346
10 Proteins of Muscle and the Cytoskeleton
embryonal heart and early embryonal skeletal
muscle. It may be that the acidic decapeptide
encoded by exon 5, which is missing from isoform
II (291 amino acids) from adult heart, is only
required for the early synthesis of sarcomeres. In
fact, the heart-specific Tn-T gene of the chicken
includes amongst its 18 exons the shortest known
exon sequence: exon 17 is only 17 bp long. In the
rat Tn-T gene the corresponding sequence is
included in the last exon; the number and position of the introns does not completely agree in
the two Tn-T genes [47].
Alternative splicing complicates comparison of
the Tn-T sequences of different species. The Tn-T
from the skeletal muscles of the rat and rabbit differ mainly in the N-terminal amino acids 1-36;
the amino acids 37-259 are identical except at
four positions. The differences between skeletalmuscle Tn-Ts of mammals and birds are also
mainly restricted to the terminal regions [25]. The
Tn-T from rabbit heart muscle (276 amino acids)
differs in particular in three regions (amino acids
17-46, 152-169 and 237-249) from that of skeletal muscle, but is in fact very similar to chicken
heart Tn-T in just these regions; it is possible that
this part of the polypeptide chain is essential for
heart function [215].
The Tn-I is also found in heart muscle, as well
as in both fast- and slow-twitch skeletal muscle in
different isoforms with up to 40 % differences in
sequence. The Tn-I from the skeletal muscle of
the chicken has a length of 182 amino acids [203],
whilst Tn-I from rabbit heart bears an N-terminal
extension of 26 amino acids; the phosphorylation
of the 20-Ser within this segment by a cAMPdependent protein kinase reduces the calcium
affinity of the troponin complex. In normal heart,
Tn-I is only 30 % phosphorylated, but after ~adrenergic stimulation it increases to 100 %. Only
one Tn-I has been sequenced from invertebrates,
from the crayfish Procambarus clarkii. It has an
acetylated N-terminus, contains two trimethyllysine residues and shows only 20-25 % agreement to vertebrate Tn-Is in its sequence of 201
amino acids; in contrast, the latter show more
than 60 % agreement with each other [140].
The Tn-C belongs, like the myosin LC, to the
protein super-family of the calmodulin-like,
calcium-binding proteins, and bears the typical
four calcium-binding sites. In the Tn-C of fasttwitch skeletal-muscle fibres, only binding sites I
and II are available for calcium regulation, whilst
sites III and IV are occupied by Mi+ under
physiological conditions. In the Tn-C from slowtwitch muscle fibres and heart muscle, only Ca 2 +binding site II is functional. The heart and slowtwitch skeletal-muscle fibres of the rabbit contain
the same Tn-C, and this differs in 56 of its 181
amino acids (35 %) from that of the fast-twitch
muscle fibres. Despite the identity of the Tn-Cs,
the troponin complexes in heart and slow-twitch
muscle fibres have differences in their calcium
affinities, determined partially by the different TnIs present. The Tn-Cs show relatively slow evolution: rabbit and bovine heart Tn-Cs differ in only
one amino acid; the skeletal muscle Tn-C of the
rabbit differs from that of man by one amino acid,
and from that of the chicken and frog Rana esculenta in 13-15 of about 160 amino acids (Table 10.1). The crayfish Procambarus clarkii has two
Tn-C isoforms which differ in 21 of 150 amino
acids, i.e. they are apparently encoded by different
genes. The giant barnacle Balanus nubilis also has
two Tn-Cs of 151 and 158 amino acids and these
agree by only 61 %. These are the only invertebrate Tn-Cs to have been described in detail, and
they have only one-third of their sequences in
common with the Tn-Cs and calmodulins of the
vertebrates. They have only two functional Ca2+binding sites [44, 141]. The annelids, arthropods,
molluscs, nematodes and some other invertebrates
have all been shown to possess Tn-Cs, but
investigations of the arthropod and mollusc molecules show only 0.75-1.3 Ca2+ ions are bound
in vitro compared with 3-4 in the vertebrates [35].
10.7 Paramyosins
Paramyosins are rod-shaped molecules with a
length of 130 nm and a diameter of 2 nm. Like
the LMM region of the myosins or the tropomyosins, they consist of two a helices associated into
a coiled-coil; the two chains are identical and
have a molecular mass of 100-110 kDa. At low
pH and in the presence of a high concentration of
divalent cations they form paracrystals; paramyosin is easily identified from its characteristic axial
periodicities of 72.5 and 14.5 nm. Higher or
lower amounts of paramyosin are to be found in
probably all invertebrate muscles, and it has been
detected in, for example, molluscs, annelids,
xiphosurans, insects, nematodes, trematodes and
cestodes; it is absent from vertebrate muscles and
non-muscle cells [103, 238]. The paramyosin gene
of Caenorhabditis elegans (unc-16) has been
sequenced. It codes for a protein of 868 amino
acids which has the heptad repeats of hydro-
10 Proteins of Muscle and the Cytoskeleton
embryonal heart and early embryonal skeletal
muscle. It may be that the acidic decapeptide
encoded by exon 5, which is missing from isoform
II (291 amino acids) from adult heart, is only
required for the early synthesis of sarcomeres. In
fact, the heart-specific Tn-T gene of the chicken
includes amongst its 18 exons the shortest known
exon sequence: exon 17 is only 17 bp long. In the
rat Tn-T gene the corresponding sequence is
included in the last exon; the number and position of the introns does not completely agree in
the two Tn-T genes [47].
Alternative splicing complicates comparison of
the Tn-T sequences of different species. The Tn-T
from the skeletal muscles of the rat and rabbit differ mainly in the N-terminal amino acids 1-36;
the amino acids 37-259 are identical except at
four positions. The differences between skeletalmuscle Tn-Ts of mammals and birds are also
mainly restricted to the terminal regions [25]. The
Tn-T from rabbit heart muscle (276 amino acids)
differs in particular in three regions (amino acids
17-46, 152-169 and 237-249) from that of skeletal muscle, but is in fact very similar to chicken
heart Tn-T in just these regions; it is possible that
this part of the polypeptide chain is essential for
heart function [215].
The Tn-I is also found in heart muscle, as well
as in both fast- and slow-twitch skeletal muscle in
different isoforms with up to 40 % differences in
sequence. The Tn-I from the skeletal muscle of
the chicken has a length of 182 amino acids [203],
whilst Tn-I from rabbit heart bears an N-terminal
extension of 26 amino acids; the phosphorylation
of the 20-Ser within this segment by a cAMPdependent protein kinase reduces the calcium
affinity of the troponin complex. In normal heart,
Tn-I is only 30 % phosphorylated, but after ~adrenergic stimulation it increases to 100 %. Only
one Tn-I has been sequenced from invertebrates,
from the crayfish Procambarus clarkii. It has an
acetylated N-terminus, contains two trimethyllysine residues and shows only 20-25 % agreement to vertebrate Tn-Is in its sequence of 201
amino acids; in contrast, the latter show more
than 60 % agreement with each other [140].
The Tn-C belongs, like the myosin LC, to the
protein super-family of the calmodulin-like,
calcium-binding proteins, and bears the typical
four calcium-binding sites. In the Tn-C of fasttwitch skeletal-muscle fibres, only binding sites I
and II are available for calcium regulation, whilst
sites III and IV are occupied by Mi+ under
physiological conditions. In the Tn-C from slowtwitch muscle fibres and heart muscle, only Ca 2 +binding site II is functional. The heart and slowtwitch skeletal-muscle fibres of the rabbit contain
the same Tn-C, and this differs in 56 of its 181
amino acids (35 %) from that of the fast-twitch
muscle fibres. Despite the identity of the Tn-Cs,
the troponin complexes in heart and slow-twitch
muscle fibres have differences in their calcium
affinities, determined partially by the different TnIs present. The Tn-Cs show relatively slow evolution: rabbit and bovine heart Tn-Cs differ in only
one amino acid; the skeletal muscle Tn-C of the
rabbit differs from that of man by one amino acid,
and from that of the chicken and frog Rana esculenta in 13-15 of about 160 amino acids (Table 10.1). The crayfish Procambarus clarkii has two
Tn-C isoforms which differ in 21 of 150 amino
acids, i.e. they are apparently encoded by different
genes. The giant barnacle Balanus nubilis also has
two Tn-Cs of 151 and 158 amino acids and these
agree by only 61 %. These are the only invertebrate Tn-Cs to have been described in detail, and
they have only one-third of their sequences in
common with the Tn-Cs and calmodulins of the
vertebrates. They have only two functional Ca2+binding sites [44, 141]. The annelids, arthropods,
molluscs, nematodes and some other invertebrates
have all been shown to possess Tn-Cs, but
investigations of the arthropod and mollusc molecules show only 0.75-1.3 Ca2+ ions are bound
in vitro compared with 3-4 in the vertebrates [35].
10.7 Paramyosins
Paramyosins are rod-shaped molecules with a
length of 130 nm and a diameter of 2 nm. Like
the LMM region of the myosins or the tropomyosins, they consist of two a helices associated into
a coiled-coil; the two chains are identical and
have a molecular mass of 100-110 kDa. At low
pH and in the presence of a high concentration of
divalent cations they form paracrystals; paramyosin is easily identified from its characteristic axial
periodicities of 72.5 and 14.5 nm. Higher or
lower amounts of paramyosin are to be found in
probably all invertebrate muscles, and it has been
detected in, for example, molluscs, annelids,
xiphosurans, insects, nematodes, trematodes and
cestodes; it is absent from vertebrate muscles and
non-muscle cells [103, 238]. The paramyosin gene
of Caenorhabditis elegans (unc-16) has been
sequenced. It codes for a protein of 868 amino
acids which has the heptad repeats of hydro-