code for the same amino acid sequence, the sea
urchin Arbacia punctulata produces two quite
similar isocalmodulins [94]. Retropseudogenes
identified in the multi-gene family of the rat,
humans and the chicken would code for calmodulins with quite deviant sequences, but are probably not expressed [142]. A protein of unknown
function, isolated from the optic lobe of the squid
Loligo pealei, is 68 % similar in its 149-aminoacid sequence to mammalian calmodulin, and
contains four calcium-binding sites [96].
Ca 2 + -calmodulin takes part in the regulation of
enzymatic and other cellular reactions through
involvement of the secondary messenger Ca 2 +,
either as a regulatory component of the enzyme
itself (MLCK, phosphorylase kinase), or indirectly, and hence more slowly, by influencing the
activity of protein kinases. Many enzymes which
are directly or indirectly regulated by Ca 2 + -
calmodulin are simultaneously substrates of
cAMP-dependent protein kinases; this is also
true for MLCK. Furthermore, as an activator of
cAMP phosphodiesterase, Ca 2 + -calmodulin interacts with the cAMP secondary messenger system.
Ca 2 + -calmodulin also has a functional relationship with contractile mechanisms and the cytoskeleton. Caldesmon of the smooth muscle alternates, according to the calcium concentration,
between calmodulin and F-actin with a sort of
flip-flop mechanism, and the microtubuleassociated tau factor does the same but with calmodulin and tubulin. Finally, calmodulin is
involved in the regulation of calcium pumps in
the plasma membrane and the endoplasmic reticulum membrane, and thus participates in the
calcium homeostasis of the cell [238].
Despite the extensive structural similarities
between all calmodulins, there are sometimes differences in function. Thus, for example, the guanylate cyclase of Tetrahymena is only activated by
the calmodulin of this ciliate, although it differs at
only 12 positions from other calmodulins [318].
The calmodulins of Tetrahymena,the mussels Patinopecten and Mytilus, and the fly Drosophila are
much less active in the activation of mammalian
brain phosphodiesterase than is mammalian calmodulin itself. Thus, is would appear that
coevolution of calmodulin and the enzymes has
occurred [86, 185]. In this connection, it is noteworthy that calmodulin from muscles of the migratory locust Schistocera gregaria can activate
phosphodiesterase from bovine brain but not the
enzyme from its own muscles.
10.8.2 Other Calcium-Binding Proteins
349
10.8.2 Other Calcium-Binding Proteins
Apart from the calmodulins, essential and regulatory myosin-LCs and troponin C, muscles and nonmuscle cells contain a series of further proteins
which may be included in this same protein superfamily on the basis of their EF-hand structure: the
parvalbumins of vertebrate muscle, the sarcoplasm
calcium-binding proteins (SCBP) of invertebrate
muscles, the large (28 kDa) and small (9 kDa)
cholecalciferol (vitamin D)-induced calbindins of
birds and mammals, the S-110 proteins, the calcium vector protein (CaVP) of the acranian Branchiostoma, the spec proteins of sea urchin
embryos, the calcium-dependent proteases (calpains), the muscle protein a actinin, and the fluorescent protein aequorin. A phylogenetic tree,
constructed by the parsimony method, of the 153
known EF-hand proteins shows that a least 12
evolutionary lines separated quite early and that
further subfamilies probably exist [190, 223]. From
comparisons of the calcium-binding domains, it
seems likely that the common ancestral form had
two calcium-binding sites, and independent
duplications in different evolutionary lines led to
proteins with four binding sites [213]. The parvalbumins lack the N-terminal 40 amino acids and,
therefore, calcium-binding site I; domain II is no
longer able to bind Ca 2 +, and sites III and IV have
increased affinities. Amongst the SCBPs of invertebrates are some with two or three functional
binding sites: I and II in the scallop Patinopecten,
I-II in the Crustacea, and I, III and IV in the polychaete Perinereis [272]. In troponin C, all four
binding sites are present but only I and II are calcium specific; III and IV have changed so much
that they preferentially bind Mg2+. Site I of the
heart type of troponin C can no longer bind Ca 2 +.
Amongst the myosin LCs, the regulatory LCs can
bind Ca 2 + at site I, whilst all four domains of the
essential LCs have completely lost the ability to
bind divalent ions. The 28-kDa calbindins of birds
and mammals contain six EF-hand structures, of
which four can bind calcium; in contrast, the 9kDa variety from mammalian gut has only two
calcium-binding sites. The two calbindins show no
great similarity, either to each other or to other
calcium-binding proteins, and quite clearly deviated early from the evolutionary line of the
calmodulin-like proteins [7, 76, 112].
The parvalbumins are acidic cytosolic proteins
of 108-110 amino acids and a molecular mass of
12 kDa. They were discovered in 1934 in frog
muscle, and are found at the highest concentrations in the white muscles of fish, amphibians and
urchin Arbacia punctulata produces two quite
similar isocalmodulins [94]. Retropseudogenes
identified in the multi-gene family of the rat,
humans and the chicken would code for calmodulins with quite deviant sequences, but are probably not expressed [142]. A protein of unknown
function, isolated from the optic lobe of the squid
Loligo pealei, is 68 % similar in its 149-aminoacid sequence to mammalian calmodulin, and
contains four calcium-binding sites [96].
Ca 2 + -calmodulin takes part in the regulation of
enzymatic and other cellular reactions through
involvement of the secondary messenger Ca 2 +,
either as a regulatory component of the enzyme
itself (MLCK, phosphorylase kinase), or indirectly, and hence more slowly, by influencing the
activity of protein kinases. Many enzymes which
are directly or indirectly regulated by Ca 2 + -
calmodulin are simultaneously substrates of
cAMP-dependent protein kinases; this is also
true for MLCK. Furthermore, as an activator of
cAMP phosphodiesterase, Ca 2 + -calmodulin interacts with the cAMP secondary messenger system.
Ca 2 + -calmodulin also has a functional relationship with contractile mechanisms and the cytoskeleton. Caldesmon of the smooth muscle alternates, according to the calcium concentration,
between calmodulin and F-actin with a sort of
flip-flop mechanism, and the microtubuleassociated tau factor does the same but with calmodulin and tubulin. Finally, calmodulin is
involved in the regulation of calcium pumps in
the plasma membrane and the endoplasmic reticulum membrane, and thus participates in the
calcium homeostasis of the cell [238].
Despite the extensive structural similarities
between all calmodulins, there are sometimes differences in function. Thus, for example, the guanylate cyclase of Tetrahymena is only activated by
the calmodulin of this ciliate, although it differs at
only 12 positions from other calmodulins [318].
The calmodulins of Tetrahymena,the mussels Patinopecten and Mytilus, and the fly Drosophila are
much less active in the activation of mammalian
brain phosphodiesterase than is mammalian calmodulin itself. Thus, is would appear that
coevolution of calmodulin and the enzymes has
occurred [86, 185]. In this connection, it is noteworthy that calmodulin from muscles of the migratory locust Schistocera gregaria can activate
phosphodiesterase from bovine brain but not the
enzyme from its own muscles.
10.8.2 Other Calcium-Binding Proteins
349
10.8.2 Other Calcium-Binding Proteins
Apart from the calmodulins, essential and regulatory myosin-LCs and troponin C, muscles and nonmuscle cells contain a series of further proteins
which may be included in this same protein superfamily on the basis of their EF-hand structure: the
parvalbumins of vertebrate muscle, the sarcoplasm
calcium-binding proteins (SCBP) of invertebrate
muscles, the large (28 kDa) and small (9 kDa)
cholecalciferol (vitamin D)-induced calbindins of
birds and mammals, the S-110 proteins, the calcium vector protein (CaVP) of the acranian Branchiostoma, the spec proteins of sea urchin
embryos, the calcium-dependent proteases (calpains), the muscle protein a actinin, and the fluorescent protein aequorin. A phylogenetic tree,
constructed by the parsimony method, of the 153
known EF-hand proteins shows that a least 12
evolutionary lines separated quite early and that
further subfamilies probably exist [190, 223]. From
comparisons of the calcium-binding domains, it
seems likely that the common ancestral form had
two calcium-binding sites, and independent
duplications in different evolutionary lines led to
proteins with four binding sites [213]. The parvalbumins lack the N-terminal 40 amino acids and,
therefore, calcium-binding site I; domain II is no
longer able to bind Ca 2 +, and sites III and IV have
increased affinities. Amongst the SCBPs of invertebrates are some with two or three functional
binding sites: I and II in the scallop Patinopecten,
I-II in the Crustacea, and I, III and IV in the polychaete Perinereis [272]. In troponin C, all four
binding sites are present but only I and II are calcium specific; III and IV have changed so much
that they preferentially bind Mg2+. Site I of the
heart type of troponin C can no longer bind Ca 2 +.
Amongst the myosin LCs, the regulatory LCs can
bind Ca 2 + at site I, whilst all four domains of the
essential LCs have completely lost the ability to
bind divalent ions. The 28-kDa calbindins of birds
and mammals contain six EF-hand structures, of
which four can bind calcium; in contrast, the 9kDa variety from mammalian gut has only two
calcium-binding sites. The two calbindins show no
great similarity, either to each other or to other
calcium-binding proteins, and quite clearly deviated early from the evolutionary line of the
calmodulin-like proteins [7, 76, 112].
The parvalbumins are acidic cytosolic proteins
of 108-110 amino acids and a molecular mass of
12 kDa. They were discovered in 1934 in frog
muscle, and are found at the highest concentrations in the white muscles of fish, amphibians and