10 Proteins of Muscle and the Cytoskeleton
10.1
10.2
10.3
10.4
10.5
10.6
10.7
10.8
10.8.1
Calcium Regulation of the Actomyosin System
Molecular Heterogeneity of Skeletal Muscle
Fibres
Myosins
Actins
Actin-Binding Proteins
Tropomyosins and Troponins
Paramyosins
Calcium-Binding Proteins
Calmodulins
The contractile elements of striated vertebrate
skeletal muscle, the myofibrils, contain thin filaments, which are 6 nm in diameter and consist
mainly of actin, and thicker myosin filaments with
a diameter of 16 nm (Fig. 10.1). During muscle
contraction, the filaments undergo a sliding
movement relative to each other (sliding filament
mechanism). This is brought about by the reversible formation of bridges between the myosin
molecules and the actin filaments, which bind,
change their conformation and then dissociate
(bridge cycle). The required energy is supplied
by the hydrolysis of ATP. The sliding distance
(step size) per molecule of ATP hydrolysed is
controversial; the most recent measurements
give a value of 40 nm [102,286]. As yet, the
molecular processes of bridge formation and
change in conformation have not been fully
defined [79, 257, 267]. Because one ATP is
hydrolysed per bridge cycle, the ATPase activity
can be used as an in vitro indication of contraction. Isolated myosin has a very low ATPase activity, but this increases up to 1000-fold after combination with actin to give actomyosin. ATPase is
inhibited in relaxed muscle; the relief of ATPase
inhibition and the triggering of contraction in
activated muscle involves an increase in the sarcoplasmic Ca2+ concentration from about 0.1 flmol/l
at rest to about 10 flmol/l on activation. The
calcium-dependent regulation of contraction is
brought about by tropomyosin and the troponin
complex, which are an integral part of the thin
filaments [238].
10.8.2 Other Calcium-Binding Proteins
10.9
Microtubule Proteins
10.10 Proteins of Intermediary Filaments
10.10.1 Cytokeratins
10.10.2 Proteins of Other Intermediary Filaments
10.10.3 Lamins
10.11 Further Intracellular Structural Proteins
10.12 Lens Proteins (Crystallins)
References
Essentially the same processes are responsible
for the contraction of all muscle types, and also
for the movement of non-muscle cells. Actins are
abundant in all cell types, making up 20-25 % of
the cell protein in muscle cells and 10-15 % in
non-muscle cells. Whereas the actin molecules in
striated muscle are all aggregated into filaments,
they exist in non-muscle cells as both actin monomers (G-actin) and filaments (F-actin). The
formation and degeneration of actin filaments is
regulated by various proteins. Further actinbinding proteins bring about the association of
actin filaments in non-muscle cells into bundles
and networks. The myosin concentration in
smooth-muscle cells (8 % of the total protein) is
significantly lower than in striated muscle (36 % ),
and is lower still in non-muscle cells (1 %).
Smooth muscle contains short myosin filaments
which, unlike the situation in striated muscle, do
not show any regular structural relationship to the
actin filaments. The myosin of non-muscle cells
can also form filaments of 12-18 molecules
in vitro; however, the nature of myosin aggregation in the cytoplasm of living cells is not known
[244].
The cytoplasm of all cells contains filamentlike structures which together make up the cytoskeleton. This is a complex system of protein
molecules which confers on the cell its typical
form, internal organization and motility [313]. In
addition to actin micro filaments (diameters of
6-8 nm), the other main components of the
cytoskeleton are the tubular microtubules (exter-
10.1
10.2
10.3
10.4
10.5
10.6
10.7
10.8
10.8.1
Calcium Regulation of the Actomyosin System
Molecular Heterogeneity of Skeletal Muscle
Fibres
Myosins
Actins
Actin-Binding Proteins
Tropomyosins and Troponins
Paramyosins
Calcium-Binding Proteins
Calmodulins
The contractile elements of striated vertebrate
skeletal muscle, the myofibrils, contain thin filaments, which are 6 nm in diameter and consist
mainly of actin, and thicker myosin filaments with
a diameter of 16 nm (Fig. 10.1). During muscle
contraction, the filaments undergo a sliding
movement relative to each other (sliding filament
mechanism). This is brought about by the reversible formation of bridges between the myosin
molecules and the actin filaments, which bind,
change their conformation and then dissociate
(bridge cycle). The required energy is supplied
by the hydrolysis of ATP. The sliding distance
(step size) per molecule of ATP hydrolysed is
controversial; the most recent measurements
give a value of 40 nm [102,286]. As yet, the
molecular processes of bridge formation and
change in conformation have not been fully
defined [79, 257, 267]. Because one ATP is
hydrolysed per bridge cycle, the ATPase activity
can be used as an in vitro indication of contraction. Isolated myosin has a very low ATPase activity, but this increases up to 1000-fold after combination with actin to give actomyosin. ATPase is
inhibited in relaxed muscle; the relief of ATPase
inhibition and the triggering of contraction in
activated muscle involves an increase in the sarcoplasmic Ca2+ concentration from about 0.1 flmol/l
at rest to about 10 flmol/l on activation. The
calcium-dependent regulation of contraction is
brought about by tropomyosin and the troponin
complex, which are an integral part of the thin
filaments [238].
10.8.2 Other Calcium-Binding Proteins
10.9
Microtubule Proteins
10.10 Proteins of Intermediary Filaments
10.10.1 Cytokeratins
10.10.2 Proteins of Other Intermediary Filaments
10.10.3 Lamins
10.11 Further Intracellular Structural Proteins
10.12 Lens Proteins (Crystallins)
References
Essentially the same processes are responsible
for the contraction of all muscle types, and also
for the movement of non-muscle cells. Actins are
abundant in all cell types, making up 20-25 % of
the cell protein in muscle cells and 10-15 % in
non-muscle cells. Whereas the actin molecules in
striated muscle are all aggregated into filaments,
they exist in non-muscle cells as both actin monomers (G-actin) and filaments (F-actin). The
formation and degeneration of actin filaments is
regulated by various proteins. Further actinbinding proteins bring about the association of
actin filaments in non-muscle cells into bundles
and networks. The myosin concentration in
smooth-muscle cells (8 % of the total protein) is
significantly lower than in striated muscle (36 % ),
and is lower still in non-muscle cells (1 %).
Smooth muscle contains short myosin filaments
which, unlike the situation in striated muscle, do
not show any regular structural relationship to the
actin filaments. The myosin of non-muscle cells
can also form filaments of 12-18 molecules
in vitro; however, the nature of myosin aggregation in the cytoplasm of living cells is not known
[244].
The cytoplasm of all cells contains filamentlike structures which together make up the cytoskeleton. This is a complex system of protein
molecules which confers on the cell its typical
form, internal organization and motility [313]. In
addition to actin micro filaments (diameters of
6-8 nm), the other main components of the
cytoskeleton are the tubular microtubules (exter-