can copolymerize with the actin monomers from
rabbit skeletal muscle. In contrast, the Acanthamoeba actin is unique in several ways: it has Nterminally a blocked residue, and not aspartic or
glutamic acid as in all other known actins
(Fig. 10.3); there is an N-trimethyl-Iysine residue
at position 326 [289]. In addition to this actin,
which makes up about 10 % of the total cytoplasmic protein, Acanthamoeba has two others, of
which one only appears to be present in the nucleus. The most deviant actins are found in the ciliates. For example, actin from Tetrahymena thermophila, with a sequence of 275 amino acids, is
25 % different to yeast actin, and even 35 % different to the small actin from Oxytricha tal/ax. Vertebrate actin differs from yeast actin by only 13 % ,
from the actins of higher plants by 11-14 %, from
Acanthamoeba actin by 10 %, and from those of
Oxytricha by 29-33 %. In addition, the Oxytricha
actin has only 356 amino acids, lacking the 18
amino acids which appear at positions 68-85 in the
other actins. The great evolutionary distance of
their actins highlights the special position of the
ciliates within the eukaryotes [51, 95, 101, 104].
Actin isoforms are so far known from only a
few invertebrates. Two electrophoretically distinct actins can be detected in the flight muscles
of the Diptera and Hymenoptera; in Drosophila,
the actins of the flight muscles, leg muscles and
larval abdominal muscles show tissue-specific differences. Five genes identified in the sea urchin
Strongylocentrotus purpuratus code for tissuespecific cytoskeleton actins [62]. Even though no
actin isoforms are detected in most invertebrates,
there are usually several actin genes present;
these are either dispersed in the genome (as in
Drosophila) or ordered in clusters (as in the sea
urchin). Like yeast, Tetrahymena has only 1 actin
gene, but there are 3 in the ciliate Euplotes and in
Acanthamoeba, 4 in Caenorhabditis, 5-6 in various insects, 5-8 in echinoderms, 15-17 in the
slime mould Dictyostelium, and 50-75 in the
roundworm Ascaris [51,62, 101, 147, 309]. Drosophila melanogaster has six actin genes, which
are expressed in a stage- and tissue-specific manner, including four which are muscle specific
[176]. The silkworm Bombyx mori also possesses
at least five actin genes, three of which have been
sequenced. One of these three genes is expressed
in the silk glands in actin filaments involved in the
ejection of the silk thread. The coded sequence is
almost identical to that of the cytoplasmic actin
from Drosophila. The other two sequenced genes
are muscle specific and differ in 8 of their 374
amino acids [196].
10.4 Actins
341
The sea urchin Strongylocentrotus purpuratus
has at least six variously expressed actin genes.
Five genes arranged in two clusters encode the
cytoplasmic actins; two have been sequenced.
Just one isolated gene codes for a muscle-specific
actin in the sea urchin, whereas in insects, as in
the vertebrates, there are always several musclespecific actins. The sea urchin muscle actin is
more similar to the cytoplasmic actins of vertebrates than to their muscle actins; in one region
of the gene there is clear evidence for a conversion event with one of the cytoplasmic-actin gene
clusters [49]. The six genes are expressed to varying degrees, e.g. in the pluteus larva there are
60-190 mRNA molecules for the cytoplasmic
actins compared with 650-1300 for the muscle
actin. The gene for muscle actin is expressed relatively late in development and only in certain
mesoderm cells. The five genes of the cytoplasmic
actins fall into three groups, one or two of which
are expressed in each embryonal cell line, but
never all three [48].
There are about 20 actin genes in the human
and murine genomes; if genes which differ by
more than 20 % are included, then the number in
the mouse may be much higher. Other mammals,
like the golden hamster, appear to possess only
five or six actin genes [218]. Why are there so
many actin genes in some mammals and yet only
six different proteins? The actins of human skeletal and heart muscles are each encoded by only
one gene, as is the cytoplasmic ~ actin. The most
plausible explanation is that among the human
and murine actin genes there are probably many
pseudo genes which are never expressed. The
arrangement of actin genes in the genome covers
the whole range of possibilities: tight linkage in
Dictyostelium, dispersed in Drosophila, and tandem clusters in sea urchins and nematodes [68].
The human and murine genes for the different a
actins are not linked [3]. In general, in the mouse
the active genes for the various muscle proteins
are widely dispersed in the genome; the genes for
myosin HC are on chromosome 11, for regulatory
myosin LC on chromosome 7, for the skeletal
actin on chromosome 3, for heart muscle on
chromosome 17, and for the non-muscle actin on
chromosome 5.
About two dozen actin genes have been
sequenced to date. In parallel to the great similarity of the encoded proteins, the actin genes themselves show such broad similarity that the genes
of widely separated organisms can hybridize. Of
course, synonymous substitutions have occurred
in the evolution of the actin genes. However,
rabbit skeletal muscle. In contrast, the Acanthamoeba actin is unique in several ways: it has Nterminally a blocked residue, and not aspartic or
glutamic acid as in all other known actins
(Fig. 10.3); there is an N-trimethyl-Iysine residue
at position 326 [289]. In addition to this actin,
which makes up about 10 % of the total cytoplasmic protein, Acanthamoeba has two others, of
which one only appears to be present in the nucleus. The most deviant actins are found in the ciliates. For example, actin from Tetrahymena thermophila, with a sequence of 275 amino acids, is
25 % different to yeast actin, and even 35 % different to the small actin from Oxytricha tal/ax. Vertebrate actin differs from yeast actin by only 13 % ,
from the actins of higher plants by 11-14 %, from
Acanthamoeba actin by 10 %, and from those of
Oxytricha by 29-33 %. In addition, the Oxytricha
actin has only 356 amino acids, lacking the 18
amino acids which appear at positions 68-85 in the
other actins. The great evolutionary distance of
their actins highlights the special position of the
ciliates within the eukaryotes [51, 95, 101, 104].
Actin isoforms are so far known from only a
few invertebrates. Two electrophoretically distinct actins can be detected in the flight muscles
of the Diptera and Hymenoptera; in Drosophila,
the actins of the flight muscles, leg muscles and
larval abdominal muscles show tissue-specific differences. Five genes identified in the sea urchin
Strongylocentrotus purpuratus code for tissuespecific cytoskeleton actins [62]. Even though no
actin isoforms are detected in most invertebrates,
there are usually several actin genes present;
these are either dispersed in the genome (as in
Drosophila) or ordered in clusters (as in the sea
urchin). Like yeast, Tetrahymena has only 1 actin
gene, but there are 3 in the ciliate Euplotes and in
Acanthamoeba, 4 in Caenorhabditis, 5-6 in various insects, 5-8 in echinoderms, 15-17 in the
slime mould Dictyostelium, and 50-75 in the
roundworm Ascaris [51,62, 101, 147, 309]. Drosophila melanogaster has six actin genes, which
are expressed in a stage- and tissue-specific manner, including four which are muscle specific
[176]. The silkworm Bombyx mori also possesses
at least five actin genes, three of which have been
sequenced. One of these three genes is expressed
in the silk glands in actin filaments involved in the
ejection of the silk thread. The coded sequence is
almost identical to that of the cytoplasmic actin
from Drosophila. The other two sequenced genes
are muscle specific and differ in 8 of their 374
amino acids [196].
10.4 Actins
341
The sea urchin Strongylocentrotus purpuratus
has at least six variously expressed actin genes.
Five genes arranged in two clusters encode the
cytoplasmic actins; two have been sequenced.
Just one isolated gene codes for a muscle-specific
actin in the sea urchin, whereas in insects, as in
the vertebrates, there are always several musclespecific actins. The sea urchin muscle actin is
more similar to the cytoplasmic actins of vertebrates than to their muscle actins; in one region
of the gene there is clear evidence for a conversion event with one of the cytoplasmic-actin gene
clusters [49]. The six genes are expressed to varying degrees, e.g. in the pluteus larva there are
60-190 mRNA molecules for the cytoplasmic
actins compared with 650-1300 for the muscle
actin. The gene for muscle actin is expressed relatively late in development and only in certain
mesoderm cells. The five genes of the cytoplasmic
actins fall into three groups, one or two of which
are expressed in each embryonal cell line, but
never all three [48].
There are about 20 actin genes in the human
and murine genomes; if genes which differ by
more than 20 % are included, then the number in
the mouse may be much higher. Other mammals,
like the golden hamster, appear to possess only
five or six actin genes [218]. Why are there so
many actin genes in some mammals and yet only
six different proteins? The actins of human skeletal and heart muscles are each encoded by only
one gene, as is the cytoplasmic ~ actin. The most
plausible explanation is that among the human
and murine actin genes there are probably many
pseudo genes which are never expressed. The
arrangement of actin genes in the genome covers
the whole range of possibilities: tight linkage in
Dictyostelium, dispersed in Drosophila, and tandem clusters in sea urchins and nematodes [68].
The human and murine genes for the different a
actins are not linked [3]. In general, in the mouse
the active genes for the various muscle proteins
are widely dispersed in the genome; the genes for
myosin HC are on chromosome 11, for regulatory
myosin LC on chromosome 7, for the skeletal
actin on chromosome 3, for heart muscle on
chromosome 17, and for the non-muscle actin on
chromosome 5.
About two dozen actin genes have been
sequenced to date. In parallel to the great similarity of the encoded proteins, the actin genes themselves show such broad similarity that the genes
of widely separated organisms can hybridize. Of
course, synonymous substitutions have occurred
in the evolution of the actin genes. However,