Three-dimensional structure of a sarcoplasmic calcium-binding protein from Nereis diversicolor

The three-dimensional structure of a sarcoplasmic Ca2(+)-binding protein from the sandworm Nereis diversicolor has been determined at 3.0 A resolution using multiple isomorphous replacement techniques. The NH2-terminal half of the molecule contains one variant Ca2(+)-binding domain with a novel helix-loop-helix conformation and one Ca2(+)-binding domain that is no longer functional because of amino acid changes. The overall conformation of this pair of domains is different from any previously described Ca2(+)-binding protein. The COOH-terminal half of the protein contains two Ca2(+)-binding domains with the usual helix-loop-helix configuration and is similar to calmodulin and troponin C. Unlike calmodulin or troponin C, there is no exposed alpha-helix connecting the two halves of the molecule, so the overall structure is much more compact.

Primary structure of the target of calcium vector protein of amphioxus

CaVPT, a target protein of Ca2(+)-vector from amphioxus muscle, was purified from its complex with CaVP after dissociation by 6 M urea and chromatographies on DEAE-cellulose and calmodulin-Sepharose. The amino acid sequence of CaVPT has been determined. The protein is composed of 243 residues and possesses an unblocked N terminus. Its molecular weight is 26,621, distinctly lower than the apparent molecular weight deduced from electrophoresis on sodium dodecyl sulfate-containing gels. CaVPT contains a potential Asn-linked glycosylation site, four potential protein kinase C phosphorylation sites, and two casein kinase II phosphorylation sites. From the sequence the following three particular domains can be inferred: a collagen-like N-terminal segment, rich in Pro and Ala, that resembles the N-terminal segment of skeletal muscle myosin light chain kinase; next to it (from residues 33 to 50) is located a strongly amphiphilic and basic alpha-helical segment which likely binds the calcium vector protein since a proteolytic cut after Arg50, occurring occasionally during the purification of CaVPT, impairs the binding to immobilized calmodulin. This segment is followed by two immunoglobulin folds. The two immunoglobulin folds typically belong to the C2 subclass and particularly resemble those present in the neural cell surface adhesion molecules NCAM, L1, F11, MAG, TAG-1, fasciclin II, and amalgam. Recently, the presence of immunoglobulin folds of this type has been reported in some intracellular muscular proteins, namely in smooth muscle myosin light chain kinase, striated muscle C protein and titin, as well as in the nematode 600-kDa protein twitchin. From this structural study we can formulate the working hypothesis that CaVPT acts on the structure of the thick filament in muscle or regulates, perhaps via other immunoglobulin fold-containing proteins.

Kinetics of conformational changes in Nereis sarcoplasmic calcium-binding protein upon binding of divalent ions

The sarcoplasmic calcium-binding protein (SCP) of the sandworm Nereis possesses three Ca2(+)-Mg2+ sites but no Ca2(+)-specific site. Binding of Mg2+, but not of Ca2+, displays a marked positive cooperativity. The apparent cooperativity of Ca2+ binding in the presence of Mg2+ results from the allostery in Mg2+ dissociation. Binding of the first Ca2+ or Mg2+ induces all the conformational change, monitored by Trp fluorescence. In displacement reactions the conformational changes occur in the step SCP.Mg3----SCP.Ca1Mg2. Stopped-flow experiments indicate that Trp fluorescence changes upon Ca2(+)-binding are instantaneous whereas Mg2(+)-binding involves a fast pre-equilibrium (Keq = 28 M-1), followed by two slow consecutive conformational changes with k1 = 13.5 s-1 and k2 = 0.21 s-1. The fluorescence change after dissociation of Ca2+ from SCP is monophasic with k = 0.02 s-1; that after Mg2+ dissociation is biphasic with k1 = 0.8 s-1 and k2 = 0.1 s-1. Trp life time measurements also indicate that Ca2(+)- and Mg2(+)-induced conformational changes are completely different. Displacement of bound Ca2+ by Mg2+ can be described by two consecutive reactions in which the first (without fluorescence change) corresponds to the dissociation of the last Ca2+ (k1 = 2.4 s-1) and the second (k2 = 0.45 s-1) to the final conformational change observed upon direct Mg2+ binding. Displacement of bound Mg2+ by Ca2+ follows the kinetic scheme of simple competition; the conformational rate constant approaches asymptotically (up to the limit of 129 s-1) the dissociation rate of Mg2+ as the concentration of Ca2+ increases. In summary, after fast dissociation of Ca2+ or Mg2+, Nereis SCP slowly converts to the metal-free configuration, but in Ca2(+)-Mg2+ exchange reactions, the conformational changes are nearly as fast as the cation dissociation reactions.

Comparative molecular modeling of Amphioxus calcium vector protein with calmodulin and troponin C

Calcium vector protein (CaVP), a new protein isolated from Amphioxus muscle, binds in a Ca2(+)-regulated manner to a 27 kd target protein, named CaVPT, whose function has not been elucidated yet. CaVP bears significant sequence homology to both calmodulin and skeletal muscle troponin C, especially in the C-terminal half of the molecule, which presumably contains the two functional Ca2(+)-binding sites. The N-terminal half contains two abortive EF-hands and is intramolecularly crosslinked with a disulfide bond. Using the crystallographic structures of calmodulin and striated muscle troponin C as a framework, we constructed two different three-dimensional models of CaVP and modeled the intramolecular disulfide bridge. The modeling based upon the coordinates of calmodulin yields a Ca2(+)-filled sites configuration in the N-terminal half of the molecule, even though no Ca2+ is bound in this half, whereas the troponin C-derived model generates a Ca2(+)-empty sites configuration. The models predict that neither is the Ca2(+)-filled nor in the Ca2(+)-empty sites conformation is there any steric and/or energetic obstacle for the formation of the disulfide bridge and that the disulfide bond is poorly accessible to reducing reagents. The optical properties of the Trp and Tyr residues of CaVP indicate that the calmodulin-derived model represents the most plausible prediction.

Amino acid sequences of four isoforms of amphioxus sarcoplasmic calcium-binding proteins

The protochordate amphioxus (Branchiostoma lanceolatum) contains different isoforms of sarcoplasmic, high-affinity Ca2(+)-binding proteins (SCP). The amino acid sequences of the two major isoforms SCP I and II, reported previously [Takagi, T., Konishi, K. & Cox, J.A. (1986) Biochemistry 25, 3585-3592], have been corrected and differ from each other by seven amino acid substitutions in a 17-residue-long segment (positions 20-36). We also report on the isolation and amino acid sequence determination of two minor isoforms, i.e. amphioxus SCP III and IV. Although they behave very differently from the major forms with respect to net charge, they differ from SCP I by only one amino acid: SCP III has Met at position 20 (Tyr in SCP I) and SCP IV has Asn at position 23 (Asp in SCP I). Together the sequence data on amphioxus SCP suggest that, in contrast with SCP of other invertebrate phyla, the isoforms are generated by alternative splicing of the primary RNA transcript with a mutually exclusive pattern.

Differential expression of excitatory amino acid receptor subtypes in cultured cerebellar neurons

Using neurotoxicity and inositol phosphate release as criteria for receptor expression, we report the differential expression of excitatory amino acid receptor subtypes in cerebellar granule cells grown in serum-free media containing either high (25 mM) or low (5 mM) KCl. NMDA receptors are expressed in neurons grown in high, but not low, KCl. In contrast, ionotropic quisqualate receptors are expressed in neurons grown in low KCl, but not in those grown in high KCl. Addition of NMDA to cultures containing low KCl appears to mimic high KCl conditions: NMDA receptors are expressed, but ionotropic quisqualate receptors are not. Glutamate and kainate are toxic to cells grown in either condition.

Calcium- and magnesium-binding properties of oncomodulin. Direct binding studies and microcalorimetry

Ca2+ binding to the wild type recombinant oncomodulin was studied by equilibrium flow dialysis in the absence and presence of 1, 2, and 10 mM Mg2+. Direct Mg2(+)-binding experiments were carried out by the Hummel-Dryer gel filtration technique. These studies revealed that in the absence of Mg2+ oncomodulin binds two Ca2+ with KCa = 2.2 x 10(7) and 1.7 x 10(6) M-1, respectively. In the absence of Ca2+ the protein binds only one Mg2+ with KMg = 4.0 x 10(3) M-1.Mg2+ antagonizes Ca2+ binding at the high affinity site according to the rule of direct competition. Ca2+ binding to the low affinity site is only slightly affected by Mg2+, so that in the presence of 2-3 mM Mg2+ the two sites have apparently an equal affinity for Ca2+. Microcalorimetry showed that, in spite of the different affinities of the two Ca2(+)-binding sites, delta H0 for the binding of each Ca2+ is identical and exothermic for -18.9 kJ/site. It follows that the entropy gain upon binding of Ca2+ is +77.1 J K-1 site-1 for the high affinity Ca2(+)-Mg2+ site and +56.0 J K-1 site-1 for the low affinity Ca2(+)-specific site. Mg2+ binding is endothermic for +13 kJ/site with an entropy change of +111 J K-1 site-1. The thermodynamic characteristics of the Ca2(+)-Mg2+ site resemble most those of site II (the so-called EF domain) of toad alpha-parvalbumin. The characteristics of Ca2+ binding to the specific site (likely the CD domain) are different from those of the Ca2+ specific sites in troponin C and in calmodulin and suggest that in oncomodulin hydrophobic forces do not play a predominant role in the binding process at the specific site.

Cardiac contusion in pediatric patients with blunt thoracic trauma

To investigate the prevalence of myocardial contusion associated with blunt chest trauma in the pediatric age group, all patients admitted to our institution during a 6-month period with blunt thoracic trauma severe enough to produce a pulmonary contusion or rib fracture were prospectively evaluated. Cardiac evaluation was undertaken, including a multiple-gated acquisition (MUGA) cardiac scan, serial electrocardiograms (ECG), and serum creatine phosphokinase (CPK) and CPK isoenzymes. Seven patients, ranging in age from 2 1/2 to 18 years, with rib fractures or pulmonary contusion by chest roentgenograph were identified. One patient was injured as a passenger in a motor vehicle accident, five were struck by automobiles as pedestrians, and one sustained traumatic asphyxia when a car, supported by a jack, fell on his chest. All had at least one other major organ system injured. All patients had pulmonary contusions as determined by chest radiograph, and two had associated rib fractures. In 43% (three of seven) of patients, a significant cardiac contusion was identified, defined by abnormal right or left ventricular wall motion and a decreased ejection fraction on MUGA scan, and confirmed by an increase in cardiac enzymes and isoenzymes. However, in contrast with adults, no patients had ECG abnormalities. This limited series suggests that cardiac contusion may occur frequently in pediatric patients who have suffered from blunt thoracic trauma significant enough to result in pulmonary contusion. An MUGA scan provides a rapid, noninvasive assessment of cardiac damage in this setting. Further studies will be required to determine the clinical significance and long-term consequences of traumatic myocardial damage in the pediatric population.

Neurotoxicity at the N-methyl-D-aspartate receptor in energy-compromised neurons. An hypothesis for cell death in aging and disease

Our results demonstrated that the neurotoxicity of glutamate and closely related agonists was mediated by the NMDA receptor in rat cerebellar granule cells. Evidence was presented to support our hypothesis that the pivotal event in the transition of these EAAs from neurotransmitters to neurotoxins is relief of the voltage-dependent Mg++ block of the NMDA channel due to changes in membrane potential which can be caused by depletion of highly phosphorylated nucleotides or by other depolarizing stimuli. Persistent stimulation of NMDA receptors whose channels are unblocked by Mg++ can permit excessive influx of Na+ and Ca++ and neuronal death can follow by a mechanism not yet understood. Glutamate is not toxic at kainate receptors although they are present on these cells. These findings underline the potential importance of perturbations in energy metabolism in a variety of neurodegenerative disorders and in the normal process of aging which share the common feature of the loss of neurons.

Excitatory amino acid neurotoxicity at the N-methyl-D-aspartate receptor in cultured neurons: pharmacological characterization

L-Glutamate neurotoxicity at the N-methyl-D-aspartate (NMDA) receptor was characterized in cultured cerebellar granule cells. When deprived of glucose for 40 min, these cells were killed by 20-60 microM L-glutamate. However, the neurons were resistant to glutamate at concentrations as high as 5 mM when glucose and Mg2+ were present throughout. Both competitive and non-competitive NMDA receptor antagonists completely blocked neurotoxicity due to glutamate and other NMDA receptor agonists. CPP [+/-)-3-(2-carboxypiperazin-4-yl)-prophyl-1-phosphonic acid) was the most effective competitive antagonist with full protection at 100 microM while MK-801 [+/-)-10,11-dihydro-5-methyl-5H-dibenzo[a,d]-cyclohepten-5,10-imin e) was the most effective non-competitive antagonist with full protection at 20 nM. Other antagonists with higher selectivity for other subtypes of glutamate receptors were ineffective. We conclude that glutamate toxicity in energy-deprived cerebellar granule cells is mediated by NMDA receptors. Results are discussed in terms of an hypothesis offering an explanation for the transition of glutamate from neurotransmitter to neurotoxin which emphasizes the responsiveness of the receptor to agonists rather than focusing on the presence of high concentrations of agonist.

Excitatory amino acid neurotoxicity at the N-methyl-D-aspartate receptor in cultured neurons: role of the voltage-dependent magnesium block

Results of the present report show that cerebellar neurons in primary culture are resistant to glutamate concentrations as high as 5 mM in the presence of glucose and Mg2+, but sensitive to glutamate concentrations lower than 35 microM when the neurons are deprived of glucose. Glutamate toxicity is also potentiated when Mg2+ is removed but glucose and EDTA are present; in this case, higher concentrations of glutamate (1 mM) are required for full toxicity. Glucose concentrations as low as 50 microM are fully protective against the toxicity of 100 microM glutamate; pyruvate and, to a lesser extent, lactate are also protective. Significantly, increasing concentrations of extracellular Mg2+ are fully protective against the toxicity of 100 microM glutamate in the absence of glucose and against the toxicity of 1 mM glutamate in the presence of glucose and EDTA. We interpret these results as support for our hypothesis that the pivotal event in glutamate's transition to neurotoxin is relief of the Mg2+ block of the N-methyl-D-aspartate (NMDA) receptor channel, which is known to be voltage-dependent. Partial depolarization in response to depletion of high-energy phosphates relieves the voltage-dependent block enabling glutamate to stimulate an excessive ion influx which results in the death of the neuron by a mechanism which is not yet understood. We propose that this mechanism may be operative in the neuronal damage associated with a variety of neurodegenerative disorders.

A new class of calcium entry blockers defined by 1,3-diphosphonates. Interactions of SR-7037 (belfosdil) with receptors for calcium channel ligands

Tetrabutyl-2(2-phenoxyethyl)-1,3-propylidene diphosphonate (SR-7037) completely displaced dihydropyridine [( 3H]PN200-110), phenylalkylamine [( 3H]D888), and benzothiazepine [( 3H]diltiazem) ligands from brain L-type calcium channels. Half-maximal inhibition of [3H]PN200-110 binding occurred at 19 nM with a Hill coefficient of 0.96. SR-7037 primarily decreased the affinity for [3H]PN200-110 with a small, but significantly, effect on the maximal binding capacity. Kinetic studies showed that this was due to an increased radioligand dissociation rate from 0.04 min-1 to 0.43 min-1 in the presence of the diphosphonate. Displacement of [3H]D888 by SR-7037 was biphasic with respective IC50 of 44 and 8400 nM. Likewise, unlabeled (-)-D888 identified two sites with IC50 values of 0.9 and 27 nM. Both SR-7037 (1000 nM) and D888 (200 nM) accelerated radioligand dissociation about 2-fold. [3H]Diltiazem binding was inhibited by SR-7037 with an IC50 value of 29 nM. The inhibition of dihydropyridine binding by SR-7037 is enhanced by most divalent cations at millimolar concentrations with the following potency: Mn2+ greater than Mg2+ greater than Ca2+ greater than Co2+. Barium has the opposite effect. The half-maximal effect of calcium occurred at 6 microM free ion. Specific binding of [3H]D888 was antagonized in the presence of 1 mM CaCl2. It is concluded that SR-7037 has allosteric interactions with the dihydropyridine receptor of the L-type calcium channel. The differential effect of Ca2+ on the potency of D888 and diltiazem relative to that of SR-7037 indicates that the three drugs may bind to nonequivalent sites. These results support specific calcium channel inhibition, possibly at a novel site, as the primary mechanism of the diphosphonate's pharmacological actions.

Voltammetric reduction and determination of hydrogen peroxide at an electrode modified with a film containing palladium and iridium

Cyclic voltammetry of a mixture containing 0.2 mM Na2IrCl6, 0.1 mM PdCl2, 0.2 M K2SO4, and 0.1 M HCl between 1.2 and -0.3 V vs Ag/AgCl for five cycles at 50 mV s-1 yields a stable film on a glassy carbon electrode. The reduction of hydrogen peroxide in 0.1 M KCl is diffusion controlled at that modified electrode. Calibration curves obtained at a 100 mV s-1 scan rate are linear in the range 0.2-1.8 mM H2O2. The slope, 28 microA L mmol-1, is independent of film thickness. Since dissolved oxygen is reduced at about the same potential as H2O2, -0.3 V, at the modified electrode, it will act as an interferent in solutions that are not deaerated; however, the currents are additive. A second limitation of the described procedure is that with the KCl electrolyte the immobilized film must be reoxidized prior to each measurement. Preliminary data are described which suggest that this problem is alleviated by switching to a basic supporting electrolyte.

Dynamic radionuclide imaging with 99mTc-sucralfate in the detection of oesophageal ulceration

Standard oesophageal scintigraphic techniques using 99mTc-colloids rarely identify oesophageal mucosal damage. Sucralfate can be labelled with 99mTc for the detection of oesophageal mucosal ulceration. This method uses two separate supine swallows of 10 MBq 99mTc-colloid in 10 ml, followed by a single supine swallow of 30 MBq 99mTc-sucralfate. The data are processed to give time-activity curves, mean transit times and condensed dynamic images. When oesophageal ulceration is detected, the time-activity curves using sucralfate show residual activity in the oesophagus after the transit time indicated by the colloid swallow. The condensed dynamic image shows a persistence of activity at the level of the ulceration. Erect sucralfate images taken immediately after the dynamic sequence show no oesophageal localisation. The results from a study of 62 patients have shown excellent correlation between the dynamic 99mTc-sucralfate images and endoscopy findings. Sequential sucralfate studies for healing also correlate well. The use of labelled sucralfate to detect oesophageal ulceration could modify the indications for endoscopy in gastrooesophageal reflux disease.

Mode of activation of bovine brain inositol 1,4,5-trisphosphate 3-kinase by calmodulin and calcium

The effect of Ca2+ and calmodulin (CaM) on the activation of purified bovine brain Ins(1,4,5)P3 kinase was quantified and interpreted according to the model of sequential equilibria generally used for other calmodulin-stimulated systems. Two main conclusions can be drawn. (i) CaM.Ca3 and CaM.Ca4 together are the biologically active species in vitro, as is the case for the great majority of other calmodulin targets. (ii) These species bind in a non-co-operative way to the enzyme with an affinity constant of 8.23 x 10(9) M-1, i.e. approx 10-fold higher than for most calmodulin-activated target enzymes. The dose-response curve of the activation of Ins(1,4,5)P3 kinase by calmodulin is not significantly impaired by melittin and trifluoperazine, whereas under very similar assay conditions the half-maximal activation of bovine brain cyclic AMP phosphodiesterase requires over 30-50-fold higher concentrations of CaM when 1 microM melittin or 20 microM-trifluoperazine is present in the assay medium. Similarly, 1 microM of the anti-calmodulin peptides seminalplasmin and gramicidin S, as well as 20 microM of N-(6-aminohexyl)-5-chloro-1-naphthalene-sulphonamide (W7), do not inhibit the activation process. These data suggest that binding and activation of Ins(1,4,5)P3 kinase require surface sites of calmodulin which are different from those involved in the binding of most other target enzymes or of model peptides.

Evidence for four capital and six auxiliary cation-binding sites on calmodulin: divalent cation interactions monitored by direct binding and microcalorimetry

Recently, Mills and Johnson [7] and our group [9] provided evidence that calmodulin contains, in addition to the four Ca2+-binding sites (capital sites), which are essential for drug- and enzyme-binding, a number of divalent cation-binding sites of different ion selectivity (auxiliary sites), which modulate drug-binding as well as the affinity of Ca2+ for the capital sites. In the present study, the number of auxiliary sites and their relationship to the capital sites were determined by equilibrium gel filtration and by flow microcalorimetry with Zn2+ and Mn2+ as selective probes for the auxiliary sites and with Cd2+ as a probe for both types of sites. In the absence of other divalent cations, 6 mol of Zn2+ bind to calmodulin with an identical affinity constant of 2,850 M-1 and a delta H0 of 106 kJ/mol calmodulin. In the presence of millimolar free Ca2+ calmodulin binds, in addition to four Ca2+, six Zn2+ with an affinity constant of 1,200 M-1 and a delta H0 of 47 kJ/mol calmodulin. The Zn2+-Ca2+ antagonism is governed by negative free energy coupling between the capital and auxiliary sites. In contrast, the Zn2+-Mg2+ antagonism follows the rule of straight competition at all six auxiliary sites. Mn2+ also binds exclusively to the auxiliary sites with affinity constants of 800 or 280 M-1 and delta H0 of 45 or 46 kJ/mol calmodulin in the absence and presence of saturating [Ca2+], respectively. Cd2+ binds to the capital sites with an affinity constant of 3.4 10(4) M-1 (delta H = 35 kJ/mol calmodulin) and to the auxiliary sites with ca. 100-fold lower affinity. The Zn2+ much greater than Mn2+ greater than or equal to Cd2+ greater than Mg2+ selectivity of the auxiliary sites corroborates the potencies of these cations in modulating drug binding. The auxiliary site-specific cations are unable to promote high-affinity complex formation between calmodulin and melittin.

Activation of human neutrophil NADPH-oxidase in vitro by the catalytic fragment of protein kinase-C

Phorbol ester treatment of intact neutrophils both stimulates protein kinase C (PK-C) and causes the rapid proteolytic conversion to a cytosolic, co-factor independent fragment, protein kinase M (PK-M). In intact neutrophils, phorbol ester treatment activates the NADPH-oxidase, the enzyme responsible for the oxidative burst. Addition of purified PK-M to resting neutrophil light density membranes activated the NADPH-oxidase in the presence of PS, ATP and Mg2+. A 3.5-fold greater stimulation of oxidase (ca. 25 nmoles O2-/min/mg membrane protein) was obtained with comparable PK-M concentrations to that observed with the reconstituted PK-C system, and approximately 1/3 that obtained with arachidonic acid (AA) or SDS. In contrast to the reconstituted system using PK-C, PMA and Ca++ were neither required nor affected activity. The effect of PS was unexpected, since PK-M does not require phospholipids for enzymatic activity, and likely represents the action of PS on the oxidase itself or on another component in the plasma membrane fraction. Our studies demonstrate for the first time that purified PK-M permits reconstitution of a physiologic phorbol ester response.

Complete amino acid sequence of the sarcoplasmic calcium-binding protein (SCP-I) from crayfish (Astacus leptodactylus)

The complete amino acid sequence of the alpha chain of the dimeric sarcoplasmic Ca2+-binding protein (SCP-I = alpha 2) from crayfish (Astacus leptodactylus) has been determined by partial automatic sequencing of the peptides derived from tryptic digests of the protein after citraconylation or treatment with 1,2-cyclohexanedione. Overlapping peptides were obtained by cleavage with o-iodosobenzoic acid, or digestion with Staphylococcus aureus protease, thermolysin and pepsin. The acetylated N-terminus was identified by fast atom bombardment mass spectrometry. The monomeric protein contains 192 amino acids and has an Mr of 21,643. The sequence shows the presence of three calcium-binding sites and perhaps of two others that may be degenerated.

Amino acid sequence of crayfish troponin I

Troponin I is the actomyosin ATPase inhibitory subunit present in the thin filament regulatory complex. The complete amino acid sequence of crayfish tail muscle troponin I has been determined. The protein is composed of 201 amino acid residues and has a molecular weight of 23,547. The N terminus is blocked, likely by an acetyl group. Crayfish troponin I shows a rather low (20-25%) sequence identity with vertebrate troponin Is as compared to the 60-82% identity within the vertebrate phylum. Similar to vertebrate cardiac troponin I, crayfish troponin I contains a 30-residue-long N-terminal extension. In crayfish troponin I, this segment bears significant sequence homology with the heavy or light chains of particular myosins. The actin-binding domain of crayfish troponin I, which displays 57% sequence homology with vertebrate troponin Is, possesses 2 unusual trimethyllysine residues. The consensus sequence of this domain in five troponin Is is as follows: D-L-R-G-K-F-X-R*-P-X-L-R*-R*-V, where R+ stands for Arg/Lys, R* for Arg/trimethyllysine, and X for any amino acid residue. Troponin I possesses two Ca2+-dependent interactive sites for troponin C; one partly overlaps with the actin binding domain and is highly conserved, and the other, corresponding to the 30-residue-long segment following the N-terminal extension in vertebrate cardiac and crayfish troponin I, is poorly conserved in the different troponin Is. Troponin I also interacts with troponin T. The consensus sequence for the interacting site on troponin I is as follows: h-D- -X-D- -R+-Y-D-h-E-h, where h stands for a hydrophobic residue, D- for Asp/Glu, R+ for Arg/Lys, and X for any residue. The five troponin Is further possess one more 15-residue-long segment of high sequence identity near the C terminus. Its evolutionary conservation suggests that this domain is involved in protein-protein interaction.