Homology of myosin light chains, troponin-C and parvalbumins deduced from comparison of their amino acid sequences

An EF-handed Ca(2+)-binding protein of Chinese liver fluke Clonorchis sinensis

A cDNA clone encoding 8 kDa protein was retrieved from an EST pool of Chinese liver fluke Clonorchis sinensis. A deduced polypeptide of the cDNA clone was similar to 8 kDa Ca(2+)-binding proteins from other parasitic trematodes, and, thus, named as CsCa8, containing two EF-hand Ca(2+)-binding sites. Homology models predicted CsCa8 to be a single globular structure having four helices and molecular folds similar to Ca(2+)-binding state of other small Ca(2+)-binding proteins. Recombinant CsCa8 protein showed specific Ca(2+)-binding affinity and shifting in native gel mobility assay. Mouse immune sera raised against recombinant CsCa8 protein recognized native CsCa8 from adult C. sinensis worm extract. CsCa8 was localized in oral and ventral suckers, vitelline follicles and subtegumental tissues. These findings suggest that CsCa8 might be involved in cellular Ca(2+) signal transduction for muscle contraction and egg production.

Fraternal twins: Swiprosin-1/EFhd2 and Swiprosin-2/EFhd1, two homologous EF-hand containing calcium binding adaptor proteins with distinct functions

Changes in the intracellular calcium concentration govern cytoskeletal rearrangement, mitosis, apoptosis, transcriptional regulation or synaptic transmission, thereby, regulating cellular effector and organ functions. Calcium binding proteins respond to changes in the intracellular calcium concentration with structural changes, triggering enzymatic activation and association with downstream proteins. One type of calcium binding proteins are EF-hand super family proteins. Here, we describe two recently discovered homologous EF-hand containing adaptor proteins, Swiprosin-1/EF-hand domain containing 2 (EFhd2) and Swiprosin-2/EF-hand domain containing 1 (EFhd1), which are related to allograft inflammatory factor-1 (AIF-1). For reasons of simplicity and concision we propose to name Swiprosin-1/EFhd2 and Swiprosin-2/EFhd1 from now on EFhd2 and EFhd1, according to their respective gene symbols. AIF-1 and Swiprosin-1/EFhd2 are already present in Bilateria, for instance in Drosophila melanogaster and Caenhorhabditis elegans. Swiprosin-2/EFhd1 arose later from gene duplication in the tetrapodal lineage. Secondary structure prediction of AIF-1 reveals disordered regions and one functional EF-hand. Swiprosin-1/EFhd2 and Swiprosin-2/EFhd1 exhibit a disordered region at the N-terminus, followed by two EF-hands and a coiled-coil domain. Whereas both proteins are similar in their predicted overall structure they differ in a non-homologous stretch of 60 amino acids just in front of the EF-hands. AIF-1 controls calcium-dependent cytoskeletal rearrangement in innate immune cells by means of its functional EF-hand. We propose that Swiprosin-1/EFhd2 as well is a cytoskeleton associated adaptor protein involved in immune and brain cell function. Pro-inflammatory conditions are likely to modulate expression and function of Swiprosin-1/EFhd2. Swiprosin-2/EFhd1, on the other hand, modulates apoptosis and differentiation of neuronal and muscle precursor cells, probably through an association with mitochondria. We suggest furthermore that Swiprosin-2/EFhd1 is part of a cellular response to oxidative stress, which could explain its pro-survival activity in neuronal, muscle and perhaps some malignant tissues.

Structural basis for diversity of the EF-hand calcium-binding proteins

The calcium binding proteins of the EF-hand super-family are involved in the regulation of all aspects of cell function. These proteins exhibit a great diversity of composition, structure, Ca2+-binding and target interaction properties. Here, our current understanding of the Ca2+-binding mechanism is assessed. The structures of the EF-hand motifs containing 11-14 amino acid residues in the Ca2+-binding loop are analyzed within the framework of the recently proposed two-step Ca2+-binding mechanism. A hypothesis is put forward that in all EF-hand proteins the Ca2+-binding and the resultant conformational responses are governed by the central structure connecting the Ca2+-binding loops in the two-EF-hand domain. This structure, named EFbeta-scaffold, defines the position of the bound Ca2+, and coordinates the function of the N-terminal (variable and flexible) with the C-terminal (invariable and rigid) parts of the Ca2+-binding loop. It is proposed that the nature of the first ligand of the Ca2+-binding loop is an important determinant of the conformational change. Additional factors, including the interhelical contacts, the length, structure and flexibility of the linker connecting the EF-hand motifs, and the overall energy balance provide the fine-tuning of the Ca2+-induced conformational change in the EF-hand proteins.

Characterization of the lanthanide ion-binding properties of calcineurin-B using laser-induced luminescence spectroscopy

Calcineurin (CaN) is a Ca2+/calmodulin-dependent protein phosphatase found in brain and other tissues. It is a heterodimer consisting of a catalytic subunit (CaN-A) and a Ca(2+)-binding regulatory subunit (CaN-B). The primary structure of CaN-B indicates that it, like calmodulin, is an EF-hand protein and binds four Ca2+ ions. Eu3+, due to its favorable spectroscopic and chemical properties, has been substituted for Ca2+ in CaN-B to determine the metal ion-binding properties of this "calmodulin-like" protein. Excitation of the 7F0-->5D0 transition of Eu3+ results in a spectrum similar to that of calmodulin, consisting of three peaks. Analysis of the spectral titration curves reveals four Eu(3+)-binding sites in CaN-B. The affinities vary: sites I and II have dissociation constants of 1.0 +/- 0.2 and 1.6 +/- 0.4 microM, respectively; the values for sites III and IV are Kd = 140 +/- 20 and Kd = 20 +/- 10 nM, respectively. Binding of Tb3+ is slightly weaker. Tb3+ luminescence, sensitized by tyrosine, reveals that for lanthanides the highest affinity sites lie in the C-terminal domain. Energy transfer distance measurements between Eu3+ and Nd3+ in sites III and IV reveal a separation of 10.5 +/- 0.5 A, which suggests that these sites are arranged in a typical EF-hand pair. This information indicates that the overall structure of CaN-B is similar to the dumbbell-shaped proteins troponin-C and calmodulin, but is more like TnC in its metal-binding properties.

Functional implications of the unusual amino acid sequence of the regulatory light chain of Acanthamoeba castellanii myosin-II

We have determined by protein chemistry methods the amino acid sequence of light chain 2 from Acanthamoeba castellanii myosin-II (ALC2). This is the first reported sequence for any protozoan myosin light chain. ALC2 consists of 154 amino acid residues, including a single residue of His and two residues each of Pro and Tyr, and lacks Cys and Trp. The N-terminus is blocked, and if an N-terminal acetyl group is assumed. ALC2 has a calculated molecular weight of 17,657. ALC2 is an acidic protein, with a calculated net charge of -7 at pH 7. The sequence of ALC2 is most similar to those of the calmodulins (identity approximately 35%), followed by myosin regulatory light chains. ALC2 appears to lack the potential N-terminal phosphorylation site and single Ca(2+)-binding site in region I which are characteristic of most myosin regulatory light chains. Instead, ALC2, unlike any other myosin light chain characterized to date, may have a functional Ca(2+)-binding site only in region II, suggesting a novel role of ALC2 in the Ca2+ regulation of the activity of Acanthamoeba myosin-II.

Amino acid sequences of myosin essential and regulatory light chains from two clam species: comparison with other molluscan myosin light chains

We have determined the amino acid sequences of the essential light chains (ELC) and regulatory light chains (RLC) of myosin from two species of clam, Mercenaria mercenaria and Macrocallista nimbosa, using protein chemistry methods. The N-termini of all four proteins were blocked, and sequencing was carried out on various chemically and enzymatically produced peptide fragments. Cleavage of either Mercenaria RLC (MRLC) or Macrocallista RLC (VLC) at its 3 Arg yielded four peptides, three of which could not be sequenced directly, due to an N-terminal blocking group and 2 Arg-Gln bonds in these proteins. The fourth peptide was partially and specifically cleaved at an unusually reactive residue, Met-64, which is invariant in all known RLC sequences. A comparison of all available molluscan ELC and RLC sequences was carried out in search of clues to functionally important features of these proteins in muscles which are regulated by a Ca(2+)-sensitive myosin. By analogy with other RLCs, VRLC and MRLC may be phosphorylated at Ser-11 by an endogenous kinase. All myosin light chains, like troponin C and calmodulin, contain four homologous regions, I to IV, each of which contains a twelve-residue potential Ca(2+)-binding loop flanked on either side by a pair of helices. All RLCs, including those from Ca(2+)-insensitive myosins, contain a divalent cation-binding site in region I. Clam and other molluscan ELCs contain a single Ca(2+)-binding site in region III. This site is present only in the ELCs of myosins that are regulated by direct binding of Ca2+. The ELC site III is likely to play a key role in the regulation of molluscan muscle contraction.

Dictyostelium discoideum essential myosin light chain: gene structure and characterization

We have used a Dictyostelium essential myosin light chain (EMLC) cDNA clone to isolate additional cDNA clones which supply a different 3' sequence from that previously described. The revised cDNA sequence encodes a polypeptide of 150 amino acids. Amino acid residues 147-167 of the previously reported sequence are replaced by new residues 147 to 150. The new cDNA encodes a polypeptide with 66% amino acid sequence identity with the Physarum polycephalum EMLC, and approximately 30% identity with mammalian EMLC sequences. These new cDNA clones were used to isolate two genomic DNA fragments which contain the entire EMLC gene. The Dictyostelium EMLC gene contains a single intron located immediately 3' of the translation initiation codon and encodes a product most similar to MLC3 isoform of vertebrates. Primer extension analysis places the transcription initiation site approximately 90 nucleotides upstream of the translation initiation site. A DNA fragment containing 350 bases of sequence upstream of the putative transcription initiation site is sufficient to drive expression of a reporter gene upon reintroduction into growing Dictyostelium cells. In addition, the CAT reporter mRNA produced by this construct showed a pattern of developmental regulation similar to that previously reported for the endogenous EMLC mRNA. Based on comparison with published EMLC sequences from a variety of sources, the Dictyostelium EMLC shows slightly higher similarity to vertebrate EMLCs from striated muscle sources than nonmuscle sources. While Dictyostelium and human nonmuscle sequences display only 28% identity over their entire sequence, the region from residue 88 to 108 shows much higher identity (67%). The high evolutionary conservation of this region of the EMLC suggests it may play an important role in EMLC function, and as such, represents a good target for future mutagenesis studies.

Fast and slow isoforms of troponin I and troponin C. Distribution in normal rabbit muscles and effects of chronic stimulation

Polyclonal antibodies were raised against troponin I (TnI) and troponin C (TnC) purified from fast-twitch and slow-twitch rabbit muscles. These antibodies were used to elucidate the distribution of fast and slow isoforms of TnI and TnC in normal and chronically stimulated rabbit hind limb muscles by immunoblots of one-dimensional and two-dimensional electrophoreses. In contrast to the multiplicity of fast and slow troponin T (TnT) isoforms, TnI and TnC were present as unique fast and slow isoforms. Whereas no charge variants were detected for slow TnI, fast TnI was present in at least three charge variants. As judged from the results of alkaline phosphatase digestion, these charge variants represent differently phosphorylated forms. Fast and slow TnC both exist as two charge variants which, however, were unaffected by alkaline phosphatase treatment. Chronic low-frequency stimulation of fast-twitch muscles induced progressive increases in the slow isoforms of TnC and TnI at the expense of their fast isoforms. The extent of the fast-to-slow transition was more pronounced in the case of TnC than in that of TnI. Long-term stimulated muscles with a complete fast-to-slow transition, at the level of the TnT isoforms, still contained fast and slow isoforms of both TnI and TnC. The coexistence of fast and slow isoforms of the three troponin subunits in the transforming muscle was interpreted as indicating the presence of hybrid troponin molecules composed of fast and slow isoforms. Studies at the mRNA level showed changes similar to those at the protein level. However, in long-term stimulated muscles, the fast-to-slow transition of TnI was more pronounced at the mRNA level than at the protein level.

Molecular cloning and expression of chicken cardiac troponin C

We have isolated a full length complementary DNA clone (pCTnC1) from a 19-day embryonic chicken heart library corresponding to cardiac troponin C (TnC). Sequence analysis demonstrated varying homologies with TnC complementary DNA clones isolated from developing chick skeletal muscle. Using pCTnC1 as a hybridization probe, we have determined that cardiac TnC is constitutively expressed in both atria and ventricles of the developing and adult heart. These data along with previous immunochemical studies of TnC expression demonstrate that the slow skeletal and cardiac muscle isoform is the only TnC expressed in the heart. In contrast, expression of slow skeletal and cardiac muscle TnC is developmentally regulated in skeletal muscles of the chicken. The tightly controlled expression of slow skeletal and cardiac muscle TnC in the varying myocyte types of the heart suggests a physiologically significant role of this regulatory protein.

Human 27-kDa calbindin complementary DNA sequence. Evolutionary and functional implications

Human 27-kDa calbindin cDNA clones were selected by antibody screening from lambda gt11 brain libraries. The sequence revealed an open reading frame coding for a protein of 261 amino acids, containing four active calcium-binding domains, and two modified domains that had presumably lost their calcium-binding capability. Comparison with chick and bovine calbindins showed that the protein was highly conserved in evolution (evolutionary rate: 0.3 x 10(-9) amino acid-1 year-1) and that active and inactive domains were equally conserved. From the data we postulate that calbindin has an important physiological function involving protein--protein interactions. Comparison of calcium-binding domains from various proteins suggested that all members of the troponin C superfamily derive from a common two-domained ancestor, but that duplications leading to calbindin and to the four-domained calcium-binding proteins took place independently on different branches of the evolutionary tree. Preliminary data showed that another calcium-binding protein, homologous to calbindin, is present in the brain and encoded by a different gene.

Total sequential solid phase synthesis of rabbit skeletal troponin C calcium binding site III

The total sequential solid phase synthesis of AcA98STnC(90-123)amide is described. The yield is comparable to that obtained previously using fragment condensation on the solid support. HPLC purification results in a product which displays greater calcium induced conformational changes than the peptide prepared by the fragment condensation method. However, the former has a dissociation constant for calcium (28 microM) seven times higher than the latter (4 microM).

Evidence that progesterone binding uteroglobin is similar to myosin alkali light chain

Using a computer program designed to detect evolutionary relationships between proteins, I find that exon 2 of rabbit uteroglobin, a progesterone binder, and part of myosin alkali light chain have a comparison score that is 7.2 standard deviations higher than that obtained with a comparison of randomized sequences of these proteins. The probability (p) of getting this score by chance is less than 10(-12). This theoretical finding that these sequences are similar has led to the experimental finding that copper, calcium and the tranquilizer trifluoperazine, a calmodulin binding ligand, affect progesterone binding to uteroglobin.

The functional nature of calcium binding units in calmodulin, troponin C and parvalbumin

Examination of the interaction of major tranquilizers with calmodulin results in the generalization that the functional nature of calcium binding helix-loop-helix regions found in several calcium binding proteins including calmodulin, troponin C and parvalbumin is dependent upon the topography of the hydrophobic and hydrophilic regions on the amphiphilic N-terminal alpha-helix of the helix-loop-helix conformation formed by the binding of the calcium cation to these proteins. The relation of the topography of this amphiphilic alpha-helix to drug binding is delineated at the molecular level and the results obtained are used to describe the interaction of beta-endorphin, dynorphin, alpha-MSH and other peptides with calmodulin. The utility of this hypothesis is further demonstrated by the description of a possible interaction between troponin C, troponin I and troponin T of the troponin complex in skeletal muscle.

Recombinant DNA approach for defining the primary structure of monoclonal antibody epitopes. The analysis of a conformation-specific antibody to myosin light chain 2

A monoclonal antibody (MF5), capable of recognizing a divalent cation-induced conformational change in myosin light chain 2 (LC2f), has been used to screen a cDNA library constructed in the expression vector lambda gt11. A clone has been isolated that contains the whole coding sequence of this myosin subunit. The light chain was synthesized as a fusion peptide linked to beta-galactosidase by ten amino acids encoded in the 5' untranslated region of its mRNA. Seven imperfect repeats were identified in the 3' untranslated region of the mRNA. The amino acids conferring specificity on the MF 5 epitope were established by first determining the nucleotide sequence of shorter subclones that expressed the epitope and then eliminating those amino acid residues shared by cardiac myosin LC2, which was unreactive with this antibody. The epitope, which becomes accessible to MF 5 upon removal of bound divalent cations, resides at the junction between the first alpha-helical domain and the metal binding site. Theoretically, this approach can be used to define the primary structure of most protein epitopes.

Use of lanthanide-induced nuclear magnetic resonance shifts for determination of protein structure in solution: EF calcium binding site of carp parvalbumin

The binding of the paramagnetic lanthanide ion ytterbium to the calcium binding protein carp parvalbumin results in a series of 1H NMR resonances which are shifted far outside the envelope of the 1H NMR spectrum of the diamagnetic form of the protein; bound Yb3+ also induces shifts in the 13C NMR spectrum of parvalbumin and in the 113Cd NMR spectrum of cadmium-substituted parvalbumin. The interpretation of these lanthanide-shifted resonances in terms of the structure of the protein surrounding the metal binding site requires the determination of the orientation and principal elements of the magnetic susceptibility tensor of the protein-bound Yb3+ ion. A previous comparison [Lee, L., & Sykes, B. D. (1982) Biomolecular Structure Determination by NMR (Bothner-By, A. A., Glickson, J. D., & Sykes, B. D., Eds.) pp 169-188, Marcel Dekker, New York] of the observed Yb3+-shifted 1H NMR spectrum of parvalbumin with a calculated spectrum, based upon the X-ray structure and an initial determination of the magnetic susceptibility tensor, led to the conclusion that there were significant differences between the solution and X-ray structures. In this paper, the magnetic susceptibility tensor has been reevaluated with the aid of newly assigned 13C and 113Cd NMR resonances. The agreement between the calculated and observed spectra is now close overall.

Calcium and cadmium binding to troponin C. Evidence for cooperativity

Proton NMR is used to compare the structural changes induced in bovine cardiac troponin C on binding of cadmium and calcium ions. The same spectral changes are observed for both ion species. The rate of the conformational changes associated with cadmium binding to the two high-affinity sites is slow, that associated with cadmium ions binding to the low-affinity site is high. 113Cd-NMR spectra of cardiac troponin C feature two signals interpreted as due to cadmium ions bound to the strong sites. Strong arguments are given in favour of cooperativity in binding of the first two cadmium or calcium ions to cardiac and skeletal muscle troponin C.

A new troponin T and cDNA clones for 13 different muscle proteins, found by shotgun sequencing

Complete amino acid sequences have been established for 19 muscle-related proteins and these proteins are each sufficiently abundant to suggest that their mRNA levels are about 0.4% or higher. Based on these considerations, a simple theoretical analysis shows that clones for most of these proteins can be identified within a complementary DNA library by sequencing cDNA inserts from 150-200 randomly selected clones. This procedure should not only rigorously identify specific clones, but it could also uncover amino acid sequence variants of major muscle proteins such as the troponins. We have determined sequences for about 20,000 nucleotides within 178 randomly selected clones of a rabbit muscle cDNA library, and report here that in addition to finding sequences encoding the two known skeletal muscle isotypes of troponin C, we have discovered sequences encoding two forms of troponin T. Over the region of nucleotide sequence overlap in the troponin T clones, the new isotype diverges significantly from its counterpart. Altogether, clones for 13 of the 19 known muscle-specific proteins were identified, in addition to the clone for the new troponin T isotype.

Malignant hyperthermia in pigs: a search for abnormalities in Ca2+ binding proteins

Malignant hyperthermia occurs in man and pigs as a hereditary disorder notably as a complication of halothane-induced anaesthesia. It involves an abnormality in the metabolism of Ca2+. A search was made for abnormalities of calcium-binding proteins. Troponin C from normal pig muscle was found to differ in 2 of 159 amino acids from rabbit Tn C and 3 from man. No differences between normal and abnormal pig muscle were found. Two-dimensional electrophoresis of red cell calmodulin from normal and abnormal pigs also failed to demonstrate a difference.

Studies on the alkali light chains of vertebrate skeletal muscle myosin. Effect of tyrosyl modification on the ability to reassociate to heavy chains

The structures of the alkali light chain subunits A1 and A2 have been studied by examining the effect of the conformationally sensitive reagent tetranitromethane, which reacts specifically with tyrosyl residues. Whereas reaction in the presence of 6 M guanidine hydrochloride results in modification of the three tyrosyl residues of both these light chains, only two tyrosyl residues are exposed to the reagent in the native conformations of these proteins. By gel chromatography of the CNBr-cleaved chains it was demonstrated that the two reactive tyrosyls are those located in the CB-1 and CB-3 segments and that these tyrosyl residues are modified simultaneously and not sequentially. The unreactive tyrosyl residue is in the CB-6 segment and is separated by two residues from the single cysteinyl residue of these chains. It is found that the modified light chains cannot be made to reassociate with the heavy chains by the NH4Cl hybridization procedure of Wagner and Weeds [J. Mol. Biol. 109, 455-470 (1977)] or by the thermal hybridization procedure [Burke and Sivaramakrishnan (1981) Biochemistry 20, 5908-5913]. Furthermore, reduction of the nitrotyrosyl groups to aminotyrosyl residues by sodium dithionite does not restore this effect. The data suggest that regions of the light chains at CB-1 and CB-3 are involved in the association to the heavy chains.

Similar amino acid sequences: chance or common ancestry?

The systemic comparison of every newly determined amino acid sequence with all other known sequences may allow a complete reconstruction of the evolutionary events leading to contemporary proteins. But sometimes the surviving similarities are so vague that even computer-based sequence comparisons procedures are unable to validate relationships. In other cases similar sequences may appear in totally alien proteins as a result of mere chance or, occasionally, by the convergent evolution of sequences with special properties.

Strategies for the uses of lanthanide NMR shift probes in the determination of protein structure in solutio. Application to the EF calcium binding site of carp parvalbumin

The homologous sequences observed for many calcium binding proteins such as parvalbumin, troponin C, the myosin light chains, and calmodulin has lead to the hypothesis that these proteins have homologous structures at the level of their calcium binding sites. This paper discusses the development of a nuclear magnetic resonance (NMR) technique which will enable us to test this structural hypothesis in solution. The technique involves the substitution of a paramagnetic lanthanide ion for the calcium ion which results in lanthanide induced shifts and broadening in the 1H NMR spectrum of the protein. These shifts are sensitive monitors of the precise geometrical orientation of each proton nucleus relative to the metal. The values of several parameters in the equation relating the NMR shifts to the structure are however known as priori. We have attempted to determine these parameters, the orientation and principal elements of the magnetic susceptibility tensor of the protein bound metal, by studying the lanthanide induced shifts for the protein parvalbumin whose structure has been determined by x-ray crystallographic techniques. The interaction of the lanthanide ytterbium with parvalbumin results in high resolution NMR spectra exhibiting a series of resonances with shifts spread over the range 32 to -19 ppm. The orientation and principal elements of the ytterbium magnetic susceptibility tensor have been determined using three assigned NMR resonances, the His-26 C2 and C4 protons and the amino terminal acetyl protons, and seven methyl groups; all with known geometry relative to the EF calcium binding site. The elucidation of these parameters has allowed us to compare the observed spectrum of the nuclei surrounding the EF calcium binding site of parvalbumin with that calculated from the x-ray structure. A significant number of the calculated shifts are larger than any of the observed shifts. We feel that a refinement of the x-ray based proton coordinates will be possible utilizing the geometric information contained in the lanthanide shifted NMR spectrum.

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.

Sodium-23 nuclear magnetic resonance as an indicator of sodium binding to troponin C and tryptic fragments, in relation to calcium content and attendant conformational changes

The relaxation rate enhancements of the 23Na nuclei for NaHCO3 solutions of troponin C and its tryptic peptides TR-1 and TR-2 indicate true binding of Na+ ions to these biomolecules. The low-affinity sites I and II of TR-1 and troponin C are the sites of competitive Na+/Ca2+ binding, below one calcium ion per molecule, with log KNa approximately 2. At low calcium content Na+ ions bind to TR-2 and to troponin C non-competitively with Ca2+ ions; binding of Ca2+ ions to the high-affinity sites III and IV allosterically affects the binding of the Na+ ions: even when sites I and II, located on TR-1 or sites I, II, III, IV of troponin C, are saturated with Ca2+ ions, Na+ ions continue to bind weakly at secondary binding sites.

An improved method for isolation of Ca2+ binding cyanogen bromide peptides from rabbit skeletal muscle troponin C

A cyanogen bromide digest of troponin C from rabbit skeletal muscle yields two biologically active Ca2+ binding peptides. A simple, one-step method for the simultaneous purification, in high yield, of these two peptides is presented.

Evolutionary diversification of structure and function in the family of intracellular calcium-binding proteins

The maximum parsimony method was used to reconstruct the genealogical history of the family of intracellular calcium-binding proteins represented by six major present-day lineages, three of which--calcium dependent modulator protein, heart and skeletal muscle troponin Cs, and alkali light chains of myosin--were found to share a closer kinship with one another than with the other lineages. Similarly, parvalbumins and regulatory light chains of myosin were depicted as more closely related, whereas the branch of intestinal calcium-binding protein proved to have the most distant separation. The computer-generated amino acid sequence for the common ancestor of these six lineages described a four domain protein in which each domain of approximately 40 amino acid residues had a mid-region. 12 residue segment that bound calcium and had properties most resembling those of the calcium dependent modulator protein. It could then be deduced that parvalbumins evolved by deletion of domain I, inactivation of calcium-binding properties in domain II, and acquisition of increased affinity for Ca++ and Mg++ in domains III and IV. Regulatory light chains of myosin lost the cation binding property from three domains, retaining it in I, whereas alkali light chains of myosin lost this ability from each of the four domains. In skeletal muscle troponin C all domains retained their calcium-binding activity; however, like parvalbumins, domains III and IV acquired high affinity properties. Cardiac troponin C lost its binding activity from domain I but otherwise resembled the skeletal muscle form. Finally, intestinal calcium-binding protein evolved by deletion of domains III and IV. Positive selection could be implicated in these evolutionary changes in that the rate of fixation of mutations substantially increased in the mid portions of those domains which were loosing calcium-binding activity. Likewise, when the cation binding sites were changing from low to high affinity, an accelerated rate of fixed mutations was observed. Once this new functional parameter was selected these regions showed a remarkable conservatism, as did those binding sites which were maintaining the lower affinity. Moreover even in sequence regions not directly involved in cation binding, the lineage of troponin C because very conservative over the past 300 million years, perhaps becuase of the necessity for maintaining specific interfaces in order for the molecule to interact with troponin I and T in a functional thin myofilament. A similar phenomenon was observed in domain II of the regulatory light chains of the myosin lineage suggesting a possible binding site with the heavy chain of myosin.

Lanthanides as probes for calcium in biological systems

Calcium ion plays an essential role in many biological processes. The environment about Ca2+may be probed by substitution of tripositive lanthanide ions, Ln3+. Ca2+proteins fall into two broad classes: those that are inhibited by Ln3+substitution and those that are not. It is suggested that although Ca2+undertakes a primarily structural role in the Ln3+non-inhibited proteins, Ca2+may be near the active site or participate in the mechanism of action of Ln3+inhibited proteins. Ca2+and Ln3+radii are similar; most Ln3+are slightly larger than Ca2+in complexes of the same coordination number, and substitution of Ln3+for Ca2+is accommodated by a slight decrease in bond distance or by an increase in coordination number. Luminescence from Tb3+has been demonstrated to be a sensitive environmental probe of Ca2+binding sites in proteins.