phobic amino acids and the 28-mer repeats of
charged amino acids that are very typical of the
myosin heavy-chain rod. The paramyosin molecule, however, differs from the myosin HC in
that it lacks the terminal non-helical regions [122].
All the muscles of molluscs contain paramyosin, with the highest amounts being found in the
smooth shell adductors, foot muscles and byssus
retractors of the mussels. The thick filaments in
these paramyosin muscles are unusual in their
thickness and length; they consist of a paracrystalline nucleus of paramyosin bearing myosin on
its surface. The ratio of paramyosin to myosin lies
between 2:1 and 10:1 [33,238]. The paramyosin
molecule, like the HC tail region of myosin, can
be reversibly phosphorylated by a cAMPdependent protein kinase; in both cases this
results in increased solubility [33]. Functionally,
the paramyosin muscles are noted for the "catch"
mechanism, by which a high mechanical tension
can be maintained over long periods with minimal energy consumption. This phenomenon can
be dramatically demonstrated with mussels,
which can keep their shells closed despite many
weeks of applied tension of several kg/cm 2 muscle
cross-section. The catch mechanism does not
involve the active generation of tension, but more
a rigor-like state which is relieved by, for example, serotonin in intact fibres and cAMP in the
case of sarcolemma-free fibres. The molecular
mechanism of the catch system has not yet been
described. One set of hypotheses assumes it to be
related to the mechanical properties of the
paramyosin-myosin complex and to be regulated
by phosphorylation, although a connection
between an increase in phosphorylation and
release of the catch has by no means been established. Other hypotheses seek to explain the
mechanism by changes in the kinetics of bridge
formation between thick and thin filaments, i.e.
increased stability or reduced breakage of these
bridges. Unambiguous evidence is also lacking in
this case, and neither hypothesis explains why the
catch phenomenon depends upon the presence of
large quantities of paramyosin [33, 238,299].
10.8 Calcium-Binding Proteins
10.8.1 Calmodulins
The protein family to which the light myosin
chains and troponin C belong also includes a
whole series of proteins from muscles and non10.8.1 Calmodulins
347
muscle cells which have the capacity to bind Ca 2 +
and Mi+ with high affinity. The prototype of
these calcium-binding proteins is calmodulin,
which was discovered at the beginning of the
1970s as the activator of certain phosphodiesterases and is now known as the universal calcium
receptor of eukaryote cells [80,263]. Calmodulin
has been found in all eukaryote cells examined,
from the mammals to the lower fungi, and even in
several bacteria [149]. Particularly high levels are
found in mammalian brain and testis as well as in
the electric organ of the electric eel Electrophorus
electricus, from which up to 1 g calmodulin per kg
organ can be obtained.
Calmodulin is made up of a polypeptide chain
of 16.7 kDa with 148 amino acids, including
approximately 50 aspartate and glutamate residues which confer the acidic nature of the molecule (pI = 4). The N-terminus is always blocked,
and many calmodulins have a post-translationally
modified NE-trimethyl-Iysine at position 115. The
calmodulin molecule has the shape of a dumbbell, with two globular regions, each bearing two
calcium-binding sites, connected by a long a helix
[8]. The amino acid sequence has a distinct internal periodicity with four homologous, calciumbinding domains, I-IV, each of 30 amino acids.
They have a characteristic tertiary structure referred to as EF-hand, consisting of a calciumbinding loop of 12 amino acids flanked by two ahelical sections arranged more-or-Iess perpendicular to each other (Fig. 10.5). The sequence
similarity is especially high between domains I
and III, and II and IV. It seems reasonable to
assume that this structure arose by the double
duplication of a gene for a primitive calciumbinding eukaryote protein. It is clear, however,
that the intronlexon structure of the gene is not
entirely compatible with this concept, as the
introns often break up Ca2+ -binding domains.
The evolution of the calmodulin gene has apparently been somewhat complicated [204, 217].
The calmodulins of different animal species are
structurally and functionally very similar, as is
shown already by analysis of the amino acid composition. Vertebrate calmodulins contain two
tyro sines , whereas in the invertebrates there is
always only one. The trimethyl-Iysine that is characteristic of most calmodulins is missing in green
plants, Dictyostelium and Neurospora, and also in
Trypanosoma, the crayfish Orconectes limosus
and the insects Bombyx mori, Drosophila melanogaster and Mamestra configurata. The amino
acid sequence of Drosophila calmodulin,
obtained via the cDNA, has a lysine in position
charged amino acids that are very typical of the
myosin heavy-chain rod. The paramyosin molecule, however, differs from the myosin HC in
that it lacks the terminal non-helical regions [122].
All the muscles of molluscs contain paramyosin, with the highest amounts being found in the
smooth shell adductors, foot muscles and byssus
retractors of the mussels. The thick filaments in
these paramyosin muscles are unusual in their
thickness and length; they consist of a paracrystalline nucleus of paramyosin bearing myosin on
its surface. The ratio of paramyosin to myosin lies
between 2:1 and 10:1 [33,238]. The paramyosin
molecule, like the HC tail region of myosin, can
be reversibly phosphorylated by a cAMPdependent protein kinase; in both cases this
results in increased solubility [33]. Functionally,
the paramyosin muscles are noted for the "catch"
mechanism, by which a high mechanical tension
can be maintained over long periods with minimal energy consumption. This phenomenon can
be dramatically demonstrated with mussels,
which can keep their shells closed despite many
weeks of applied tension of several kg/cm 2 muscle
cross-section. The catch mechanism does not
involve the active generation of tension, but more
a rigor-like state which is relieved by, for example, serotonin in intact fibres and cAMP in the
case of sarcolemma-free fibres. The molecular
mechanism of the catch system has not yet been
described. One set of hypotheses assumes it to be
related to the mechanical properties of the
paramyosin-myosin complex and to be regulated
by phosphorylation, although a connection
between an increase in phosphorylation and
release of the catch has by no means been established. Other hypotheses seek to explain the
mechanism by changes in the kinetics of bridge
formation between thick and thin filaments, i.e.
increased stability or reduced breakage of these
bridges. Unambiguous evidence is also lacking in
this case, and neither hypothesis explains why the
catch phenomenon depends upon the presence of
large quantities of paramyosin [33, 238,299].
10.8 Calcium-Binding Proteins
10.8.1 Calmodulins
The protein family to which the light myosin
chains and troponin C belong also includes a
whole series of proteins from muscles and non10.8.1 Calmodulins
347
muscle cells which have the capacity to bind Ca 2 +
and Mi+ with high affinity. The prototype of
these calcium-binding proteins is calmodulin,
which was discovered at the beginning of the
1970s as the activator of certain phosphodiesterases and is now known as the universal calcium
receptor of eukaryote cells [80,263]. Calmodulin
has been found in all eukaryote cells examined,
from the mammals to the lower fungi, and even in
several bacteria [149]. Particularly high levels are
found in mammalian brain and testis as well as in
the electric organ of the electric eel Electrophorus
electricus, from which up to 1 g calmodulin per kg
organ can be obtained.
Calmodulin is made up of a polypeptide chain
of 16.7 kDa with 148 amino acids, including
approximately 50 aspartate and glutamate residues which confer the acidic nature of the molecule (pI = 4). The N-terminus is always blocked,
and many calmodulins have a post-translationally
modified NE-trimethyl-Iysine at position 115. The
calmodulin molecule has the shape of a dumbbell, with two globular regions, each bearing two
calcium-binding sites, connected by a long a helix
[8]. The amino acid sequence has a distinct internal periodicity with four homologous, calciumbinding domains, I-IV, each of 30 amino acids.
They have a characteristic tertiary structure referred to as EF-hand, consisting of a calciumbinding loop of 12 amino acids flanked by two ahelical sections arranged more-or-Iess perpendicular to each other (Fig. 10.5). The sequence
similarity is especially high between domains I
and III, and II and IV. It seems reasonable to
assume that this structure arose by the double
duplication of a gene for a primitive calciumbinding eukaryote protein. It is clear, however,
that the intronlexon structure of the gene is not
entirely compatible with this concept, as the
introns often break up Ca2+ -binding domains.
The evolution of the calmodulin gene has apparently been somewhat complicated [204, 217].
The calmodulins of different animal species are
structurally and functionally very similar, as is
shown already by analysis of the amino acid composition. Vertebrate calmodulins contain two
tyro sines , whereas in the invertebrates there is
always only one. The trimethyl-Iysine that is characteristic of most calmodulins is missing in green
plants, Dictyostelium and Neurospora, and also in
Trypanosoma, the crayfish Orconectes limosus
and the insects Bombyx mori, Drosophila melanogaster and Mamestra configurata. The amino
acid sequence of Drosophila calmodulin,
obtained via the cDNA, has a lysine in position