tions between the cytoskeleton and the plasma
membrane. They also act extracellularly: the ~~
homodimer is identical to the neurite extension
factor (NEF) which stimulates neurite growth;
the a~ heterodimer has no such activity [135].
The sea urchin Strongylocentrotus purpuratus has
about eight spec genes which are expressed
exclusively in the aboral ectodermal cells of the
embryo. They encode about ten different spec
proteins of 14-17 kDa with four EF-hand
domains, which are produced during embryogenesis in the same cells but at different times and
in different amounts. The specl protein is, for a
time, one of the most predominant proteins of the
embryo. Its precise function is not known; however, it appears to bind to actin and effect regular
changes in the cell form. If the spec proteins
really play an important role in embryo development, corresponding proteins would be expected
in other sea urchin species. In fact, a protein of
34 kDa with eight EF-hand domains in Lytechinus pictus could have arisen by the duplication of
a spec-like protein [93, 314].
The duodenum of birds and mammals contains
calcium-binding proteins with a similar function
but different structures; these are involved in
vitamin D-dependent calcium absorption and are
now mostly known as calbindins. The so-called
28-kDa calbindin, isolated from the gut wall of
the chicken and sequenced via the cDNA, consists of 262 amino acids, corresponding to a molecular mass of 30 kDa, and includes six functional
EF-hand domains [159]. In the gene there are 11
exons whose borders do not coincide with the 6
domains; 3 poly(A) signals are present and 3
mRNAs of different length are produced [306].
Sequence comparisons of human, bovine and
chicken 28-kDa calbindin suggest a low rate of
evolution of 0.3 . 10- 9 substitutions per amino
acid per year [213]. Instead of the 28-kDa calbindin, the mammalian gut contains an equivalent
molecule of 9 kDa. The small calbindin isolated
from rat gut consists of 79 amino acids and possesses only two calcium-binding domains, which
apparently correspond to domains I and II of calmodulin. The 9-kDa calbindin from porcine and
bovine intestine differ by about 20 % from that of
the rat [52]. The kidneys of mammals, birds,
reptiles and amphibians express the 28-kDa calbindin, and only in the bat Rousettus aegyptiacus
has a 9-kDa version been found. The 28-kDa
molecule is also present in the cerebellum, and
the 9-kDa form occurs in the placenta [209].
There are several families of calcium-binding
proteins which do not belong to the super-family
10.8.2 Other Calcium-Binding Proteins
351
of the EF-hand proteins. The calsequestrins are
the predominant Ca2+ -binding protein of the cisternae of the sarcoplasmic reticulum in the skeletal muscle of the mammals, birds and amphibians, and in mammalian heart. They bind 40-50
calcium ions per molecule with a relatively low
affinity, and ensure that the Ca 2 + concentration in
the lumen of the endoplasmic reticulum does not
exceed 1 mmol/l, thereby reducing the task of the
calcium pump. Several such proteins have been
sequenced via cDNA. The calsequestrin from the
fast-twitch skeletal muscle of the rabbit is an
aspartate/glutamate-rich glycoprotein of 63 kDa
with 367 amino acids. The calsequestrins from the
slow-twitch skeletal muscles and heart muscle differ in their glycosylation and molecular mass
[317,319]. The sarcoplasmic reticulum of smooth
muscle and the endoplasmic reticulum of the nonmuscle cells contain no calsequestrin; the predominant Ca2+ -binding proteins in this case are the
caireticuiins, which are located in their own storage compartments, the calciosomes. However,
calreticulins are also found together with calsequestrins in the sarcoplasmic reticulum of striated
muscle. Calreticulin from rabbit skeletal muscle
consists of 418 amino acids, 109 of them acidic,
and binds 43 Ca2+ ions with low affinity. It shows
clear agreement with calsequestrin in two parts of
its sequence [69, 184, 287]. Calsequestrin-like
proteins have also been found in plant cells [146].
More recently, a further group of calcium-binding
proteins has been discovered; these have no EFhand structures in their sequences and have a
high affinity for phospholipids. These proteins
were originally given names like lipocortin, calpactin, calelectrin, endonexin, synexin, chromobindin and calcimedin; the members of the new
super-family of calcium-binding proteins are now
referred to as annexins [118]. The annexins are
found in very different types of vertebrate cells,
but have also been reported in Drosophila and
are probably widely distributed in the animal
kingdom. Some annexins are highly tissue specific; for example, annexin IV (32.5-kDa calelectrin) is found only in the epithelial cells of the gall
and pancreas ducts, and annexin II (lipocortin 2)
is found mainly in the microvilli. In contrast,
annexin VI (67-kDa calelectrin) is found in all
cells. The amino acid sequences of the smaller
annexins (ca. 32 kDa) always show four repeats
of about 80-85 amino acids, and in the larger
annexins (ca. 67 kDa) there are eight such
repeats [14, 82, 118, 138].
membrane. They also act extracellularly: the ~~
homodimer is identical to the neurite extension
factor (NEF) which stimulates neurite growth;
the a~ heterodimer has no such activity [135].
The sea urchin Strongylocentrotus purpuratus has
about eight spec genes which are expressed
exclusively in the aboral ectodermal cells of the
embryo. They encode about ten different spec
proteins of 14-17 kDa with four EF-hand
domains, which are produced during embryogenesis in the same cells but at different times and
in different amounts. The specl protein is, for a
time, one of the most predominant proteins of the
embryo. Its precise function is not known; however, it appears to bind to actin and effect regular
changes in the cell form. If the spec proteins
really play an important role in embryo development, corresponding proteins would be expected
in other sea urchin species. In fact, a protein of
34 kDa with eight EF-hand domains in Lytechinus pictus could have arisen by the duplication of
a spec-like protein [93, 314].
The duodenum of birds and mammals contains
calcium-binding proteins with a similar function
but different structures; these are involved in
vitamin D-dependent calcium absorption and are
now mostly known as calbindins. The so-called
28-kDa calbindin, isolated from the gut wall of
the chicken and sequenced via the cDNA, consists of 262 amino acids, corresponding to a molecular mass of 30 kDa, and includes six functional
EF-hand domains [159]. In the gene there are 11
exons whose borders do not coincide with the 6
domains; 3 poly(A) signals are present and 3
mRNAs of different length are produced [306].
Sequence comparisons of human, bovine and
chicken 28-kDa calbindin suggest a low rate of
evolution of 0.3 . 10- 9 substitutions per amino
acid per year [213]. Instead of the 28-kDa calbindin, the mammalian gut contains an equivalent
molecule of 9 kDa. The small calbindin isolated
from rat gut consists of 79 amino acids and possesses only two calcium-binding domains, which
apparently correspond to domains I and II of calmodulin. The 9-kDa calbindin from porcine and
bovine intestine differ by about 20 % from that of
the rat [52]. The kidneys of mammals, birds,
reptiles and amphibians express the 28-kDa calbindin, and only in the bat Rousettus aegyptiacus
has a 9-kDa version been found. The 28-kDa
molecule is also present in the cerebellum, and
the 9-kDa form occurs in the placenta [209].
There are several families of calcium-binding
proteins which do not belong to the super-family
10.8.2 Other Calcium-Binding Proteins
351
of the EF-hand proteins. The calsequestrins are
the predominant Ca2+ -binding protein of the cisternae of the sarcoplasmic reticulum in the skeletal muscle of the mammals, birds and amphibians, and in mammalian heart. They bind 40-50
calcium ions per molecule with a relatively low
affinity, and ensure that the Ca 2 + concentration in
the lumen of the endoplasmic reticulum does not
exceed 1 mmol/l, thereby reducing the task of the
calcium pump. Several such proteins have been
sequenced via cDNA. The calsequestrin from the
fast-twitch skeletal muscle of the rabbit is an
aspartate/glutamate-rich glycoprotein of 63 kDa
with 367 amino acids. The calsequestrins from the
slow-twitch skeletal muscles and heart muscle differ in their glycosylation and molecular mass
[317,319]. The sarcoplasmic reticulum of smooth
muscle and the endoplasmic reticulum of the nonmuscle cells contain no calsequestrin; the predominant Ca2+ -binding proteins in this case are the
caireticuiins, which are located in their own storage compartments, the calciosomes. However,
calreticulins are also found together with calsequestrins in the sarcoplasmic reticulum of striated
muscle. Calreticulin from rabbit skeletal muscle
consists of 418 amino acids, 109 of them acidic,
and binds 43 Ca2+ ions with low affinity. It shows
clear agreement with calsequestrin in two parts of
its sequence [69, 184, 287]. Calsequestrin-like
proteins have also been found in plant cells [146].
More recently, a further group of calcium-binding
proteins has been discovered; these have no EFhand structures in their sequences and have a
high affinity for phospholipids. These proteins
were originally given names like lipocortin, calpactin, calelectrin, endonexin, synexin, chromobindin and calcimedin; the members of the new
super-family of calcium-binding proteins are now
referred to as annexins [118]. The annexins are
found in very different types of vertebrate cells,
but have also been reported in Drosophila and
are probably widely distributed in the animal
kingdom. Some annexins are highly tissue specific; for example, annexin IV (32.5-kDa calelectrin) is found only in the epithelial cells of the gall
and pancreas ducts, and annexin II (lipocortin 2)
is found mainly in the microvilli. In contrast,
annexin VI (67-kDa calelectrin) is found in all
cells. The amino acid sequences of the smaller
annexins (ca. 32 kDa) always show four repeats
of about 80-85 amino acids, and in the larger
annexins (ca. 67 kDa) there are eight such
repeats [14, 82, 118, 138].