344
10 Proteins of Muscle and the Cytoskeleton
(15-20 kDa) have been isolated from protozoans,
echinoderms and vertebrates, but in the absence
of sequence data little can be said about their
relationships to each other [46, 224].
The proteins that bind sideways on a single
actin filament include, in particular, tropomyosin.
The rod-shaped tropomyosin molecule is monovalent, i.e. it cannot make either bundles or lattices from actin filaments, but it probably serves to
stiffen them and increase their mechanical
strength [114]. Bundling of actin filaments is
observed in microvilli and some other cytoplasmic
structures. The actin-bundling proteins include
the fascins, found as monomers of 57 kDa in
Acanthamoeba and slime moulds, in the eggs,
sperm and coelomocytes of echinoderms, in
Limulus sperm and in mammalian brain; fimbrim
(68 kDa) from various mammalian cells; and villin (95 kDa) from the microvilli of the brushborder cells in the gut and kidney [15]. Actin
cross-linking proteins were discovered through
their drastic effect on the viscosity of actin solutions. The filamins of smooth muscle and the
"actin-binding proteins" (ABPs) from non-muscle
cells are homodimers (two units of 250 kDa)
which carry an actin-binding site on each subunit,
and hence have filament-bundling or filamentcross-linking activity [106,224]. There are two
types of thin fdarnent in the smooth muscle of the
chicken crop; one consists of actin, tropomyosin
and caldesmon, and the other of actin, tropomyosin and filamin. The biological significance of
this heterogeneity is not known [163].
Specific structural proteins link the actin microfilaments of the cytoskeleton to the plasma membrane. For example, the actin filaments of the fibroblasts are bundled into so-called stress fibres
which end in adhesion plaques on the plasma
membrane. The stress fibres contain periodically
arranged a actinin in addition to filamin, and the
plaques contain vinculin (130 kDa) and talin
(215 kDa). Vinculin and talin are also found in
muscle [125,229, 302]. Desmosomes and other
structures linking epithelial cells contain placoglobin (83 kDa), which is also found in cells that
lack desmosomes [123]. The a actinins were discovered in vertebrate striated-muscle cells as a component of the Z lines, but they are also present in
the functionally equivalent "thick bodies" of
smooth muscles, in the non-muscle cells of vertebrates, and in sea urchin eggs and slime moulds.
They are homodimers of 100-kDa subunits with
almost 900 amino acids [6, 183, 224]. It has only
recently become clear via sequence comparisons
that a actinin has homology to the spectrins of the
membrane skeleton. The spectrins are composed
of 106-amino-acid repeats; three such repeats with
significant (25 %) sequence similarity to spectrin
make up the C-terminal domain of chicken a actinino In contrast, the N-terminal, actin-binding
domain has no similarity to spectrin [24, 298].
10.6 Tropomyosins and lroponins
The tropornyosins are a-helical polypeptides with
a length of 284 amino acids and a mass of
34-46 kDa; they occur in the muscles of vertebrates and invertebrates, and may be smaller in nonmuscle cells. Usually, two equal or different
chains are intertwined to give a rod-like dimer
that is 40 nm long (coiled-coil structure). Tropomyosin has been detected in all muscle types of
the vertebrates, and also in the muscles of various
invertebrates and in many non-muscle cells. The
thin filaments of vertebrate skeletal muscle contain actin monomers, tropomyosin and the troponin complex in the ratio 7: 1 : 1. The tropomyosin
molecules lie in the grooves between the actin
strands of the thin filaments, and each carries one
troponin complex. This conformation is responsible for the actin-specific calcium regulation of the
contractile apparatus. Tropomyosin is also found
in myosin-specific regulated muscle and nonmuscle cells, but is then not associated with a troponin complex; here, it appears to have the function of increasing the mechanical strength and
stiffness of the thin filaments [114]. The tropomyosin genes have quite different lengths (rat a-TM
28 kb; rat ~-TM 10 kb; Drosophila Tml 22 kb and
Tm2 28 kb), but always have exons of either 21 or
42 codons. One large and two small exons encode
each of the actin-binding sites, of which there are
seven regularly distributed over the whole length
of 284 amino acids [289]. The shorter tropomyosins from equine blood platelets (247 amino
acids), human fibroblasts (247 amino acids) and
Drosophila non-muscle cells (252 amino acids)
extend over only six actin monomers [92].
Just like other muscle proteins, tropomyosins
exist as several isoforms, which are either
encoded by different genes or derived by alternative splicing of the same primary transcript. Additional heterogeneity can arise by posttranslational phosphorylation. There are two tropomyosin subunits, a and ~, in striated and
smooth vertebrate muscle; these are encoded by
different genes but differ only slightly in their
sequences. They mostly exist as a~ heterodimers
10 Proteins of Muscle and the Cytoskeleton
(15-20 kDa) have been isolated from protozoans,
echinoderms and vertebrates, but in the absence
of sequence data little can be said about their
relationships to each other [46, 224].
The proteins that bind sideways on a single
actin filament include, in particular, tropomyosin.
The rod-shaped tropomyosin molecule is monovalent, i.e. it cannot make either bundles or lattices from actin filaments, but it probably serves to
stiffen them and increase their mechanical
strength [114]. Bundling of actin filaments is
observed in microvilli and some other cytoplasmic
structures. The actin-bundling proteins include
the fascins, found as monomers of 57 kDa in
Acanthamoeba and slime moulds, in the eggs,
sperm and coelomocytes of echinoderms, in
Limulus sperm and in mammalian brain; fimbrim
(68 kDa) from various mammalian cells; and villin (95 kDa) from the microvilli of the brushborder cells in the gut and kidney [15]. Actin
cross-linking proteins were discovered through
their drastic effect on the viscosity of actin solutions. The filamins of smooth muscle and the
"actin-binding proteins" (ABPs) from non-muscle
cells are homodimers (two units of 250 kDa)
which carry an actin-binding site on each subunit,
and hence have filament-bundling or filamentcross-linking activity [106,224]. There are two
types of thin fdarnent in the smooth muscle of the
chicken crop; one consists of actin, tropomyosin
and caldesmon, and the other of actin, tropomyosin and filamin. The biological significance of
this heterogeneity is not known [163].
Specific structural proteins link the actin microfilaments of the cytoskeleton to the plasma membrane. For example, the actin filaments of the fibroblasts are bundled into so-called stress fibres
which end in adhesion plaques on the plasma
membrane. The stress fibres contain periodically
arranged a actinin in addition to filamin, and the
plaques contain vinculin (130 kDa) and talin
(215 kDa). Vinculin and talin are also found in
muscle [125,229, 302]. Desmosomes and other
structures linking epithelial cells contain placoglobin (83 kDa), which is also found in cells that
lack desmosomes [123]. The a actinins were discovered in vertebrate striated-muscle cells as a component of the Z lines, but they are also present in
the functionally equivalent "thick bodies" of
smooth muscles, in the non-muscle cells of vertebrates, and in sea urchin eggs and slime moulds.
They are homodimers of 100-kDa subunits with
almost 900 amino acids [6, 183, 224]. It has only
recently become clear via sequence comparisons
that a actinin has homology to the spectrins of the
membrane skeleton. The spectrins are composed
of 106-amino-acid repeats; three such repeats with
significant (25 %) sequence similarity to spectrin
make up the C-terminal domain of chicken a actinino In contrast, the N-terminal, actin-binding
domain has no similarity to spectrin [24, 298].
10.6 Tropomyosins and lroponins
The tropornyosins are a-helical polypeptides with
a length of 284 amino acids and a mass of
34-46 kDa; they occur in the muscles of vertebrates and invertebrates, and may be smaller in nonmuscle cells. Usually, two equal or different
chains are intertwined to give a rod-like dimer
that is 40 nm long (coiled-coil structure). Tropomyosin has been detected in all muscle types of
the vertebrates, and also in the muscles of various
invertebrates and in many non-muscle cells. The
thin filaments of vertebrate skeletal muscle contain actin monomers, tropomyosin and the troponin complex in the ratio 7: 1 : 1. The tropomyosin
molecules lie in the grooves between the actin
strands of the thin filaments, and each carries one
troponin complex. This conformation is responsible for the actin-specific calcium regulation of the
contractile apparatus. Tropomyosin is also found
in myosin-specific regulated muscle and nonmuscle cells, but is then not associated with a troponin complex; here, it appears to have the function of increasing the mechanical strength and
stiffness of the thin filaments [114]. The tropomyosin genes have quite different lengths (rat a-TM
28 kb; rat ~-TM 10 kb; Drosophila Tml 22 kb and
Tm2 28 kb), but always have exons of either 21 or
42 codons. One large and two small exons encode
each of the actin-binding sites, of which there are
seven regularly distributed over the whole length
of 284 amino acids [289]. The shorter tropomyosins from equine blood platelets (247 amino
acids), human fibroblasts (247 amino acids) and
Drosophila non-muscle cells (252 amino acids)
extend over only six actin monomers [92].
Just like other muscle proteins, tropomyosins
exist as several isoforms, which are either
encoded by different genes or derived by alternative splicing of the same primary transcript. Additional heterogeneity can arise by posttranslational phosphorylation. There are two tropomyosin subunits, a and ~, in striated and
smooth vertebrate muscle; these are encoded by
different genes but differ only slightly in their
sequences. They mostly exist as a~ heterodimers