According to the results of X-ray analysis, the molecular
arrangement of the thick filaments of the invertebrates differs somewhat from that of the vertebrates. Myosin I, which does not form filaments,
consists of a single HC of 110-140 kDa and one
or two LCs. As in myosin II, the N-terminal
domain of the HC also forms the head, but the
tail region has a completely different structure.
The function of myosin I in living cells has not
been determined.
Due to the relatively low myosin concentrations in smooth muscle, and especially in nonmuscle cells, the myosins of the vertebrates were
initially investigated in the various types of fibre
found in skeletal and heart muscle. More
recently, the possibilities for cDNA and gene
sequencing have significantly expanded our
knowledge of the myosins of vertebrate smooth
muscle and non-muscle cells as well as of inver-
reflected by classification into types. In addition, it
should not be forgotten that each skeletal muscle
fibre possesses many nuclei which may exhibit
different patterns of gene expression [242, 258].
There are also characteristic differences between
the fibre types in the enzymes of energy-providing
metabolism, and these are the basis of a more
exact classification, e.g. slow-twitch oxidative
(SO), or fast-twitch glycolytic (FG) and fast-twitch
oxidative glycolytic (FOG), or in the acid stability
of the myosin ATPase (subclasses IIA and lIB)
[220]. The heterogeneity of invertebrate muscle
fibres is less marked. Thus, for example, in the
mussel Spisula solidissima the adductor, foot
muscle, mantle and heart all contain the same
forms of actin, myosin and tropomyosin. On the
other hand, the outer muscle cylinder of the earthworm Lumbricus terrestris has two different types
of fibre which differ in the light myosin chain and
in succinate dehydrogenase activity [55]. The clawclosing muscles of the lobster Homarus americanus contain three types of muscle fibre, which
show qualitative and quantitative differences in
their protein spectra, and are distributed characteristically between the functionally different cutter and crusher claws. Altogether, two paramyosins, three troponin-Ts, five troponin-Is, two
troponin-Cs, three myosin-aLCs and one myosin-~LC can be isolated from these muscles [201].
The existence of many isoforms of almost all
muscle proteins is partly due to the expression of
different genes; multi gene families are known in
particular for the actins, the heavy and light
chains of the myosins, and the tropomyosins. A
further mechanism for the production of protein
heterogeneity is particularly characteristic of the
muscle proteins, namely, the variable splicing of
the primary transcript of one gene. Alternative
splicing has in fact been shown for almost all
muscle proteins: actins, heavy myosin chains,
light myosin chains, tropomyosins, troponin T,
troponin I and calsequestrin. Finally, muscle proteins may be post-translationally modified in different ways. Phosphorylation, for example, is
found in light and heavy myosin chains, troponin I and T, and caldesmon, although the functional meaning is in many instances by no means
clear. Methylation is also very common amongst
the muscle proteins, Monomethyl-Iysine and -histidine, trimethyl-Iysine and histidine have been
demonstrated in myosins [175]; dimethyl- and
trimethyl-Iysine are found in calmodulins and
other calcium-binding proteins [139, 243], as well
as in the actin-binding protein profilin [4].
10.3 Myosins
335
10.3 Myosins
Two fundamentally different types of myosin may
be distinguished. The myosins of type II are
soluble at a salt concentration of 600 mm01ll
and at physiological salt concentrations of
100-250 mm01ll they aggregate to form filaments. The molecules of myosin II have a rodshaped tail of about 150 nm and two globular
heads of about 20 x 7 nm. They are hexamers of
460-500 kDa, consisting of two heavy chains
(HCs) of about 200 kDa and four light chains
(LCs) of 15-27 kDa. The C-terminal ends of both
HCs are twisted into an a-helical coiled-coil and
form the tail region. The head region is made up
of the N-terminal half of each HC together with
one each of the regulatory and essential LCs, and
carries binding sites for actin and adenine nucleotide (Fig. 10.1). The myosin molecule can be
cleaved in various ways by partial proteolysis: the
lighter fragment, LMM, corresponds to the Cterminal two-thirds of the tail, and the heavier
HMM to the rest of the tail and the two heads.
The subfragment designated S-l consists ofthe Nterminal half of an HC, folded into a head, together with the two associated LCs; S-2 is the tail
section between the LMMlHMM border and the
heads. The thick filaments of vertebrate skeletal
muscle each contain several hundred myosin
molecules arranged in a bipolar fashion, such that
the heads of both filament halves point to their
respective ends. There are further proteins in
the thick filaments, e.g. the C protein, found
in several muscle-type-specific isoforms of
135-150 kDa, and the similar X protein. According to the results of X-ray analysis, the molecular
arrangement of the thick filaments of the invertebrates differs somewhat from that of the vertebrates. Myosin I, which does not form filaments,
consists of a single HC of 110-140 kDa and one
or two LCs. As in myosin II, the N-terminal
domain of the HC also forms the head, but the
tail region has a completely different structure.
The function of myosin I in living cells has not
been determined.
Due to the relatively low myosin concentrations in smooth muscle, and especially in nonmuscle cells, the myosins of the vertebrates were
initially investigated in the various types of fibre
found in skeletal and heart muscle. More
recently, the possibilities for cDNA and gene
sequencing have significantly expanded our
knowledge of the myosins of vertebrate smooth
muscle and non-muscle cells as well as of inver-