common with the smaller myosin-I HCs [111].
The ATPase activity of myosin I is only activated
by actin when a specific serine or threonine residue of the HC is phosphorylated by a specific protein kinase (MHCK). The MHCK can bind 7-8
moles of phosphate per mole of enzyme by autophosphorylation, which is promoted by phosphatidylserine but not by calcium [27]. Only a few
proteins comparable with type-I myosin from
A. castellanii are known from other animals. The
product of the ninAc gene, expressed in the eyes
of Drosophila melanogaster, has a C-terminal
domain homologous to the head and tail of type-I
myosin [129]. The brush-border myosin I (BBMI), which is present in the microvilli of the gut
epithelia and binds the central actin-filament
bundle to the plasma membrane, apparently has a
motor function. BB-MI from the chicken includes
a heavy chain with a myosin-like globular head of
82 kDa and a 37-kDa tail domain which bears no
resemblance to the conventional myosins [91].
The myosin genes sequenced so far show large
variation in their exonlintron organization: the HC
genes from vertebrate skeletal-muscle myosins
mostly have 41 exons, the HC gene unc-54 from
Caenorhabditis elegans has 8, and the HC gene of
the muscle myosin of Drosophila has 9 exons. In
A. castellanii, the HC gene of myosin II has only 2
very short introns, whereas the HMWMI gene has
17. The mammalian LC-l/3 gene has nine exons, in
contrast to the six exons of the essential LC of
Drosophila. The regulatory LC of mammals also
has more exons (seven) than does that of Drosophila (three); the last exon of the fly corresponds
to exons 3-7 of the rat[65, 71, 111, 189].
10.4 Actins
Actin is a globular protein of 43 kDa. The polypeptide chain consists of 374-376 amino acids and
is always N-terminal acetylated. The amino acid
sequence is very conservative. Several acidic
amino acids at the N-terminus are variable, but
otherwise the animal actins vary at most by 5 % .
There are also multiple actins in plants which, in
contrast, vary both amongst themselves and compared with those of the animals by more than
10 %. The actin monomer is divided into two
lobes, between which is found the ATP-binding
site; one lobe is formed from the N-terminal150
amino acids and the other is formed from the
remaining 225 amino acids; each domain consists
of one ~ folded sheet surrounded by several a
10.4 Actins
339
helices. Under physiological conditions, the actin
monomers (G-actin) aggregate to form actin maments (F-actin). Actin in the muscle is almost
entirely in the F form, but in non-muscle cells
both G- and F-actin are found. The arrangement
of the monomers in an actin filament is usually
described as a right-hand-threaded double helix
with a pitch of 71.5 nm. Considering the tight
binding between the monomers of both strands,
the filament can also be seen as a simple lefthanded helix with a pitch of 5.9 nm [224].
Polymerization and depolymerization of the
actin monomers depends in a complex way on the
external conditions, and is influenced especially
by ATP, ADP, Mg2+ and Ca 2 +. Because all the
monomers in a filament are orientated in the
same direction, the filament ends vary. In view of
the arrowhead forms, which result from the binding of myosin head pieces (HMM), the ends are
referred to as "pointed" (P) or "barbed" (B). The
association rate is greater at B ends than at P
ends, and is greater with ATP-actin than with
ADP-actin. At a certain (critical) actin concentration, the association (particularly at B ends) and
dissociation (particularly at P ends) are in equilibrium, and the filament length remains constant.
The critical actin concentration depends upon the
actin type and the external conditions and is usually 0.1-1 I-tmol/l [30, 224].
At least six tissue-specific isoactins can be distinguished in mammals and birds; these can be
classified into three groups, a, ~ and y, according
to increasing pI values in isoelectric focusing.
There are four isoforms of the muscle-specific a
actins. All skeletal muscles contain the same a
actin and this is therefore the only muscle protein
that shows no differences amongst the various
fibre types. Further a actins are found in heart
muscle and in the smooth muscle of the stomach
and blood-vessel walls; ~ and y actins are typical
of non-muscle cells. The sequences of these different actins are very similar; the a actins differ in
only 4-6 out of 375 amino acids, and ~ and y
actins differ from each other at only 4 positions
and from a actin at 25-27 positions [188].
Species-specific differences between actins of one
type are rare in the warm-blooded vertebrates;
for example, the a-actin sequences of cattle, rabbit, rat and chicken are identical, as are the y
actins of man, the mouse and rat [186, 288]. Only
three amino acid substitutions separate the
skeletal-muscle actins of agnathans and mammals
[288]. Thus, the actins are changing no quicker
than histone H4, which is considered to be a
model of slow evolution (see Table 4.12, p.161).
The ATPase activity of myosin I is only activated
by actin when a specific serine or threonine residue of the HC is phosphorylated by a specific protein kinase (MHCK). The MHCK can bind 7-8
moles of phosphate per mole of enzyme by autophosphorylation, which is promoted by phosphatidylserine but not by calcium [27]. Only a few
proteins comparable with type-I myosin from
A. castellanii are known from other animals. The
product of the ninAc gene, expressed in the eyes
of Drosophila melanogaster, has a C-terminal
domain homologous to the head and tail of type-I
myosin [129]. The brush-border myosin I (BBMI), which is present in the microvilli of the gut
epithelia and binds the central actin-filament
bundle to the plasma membrane, apparently has a
motor function. BB-MI from the chicken includes
a heavy chain with a myosin-like globular head of
82 kDa and a 37-kDa tail domain which bears no
resemblance to the conventional myosins [91].
The myosin genes sequenced so far show large
variation in their exonlintron organization: the HC
genes from vertebrate skeletal-muscle myosins
mostly have 41 exons, the HC gene unc-54 from
Caenorhabditis elegans has 8, and the HC gene of
the muscle myosin of Drosophila has 9 exons. In
A. castellanii, the HC gene of myosin II has only 2
very short introns, whereas the HMWMI gene has
17. The mammalian LC-l/3 gene has nine exons, in
contrast to the six exons of the essential LC of
Drosophila. The regulatory LC of mammals also
has more exons (seven) than does that of Drosophila (three); the last exon of the fly corresponds
to exons 3-7 of the rat[65, 71, 111, 189].
10.4 Actins
Actin is a globular protein of 43 kDa. The polypeptide chain consists of 374-376 amino acids and
is always N-terminal acetylated. The amino acid
sequence is very conservative. Several acidic
amino acids at the N-terminus are variable, but
otherwise the animal actins vary at most by 5 % .
There are also multiple actins in plants which, in
contrast, vary both amongst themselves and compared with those of the animals by more than
10 %. The actin monomer is divided into two
lobes, between which is found the ATP-binding
site; one lobe is formed from the N-terminal150
amino acids and the other is formed from the
remaining 225 amino acids; each domain consists
of one ~ folded sheet surrounded by several a
10.4 Actins
339
helices. Under physiological conditions, the actin
monomers (G-actin) aggregate to form actin maments (F-actin). Actin in the muscle is almost
entirely in the F form, but in non-muscle cells
both G- and F-actin are found. The arrangement
of the monomers in an actin filament is usually
described as a right-hand-threaded double helix
with a pitch of 71.5 nm. Considering the tight
binding between the monomers of both strands,
the filament can also be seen as a simple lefthanded helix with a pitch of 5.9 nm [224].
Polymerization and depolymerization of the
actin monomers depends in a complex way on the
external conditions, and is influenced especially
by ATP, ADP, Mg2+ and Ca 2 +. Because all the
monomers in a filament are orientated in the
same direction, the filament ends vary. In view of
the arrowhead forms, which result from the binding of myosin head pieces (HMM), the ends are
referred to as "pointed" (P) or "barbed" (B). The
association rate is greater at B ends than at P
ends, and is greater with ATP-actin than with
ADP-actin. At a certain (critical) actin concentration, the association (particularly at B ends) and
dissociation (particularly at P ends) are in equilibrium, and the filament length remains constant.
The critical actin concentration depends upon the
actin type and the external conditions and is usually 0.1-1 I-tmol/l [30, 224].
At least six tissue-specific isoactins can be distinguished in mammals and birds; these can be
classified into three groups, a, ~ and y, according
to increasing pI values in isoelectric focusing.
There are four isoforms of the muscle-specific a
actins. All skeletal muscles contain the same a
actin and this is therefore the only muscle protein
that shows no differences amongst the various
fibre types. Further a actins are found in heart
muscle and in the smooth muscle of the stomach
and blood-vessel walls; ~ and y actins are typical
of non-muscle cells. The sequences of these different actins are very similar; the a actins differ in
only 4-6 out of 375 amino acids, and ~ and y
actins differ from each other at only 4 positions
and from a actin at 25-27 positions [188].
Species-specific differences between actins of one
type are rare in the warm-blooded vertebrates;
for example, the a-actin sequences of cattle, rabbit, rat and chicken are identical, as are the y
actins of man, the mouse and rat [186, 288]. Only
three amino acid substitutions separate the
skeletal-muscle actins of agnathans and mammals
[288]. Thus, the actins are changing no quicker
than histone H4, which is considered to be a
model of slow evolution (see Table 4.12, p.161).