Troponin-like proteins from muscles of the scallop, Aequipecten irradians

Molecular evolution of troponin I and a role of its N-terminal extension in nematode locomotion

The troponin complex, composed of troponin T (TnT), troponin I (TnI), and troponin C (TnC), is the major calcium-dependent regulator of muscle contraction, which is present widely in both vertebrates and invertebrates. Little is known about evolutionary aspects of troponin in the animal kingdom. Using a combination of data mining and functional analysis of TnI, we report evidence that an N-terminal extension of TnI is present in most of bilaterian animals as a functionally important domain. Troponin components have been reported in species in most of representative bilaterian phyla. Comparison of TnI sequences shows that the core domains are conserved in all examined TnIs, and that N- and C-terminal extensions are variable among isoforms and species. In particular, N-terminal extensions are present in all protostome TnIs and chordate cardiac TnIs but lost in a subset of chordate TnIs including vertebrate skeletal-muscle isoforms. Transgenic rescue experiments in Caenorhabditis elegans striated muscle show that the N-terminal extension of TnI (UNC-27) is required for coordinated worm locomotion but not in sarcomere assembly and single muscle-contractility kinetics. These results suggest that N-terminal extensions of TnIs are retained from a TnI ancestor as a functional domain.

Thin Filament Structure and the Steric Blocking Model

By interacting with the troponin-tropomyosin complex on myofibrillar thin filaments, Ca2+ and myosin govern the regulatory switching processes influencing contractile activity of mammalian cardiac and skeletal muscles. A possible explanation of the roles played by Ca2+ and myosin emerged in the early 1970s when a compelling "steric model" began to gain traction as a likely mechanism accounting for muscle regulation. In its most simple form, the model holds that, under the control of Ca2+ binding to troponin and myosin binding to actin, tropomyosin strands running along thin filaments either block myosin-binding sites on actin when muscles are relaxed or move away from them when muscles are activated. Evidence for the steric model was initially based on interpretation of subtle changes observed in X-ray fiber diffraction patterns of intact skeletal muscle preparations. Over the past 25 years, electron microscopy coupled with three-dimensional reconstruction directly resolved thin filament organization under many experimental conditions and at increasingly higher resolution. At low-Ca2+, tropomyosin was shown to occupy a "blocked-state" position on the filament, and switched-on in a two-step process, involving first a movement of tropomyosin away from the majority of the myosin-binding site as Ca2+ binds to troponin and then a further movement to fully expose the site when small numbers of myosin heads bind to actin. In this contribution, basic information on Ca2+-regulation of muscle contraction is provided. A description is then given relating the voyage of discovery taken to arrive at the present understanding of the steric regulatory model.

Gene expression patterns of abalone, Haliotis tuberculata, during successive infections by the pathogen Vibrio harveyi

Since 1998, episodic mass mortality of the abalone Haliotistuberculata has been observed along the northern Brittany coast of France caused by a complex interaction among the host, pathogen and environmental factors. In the present study, abalone were submitted to two successive infections with the pathogen Vibrioharveyi under controlled conditions. During the first challenge, infection by V.harveyi resulted in 64% mortality of mature abalone. After a second infection of those surviving the first challenge, only 44% mortality was observed. Physiological variability in the host response appears to be a major determinant in susceptibility to V.harveyi. In order to isolate differentially expressed genes in H.tuberculata challenged with this bacterium, suppression subtractive hybridization (SSH) cDNA libraries were constructed from muscle of moribund abalone (susceptibles), surviving individuals (apparently resistant to the bacterium) and control (unexposed) animals. Of the 1152 clones sequenced, 218 different partial cDNA sequences were obtained and represented 69 known genes. Of these, 65 were identified for the first time in H.tuberculata. Using real-time PCR, a time-course study was conducted on 19 of the genes identified by SSH. A majority of differentially expressed transcripts were down-regulated in susceptible individuals as compared to their resistant counterparts. Bacterial challenge of abalone resulted in the up-regulation of three transcripts (encoding ferritin, heat shock protein HSP84 and fatty acid binding protein FABP) in those that survived exposure to V.harveyi. This study has identified potential candidates for further investigation into the functional basis of resistance and susceptibility to summer vibriosis outbreaks in abalone.

Ca2+ -induced tropomyosin movement in scallop striated muscle thin filaments

Striated muscle contraction in most animals is regulated at least in part by the troponin-tropomyosin (Tn-Tm) switch on the thin (actin-containing) filaments. The only group that has been suggested to lack actin-linked regulation is the mollusks, where contraction is regulated through the myosin heads on the thick filaments. However, molluscan gene sequence data suggest the presence of troponin (Tn) components, consistent with actin-linked regulation, and some biochemical and immunological data also support this idea. The presence of actin-linked (in addition to myosin-linked) regulation in mollusks would simplify our general picture of muscle regulation by extending actin-linked regulation to this phylum as well. We have investigated this question structurally by determining the effect of Ca(2+) on the position of Tm in native thin filaments from scallop striated adductor muscle. Three-dimensional reconstructions of negatively stained filaments were determined by electron microscopy and single-particle image analysis. At low Ca(2+), Tm appeared to occupy the "blocking" position, on the outer domain of actin, identified in earlier studies of regulated thin filaments in the low-Ca(2+) state. In this position, Tm would sterically block myosin binding, switching off filament activity. At high Ca(2+), Tm appeared to move toward a position on the inner domain, similar to that induced by Ca(2+) in regulated thin filaments. This Ca(2+)-induced movement of Tm is consistent with the hypothesis that scallop thin filaments are Ca(2+) regulated.

Invertebrate muscles: thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

This is the second in a series of canonical reviews on invertebrate muscle. We cover here thin and thick filament structure, the molecular basis of force generation and its regulation, and two special properties of some invertebrate muscle, catch and asynchronous muscle. Invertebrate thin filaments resemble vertebrate thin filaments, although helix structure and tropomyosin arrangement show small differences. Invertebrate thick filaments, alternatively, are very different from vertebrate striated thick filaments and show great variation within invertebrates. Part of this diversity stems from variation in paramyosin content, which is greatly increased in very large diameter invertebrate thick filaments. Other of it arises from relatively small changes in filament backbone structure, which results in filaments with grossly similar myosin head placements (rotating crowns of heads every 14.5 nm) but large changes in detail (distances between heads in azimuthal registration varying from three to thousands of crowns). The lever arm basis of force generation is common to both vertebrates and invertebrates, and in some invertebrates this process is understood on the near atomic level. Invertebrate actomyosin is both thin (tropomyosin:troponin) and thick (primarily via direct Ca(++) binding to myosin) filament regulated, and most invertebrate muscles are dually regulated. These mechanisms are well understood on the molecular level, but the behavioral utility of dual regulation is less so. The phosphorylation state of the thick filament associated giant protein, twitchin, has been recently shown to be the molecular basis of catch. The molecular basis of the stretch activation underlying asynchronous muscle activity, however, remains unresolved.

Invertebrate Muscles: Muscle Specific Genes and Proteins

This is the first of a projected series of canonic reviews covering all invertebrate muscle literature prior to 2005 and covers muscle genes and proteins except those involved in excitation-contraction coupling (e.g., the ryanodine receptor) and those forming ligand- and voltage-dependent channels. Two themes are of primary importance. The first is the evolutionary antiquity of muscle proteins. Actin, myosin, and tropomyosin (at least, the presence of other muscle proteins in these organisms has not been examined) exist in muscle-like cells in Radiata, and almost all muscle proteins are present across Bilateria, implying that the first Bilaterian had a complete, or near-complete, complement of present-day muscle proteins. The second is the extraordinary diversity of protein isoforms and genetic mechanisms for producing them. This rich diversity suggests that studying invertebrate muscle proteins and genes can be usefully applied to resolve phylogenetic relationships and to understand protein assembly coevolution. Fully achieving these goals, however, will require examination of a much broader range of species than has been heretofore performed.

Evaluation of the symmetric model for myosin-linked regulation: effect of site-directed mutations in the regulatory light chain on scallop myosin

Regulatory myosins are controlled through mechanisms intrinsic to their structures and can alternate between activated and inhibited states. However, the structural difference between these two states is unclear. Scallop (Pecten maximus) striated adductor myosin is activated directly by calcium. It has been proposed that the two heads of scallop myosin are symmetrically arranged and interact through their regulatory light chains [Offer and Knight (1996) J. Mol. Biol. 256, 407-416], the interface being strengthened in the inhibited state. By contrast, vertebrate smooth-muscle myosin is activated by phosphorylation. Its structure in the inhibited state has been determined from two-dimensional crystalline arrays [Wendt, Taylor, Trybus and Taylor (2001) Proc. Natl. Acad. Sci. U.S.A. 98, 4361-4366] and is asymmetric, requiring no interaction between regulatory light chains. Using site-directed mutagenesis of the scallop regulatory light chain, we have tested the symmetric model for scallop adductor muscle myosin. Specifically, we have made myosin hybrid molecules from scallop (P. maximus) myosin, in which the normal regulatory light chains have been replaced by expressed light chains containing mutations in three residues proposed to participate in the interaction between regulatory light chains. The mutations were R126A (Arg126-->Ala), K130A and E131A; made singly, in pairs or all three together, these mutations were designed to eliminate hydrogen bonding or salt linkages between heads, which are key features of this model. Functional assays to address the competence of these hybrid myosins to bind calcium specifically, to exhibit a calcium-regulated myofibrillar Mg-ATPase and to display calcium-dependent actin sliding were performed. We conclude that the symmetrical model does not describe the inhibited state of scallop regulatory myosin and that an asymmetric structure is a plausible alternative.

Akazara scallop troponin C: Ca(2+)-induced conformational change and interaction with rabbit troponin subunits

The number of specific Ca2+ bound to Akazara scallop troponin C was estimated to be 0.7 with an apparent binding constant of 5 x 10(5) M-1 (T. Ojima and K. Nishita, 1986, J. Biol. Chem. 261, 16749-16754). In the present paper, we report on the Ca(2+)-induced conformational changes in the troponin C and the interaction of the troponin C with rabbit troponin subunits. The Ca2+ binding to the troponin C caused a marked change in difference uv absorption spectra and a retardation of elution on Sephacryl S-200 gel filtration. However, its circular dichroism spectrum was hardly changed by the Ca2+ binding. These results suggest that the Ca2+ binding to the troponin C induced changes predominantly in tertiary structure rather than in secondary structure. Akazara scallop troponin C was shown to be able to bind to rabbit troponin I-Cellulofine affinity column, but the affinity was not greatly increased by Ca2+ unlike the case of rabbit troponin C. On hybridizing with rabbit troponin T and I, Akazara scallop troponin C was shown to be incapable of substituting rabbit troponin C; i.e., the hybrid troponin strongly inhibited the Mg-ATPase activity of rabbit actomyosin-tropomyosin irrespective of the presence or absence of Ca2+, thus recovering no Ca2+ sensitivity.

Generation of multiple troponin T isoforms is a common feature of the muscles in various chordate animals

1. Troponin T (TNT) expressed in various vertebrate skeletal and ascidian smooth muscles was examined by two-dimensional electrophoresis in combination with immunoblotting. 2. A monoclonal anti-TNT antibody, NT-302, exhibited binding ability to various TNT variants in the vertebrate and protochordate animals. 3. TNT isoform pattern differed among the animals, but the existence of multiple TNT isoforms in a single muscle tissue was the general feature of all the animals examined.

Arthrin: a new actin-like protein in insect flight muscle

There are one or more proteins of 50,000 to 60,000 Mr in the thin filaments of insect flight muscle. A protein of 55,000 Mr has been isolated from insect fibrillar flight muscle and called arthrin. Despite its higher molecular weight, arthrin is in many ways like actin. The amino acid composition of arthrin was similar to that of actin. There were similarities in the peptides produced by digesting the denatured proteins and mild digestion of polymerized proteins cleaved similar-sized fragments from arthrin and actin. Polymerized arthrin activated the Mg2+ ATPase of myosin to the same extent as actin and the ATPase was regulated by rabbit or Lethocerus troponin and tropomyosin. Arthrin did not itself act as troponin-T. Electron microscopy of negatively stained specimens showed that arthrin and actin filaments were similar in structure and that arthrin could be decorated by rabbit subfragment-1 to form normal-looking arrowheads. Arthrin formed paracrystals at an optimum concentration of MgCl2 (25 mM) that was somewhat lower than the optimum for actin paracrystals. Optical diffraction showed that the structure of the paracrystals was similar to those formed from actin. The mass of arthrin and actin filaments relative to phage fd was measured by scanning transmission electron microscopy; the relative mass of arthrin and actin was 1.33, in agreement with molecular weight estimations. Therefore arthrin has the properties of a heavy form of actin. The proportion of actin, arthrin and troponin-T in Lethocerus myofibrils was six moles of actin to one mole of arthrin and one mole of troponin-T. The function of arthrin is not known.

A mechanical study of regulation in the striated adductor muscle of the scallop

Chemically skinned fibre bundles were prepared from the striated adductor muscle of the sea scallop, Placopecten magellanicus. The relation between tension and calcium concentration was determined in activating solutions containing 5 mM-MgATP, ionic strength 0.2, pH 7.1 at 20 degrees C. The isometric tension rose from zero to its maximum value between pCa 6.0 and 5.2. The steepness of the relation cannot be accounted for in terms of the binding of calcium to the two known sites on myosin and suggests that there must be an additional, co-operative mechanism. The regulatory light chain content of the fibre bundles was determined by urea gel electrophoresis and was found to be approximately 2 light chains per myosin molecule. The regulatory light chains were removed completely by treatment with EDTA at 25-30 degrees C. Fibre bundles then showed a total loss of control over contraction; a high tension was generated whether or not calcium was present in the bathing solution. Complete removal of the regulatory light chains did not greatly affect the tension generated or the stiffness in the rigor state. Control of contraction could be restored completely by the addition of regulatory light chains from scallop muscle. Treatment with EDTA at 0-12 degrees C resulted in the removal of 0.76-2.0 regulatory light chains per myosin molecule. Fibre bundles for which removal was less than complete were partially sensitive to calcium, i.e. tension was higher in the presence of calcium than in its absence. The results indicate that the normal mechanism of tension generation in scallop muscle is mediated primarily through myosin and not thin filament control. This finding is consistent with previous studies of the ATPase activity of myofibrils from scallop muscle.

The characterization of invertebrate troponin C

TNCs from lobster, mussel, and squid migrated with rabbit TNC at an apparent mol. wt of 18,000. Electrophoretic mobilities in the presence or absence of Ca2+ were compared: the electrophoretic mobility of rabbit TNC was greater in the presence of Ca2+ than it its absence, but all invertebrate TNCs tested migrated identically, whether Ca2+ was present or not. The Ca2+-binding capacity of invertebrate TNCs was only one Ca2+ ion per molecule. The alpha-helix contents in the presence or absence of Ca2+ were compared: rabbit TNC changed by a value of 16% and invertebrate TNCs by 4%. Antibodies to loligo TNC did not cross-react with rabbit TNC, but did interact with their molluscan TNCs.

Structural and functional properties of the non-muscle tropomyosins

The non-muscle tropomyosins (TMs), isolated from such tissues as platelets, brain and thyroid, are structurally very similar to the muscle TMs, being composed of two highly alpha-helical subunits wound around each other to form a rod-like molecule. The non-muscle TMs are shorter than the muscle TMs; sequence analysis demonstrates that each subunit of equine platelet TM consists of 247 amino acids, 37 fewer than for skeletal muscle TM. The major differences in sequence between platelet and skeletal muscle TM are found near the amino and carboxyl terminal ends of the proteins. Probably as the result of such alterations, the non-muscle TMs aggregate in a linear end-to-end manner much more weakly than do the muscle TMs. Since end-to-end interactions are responsible for the highly cooperative manner in which TM binds to actin, the non-muscle TMs have a lower affinity for actin filaments than do the muscle TMs. However, the attachment of other proteins to actin (e.g. the Tn-I subunit of skeletal muscle troponin or the S-1 subfragment of skeletal muscle myosin) can increase the affinity of actin filaments for non-muscle TM. The non-muscle TMs interact functionally with the Tn-I component of skeletal muscle troponin to inhibit the ATPase activity of muscle actomyosin and with whole troponin to regulate the muscle actomyosin ATPase in a Ca++-dependent manner, even though one of the binding sites for troponin on skeletal TM is missing in non-muscle TM. A novel actomyosin regulatory system can be produced using Tn-I, calmodulin and non-muscle TM; in this case inhibition is released when the non-muscle TM detaches from the actin filament in the presence of Ca++. Although it has not yet been demonstrated that the non-muscle TMs participate in a Ca++-dependent contractile regulatory system in vivo it does appear that they are associated with actin filaments in vivo.

The distribution of troponin-like proteins on thin filaments of the bay scallop, aequipecten irradians

Antibodies reacting either with bay scallop troponin-C-like protein or both bay scallop troponin-I-like protein and a possible scallop troponin-T were used to determine the distribution of the troponin complex on bay scallop striated muscle thin filaments. Both antibodies cause thin filaments to associate laterally and electron microscopy of such aggregates indicates a periodicity of approximately 38 nm, a distribution characteristic of proteins comprising the troponin complex in vertebrate and arthropod striated muscle. These studies therefore provide additional evidence for the presence of thin filament-linked regulation in bay scallops.

The ionic requirements for regulation by molluscan thin filaments

Initial studies on molluscan muscle regulation indicated that thin filaments do not confer Ca2+-dependence on vertebrate myosin ATPase, and hence that molluscan muscles do not possess thin filament-linked regulatory systems. Subsequently it was shown that molluscan thin filaments do, in fact, impart Ca2+-sensitivity but only at Mg2+ concentrations greater than those used in the earlier studies. In the present study it is shown that Mg2+ prevents significant dissociation of tropomyosin and troponin subunits from thin filaments at the low monovalent ion concentrations typically employed to assay actomyosin ATPase; as a result Mg2+ allows expression of the molluscan thin filament regulatory system under these conditions.

Depressant effect of active shortening in the anterior byssus retractor muscle of Mytilus edulis

The effect of shortening during activity, previously characterized in vertebrate striated muscle, was investigated in the anterior byssus retractor muscle (ABRM) of the mollusc Mytilus edulis. This muscle is considered to have an essentially myosin-linked Ca2+-regulatory system. Release steps of different amplitude were performed during isometric phasic contraction, and force redevelopment was recorded at a muscle length L1, defined as 90% of the muscle length at which a slight resting tension, approximately 1 mN, appeared in the presence of 2.5 X 10(-5) M 5-HT. Active shortening caused a graded depression of the contractile force without affecting the total duration of the mechanical response. Peak redeveloped force after muscle shortening of 0.06 L1 and 0.18 L1 was reduced by approximately 1.5% and 7.0%, respectively, of the isometric tension value at L1. The shortening effect was fully reversible, and had a lifetime of approximately 8 to 9 s. The depressant effect of active shortening was augmented at a reduced degree of activation of the muscle. The presence of caffeine and dantrolene and altered tonicity of the extracellular medium (0.9 T-1.2 T) did not significantly affect the shortening induced depression obtained at maximum phasic activation of the preparation. The nature of the shortening effect is compared to that obtained in vertebrate striated muscle and is discussed on the basis of differences in Ca2+-regulation of the contractile system in these two muscles.

Myosin and actin from ascidian smooth muscle and their interaction

Myosin and actin were purified from ascidian smooth muscle. Ascidian myosin contained two classes of light chains and the pH dependence of Ca2+-activated ATPase and the KCl dependence of actin-activated ATPase of ascidian myosin differed from those of vertebrate skeletal myosin. Troponin-tropomyosin complex from ascidian increased the ATPase activity of ascidian reconstituted actomyosin in a Ca2+-dependent manner. Ascidian myosin provided the reconstituted actomyosin with the responsiveness to calcium ions. Two actin isoforms were present in ascidian, which were distinguished by isoelectric points.

Properties of tropomyosin from the dual-regulated obliquely striated body wall muscle of the earthworm (Lumbricus terrestris L.)

The obliquely striated body wall muscle of the earthworm Lumbricus terrestris L. possesses a dual actin-linked and myosin-linked regulatory system. Tropomyosin from this muscle has now been purified and its functional properties compared to tropomyosin from vertebrate skeletal muscle. Earthworm tropomyosin has a molecular weight of about 70 000 and is composed of two polypeptide chains of molecular weight of 34 000 and 37 000. Structural and functional similarities to skeletal muscle tropomyosin were demonstrated with respect to the formation and periodicity of paracrystals and nets and the potentiation of skeletal muscle acto-SF1 ATPase activity at low ATP concentration. Likewise, earthworm tropomyosin inhibited skeletal muscle acto-HMM ATPase activity at normal ATP concentrations but to a much greater extent than skeletal muscle tropomyosin; this inhibition was removed by skeletal muscle troponin, in the presence of Ca2+. In a system containing earthworm myosin and skeletal muscle actin, earthworm tropomyosin had no detectable influence on the actin-activated ATPase activity. It is concluded that earthworm tropomyosin plays an active role in the actin-linked troponin-dependent regulatory system and has no measurable effect on the regulation via myosin.

Calcium-regulatory mechanisms. Functional classification using skinned fibers

The primary purpose of this study was to determine whether various agents (adenosine 3-thiotriphosphate [ATP gamma S], trifluoperazine [TFP], troponin I, the catalytic subunit of the cyclic adenosine 3',5'-monophosphate dependent protein kinase [C-subunit], and calmodulin [CaM]) could be used to classify skinned fiber types, and then to determine whether the proposed mechanisms for Ca2+ regulation were consistent with the results. Agents (ATP gamma S, TFP, C-subunit, CaM) expected to alter a light chain kinase-phosphatase system strongly affect the Ca2+-activated tension in skinned gizzard smooth muscle fibers, whereas these agents have no effect on skinned mammalian striated and scallop adductor fibers. Troponin I, which is known to bind strongly to troponin C and CaM, inhibits Ca2+ activation of skinned mammalian striated and gizzard fibers but not scallop adductor muscle. The results in different types of skinned fibers are consistent with proposed mechanisms for Ca2+ regulation.

The regulation of tension in a chemically skinned molluscan smooth muscle: effect of Mg2+ on the Ca2+-activated tension generation

Chemically skinned anterior byssus retractor muscle (ABRM) preparations were prepared by treatment with the nonionic detergents saponin and Triton X-100. Both maximum peak tension and rate of contraction were found to be greater in saponin-treated ABRM than in ABRM treated with Triton X-100. Active tension was initiated at a concentration of free Ca2+ above 0.1 microM, and maximum tension development was found at a [Ca2+] = approximately 32 microM. During exposure of the muscle preparation to optimal Ca2+ concentration, a high and almost constant tension level was sustained. The force recovery was high after a quick release during this period indicating the presence of an "active" state rather than a "catch" state. Actually, a state equivalent to the catch state in the living ABRM could not be induced, if the Ca2+ concentration was above 0.1 microM. Variations in the ionic strength in the range of 0.07--0.28 M had no influence on active state and only slightly affected the maximum tension developed. The influence of Mg2+ on the Ca2+-activated tension was examined by studying the tension-pCa relation at two concentrations of free Mg2+ (0.43 and 4.0 mM). The tension-pCa relation was found to be S-shaped with tension increasing steeply over approximately 1 pCa unit, indicating the existence of cooperativity between Ca2+ sites. Increasing the free concentration of Mg2+ shifted the tension-pCa relation to lower pCa as in striated muscles, demonstrating a decreasing Ca2+ sensitivity with increasing Mg2+. At [Mg2+] = 4.0 mM the half-maximum tension was found at [Ca2+] = 0.43 microM, decreasing to 0.20 microM at [Mg2+] = 0.43 mM. At both Mg2+ concentrations studied, plots of log Prel/(1--Prel) vs. log [Ca2+] were nonlinear with a shape indicating a rather complicated model for cooperativity, probably involving four sites for Ca2+. These Ca2+--Mg2+ interactions are most probably taking place at the myosin head itself because troponin is absent in this myosin-regulated muscle.

The stoichiometry and location of troponin I- and troponin C-like proteins in the myofibril of the bay scallop, Aequipecten irradians

Localization and quantification studies were carried out on bay-scallop (Aequipecten irradians) striated-muscle troponin C- and troponin I-like proteins. Indirect immunofluorescence microscopy of scallop myofibrils stained with either rabbit anti-(scallop troponin I) or anti-(scallop troponin C) antibodies shows staining of all I-bands observed. The results of quantification studies using sodium dodecyl sulfate poly-acrylamide-gel electrophoresis of untreated scallop myofibrils, washed scallop myofibrils, and isolated scallop thin filaments indicate an actin/tropomyosin/troponin-C molar rationn of 7:1:1. The molar ratio for troponin I could not be determined in untreated myofibrils because of interfering bands; in washed myofibrils a value of 0.6 mol of troponin I/mol of tropomyosin was found. Purified scallop troponin C binds Ca2+ and interacts with scallop troponin I to relieve troponin I-induced inhibition of actomyosin ATPase. Although scallop troponin C is an acidic protein, it appears to be less acidic than troponin C from higher organisms. A calmodulin-like protein has been isolated from scallop striated muscle that activates bovine brain phosphodiesterase to the same extent as does brain calmodulin. Its amino acid composition and its electrophoretic mobility on alkaline 6 M-urea/polyacrylamide gels differs from that of scallop troponin C, and it appears not to be associated with thin filaments.

The Croonian lecture, 1979: Regulation of muscle contraction

In this lecture I review briefly the history of the recognition of calcium ion as the sole regulatory factor of muscle contraction at the molecular level and how this led to the discovery of the troponin-tropomyosin system, which is the regulatory system of striated muscles of almost all deuterostomias and some protostomias. This is followed by a brief comment on the myosin-linked regulation, which plays a dominating role in many protostomian muscles. The regulatory mechanism in vertebrate smooth muscle is then discussed; the view is advanced that the leiotonin-tropomyosin system may be the only regulatory device for this muscle. Ca-binding components of troponin and smooth muscles of vertebrates are compared with modulator protein, an omnipresent Ca-binding protein of very conservative nature throughout evolution. Finally, the modes of action of Ca ion in different kinds of cell motility are discussed from an evolutionary point of view.

Structure and evolution of calcium-modulated proteins

This review suggests that the intracellular functions of calcium are best understood in terms of calcium's functioning as a second messenger. Further, when functioning as a second messenger, calcium completes its mission not by transferring charge nor by binding to lipid but by binding to specific targets, calcium-modulated proteins. This concept is broadly interpreted to include proteins involved in calcium transport. There is strong evidence that many, if not all, of these calcium-modulated proteins are homologs. Their structures and properties are contrasted to those of extracellular calcium-binding proteins which are not homologous to one another or to the intracellular calcium-modulated proteins. Finally, this line of thought leads to a suggestion of the evolutionary reason for the choice of calcium as the sole inorganic second messenger.