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.     

Calcium binding and conformation of regulatory light chains of smooth muscle myosin of scallop

Calcium binding was studied with two regulatory light chains (RLC-a and RLC-b) of smooth muscle myosin of scallop. With the equilibrium dialysis method, the binding of 0.98 mol Ca2+ per mol of RLC-b was observed with a dissociation constant of 2.3 X 10(-5) M. Similar values for RLC-b, 1.9 X 10(-5) M, and RLC-a, 1.5 X 10(-5) M, were obtained by measuring the difference absorption spectrum induced by Ca2+. The difference molar absorption coefficient at 288 nm was 159 and 209 M-1 X cm-1 for RLC-a and RLC-b, respectively, while it was -34 M-1 X cm-1 for the regulatory light chain of striated muscle myosin of scallop (RLC-st). Proton NMR spectra of the three light chains were very similar to each other and were broader than those of other Ca2+ binding proteins, parvalbumin and calmodulin. The regulatory light chains may be more rigid than in these Ca2+ binding proteins. CD spectra were measured for the three light chains, and the estimated helix contents were 27, 29, and 24%, respectively, for RLC-a, RLC-b, and RLC-st. All these results in comparison with the primary structures led us to suppose that the polypeptide of regulatory light chains is folded in such a way that domain 4 becomes near to the calcium binding site of domain 1. The decrease in intact light chains on trypsin digestion was determined for the gel electrophoretic patterns. RLC-a was 6 times more susceptible to the tryptic digestion than RLC-b.(ABSTRACT TRUNCATED AT 250 WORDS)     

Low-temperature studies of the sarcoplasmic reticulum calcium pump. Mechanisms of calcium binding

The mechanism of the sarcoplasmic reticulum Ca2+-ATPase was investigated at low temperatures (0 to -12 degrees C). Transient states of the enzyme were studied by two complementary techniques: intrinsic protein fluorescence and rapid filtration on Millipore filters. Intrinsic fluorescence was used to distinguish conformational states of the protein and to evaluate the rate of conversion between these states. Filtrations were used to measure the evolution of the active sites during the transition; the time resolution was 2-5 s. At sub-zero temperatures this time is shorter than the lifetime of most of the enzymatic states which have been detected. In this paper the mechanism of Ca2+ binding to the protein is investigated in the absence of nucleotides. Two basic experiments are described; (1) Kinetics of calcium binding and dissociation over a wide range of calcium concentration. (2) Kinetics of calcium exchange (45Ca2+ in equilibrium 40Ca2+) at constant concentration. The results obtained in the first series of experiments are consistent with a sequential binding to two interacting Ca2+ binding sites. Calcium ions have very fast access to a site with low apparent affinity (Kd approximately 25 microM). Occupation of this site induces a slow conformational change which increased its apparent affinity and reveals a second site of high apparent affinity. At equilibrium the two sites are not equivalent in terms of rate of exchange. Two different rates were detected k fast greater than 0.2 s-1, k slow approximately 0.015 s-1 at -10 degrees C. Removal of Ca2+ from the fast exchanging site by addition of EGTA accelerates the rate of release of the slow exchanging one. A model is proposed with two interacting Ca2+-binding sites. A set of parameters has been obtained which produces correctly the Ca2+-binding curve and the fluorescence level at equilibrium as well as the rate constants of the calcium-induced fluorescence changes over a very wide range of Ca2+ concentrations (0.02 to 150 microM). The non-equivalence of the two classes of site and the meaning of the initial low-affinity binding are discussed.     

Properties of calcium binding by Myxicola axoplasmic protein

The 45Ca2+ binding properties of axoplasmic protein from the Myxicola giant axon have been investigated using a centrifugal/concentration-dialysis technique. Scatchard plot analysis of these binding data suggest that Ca2+ is attached to a site with an equilibrium dissociation constant of 7.7 +/- 0.5 microM and a capacity of 4.4 +/- 0.2 mumol/g axoplasmic protein (n = 11). Addition of other cations--Cd2+, Mn2+, Al3+, Cu2+, Ba2+, and Zn2(+)--at concentrations up to 10 microM did not displace 0.2 microM 45Ca2+ from its binding site, probably because of buffering of these cations by amino acid residues within the protein solutions. The protein could be stored at 4 degrees C for up to 16 days with no appreciable change in the number of calcium sites. Ca2+ binding equilibrium took place in less than 30 min of incubation. Increasing the incubation temperature from 4 degrees C to 37 degree C reduced the number of Ca2+ sites. The binding capacity was reduced by one-half when the protein was dialyzed with 4 M urea or high ionic strength KCl (2 M). Calcium binding was examined as a function of pH. When the protein was dialyzed overnight at different pH values and all the binding was done at pH 7.0, the apparent number of Ca2+ sites decreased as the pH of the dialysis medium was increased. When the protein was dialyzed overnight at pH 7.0 and the binding was done at different pH values, the apparent binding capacity increased as pH increased.(ABSTRACT TRUNCATED AT 250 WORDS)     

Induction of thermal and chemical stability of O6-methylguanine-DNA methyltransferase by Ca2+

Divalent cations stabilized rat recombinant O6-methylguanine-DNA methyltransferase (rMGMT) protein against heat treatment. Activity of rMGMT was completely abolished by incubating at 45 degrees C for 30 min, however, addition of 1.0 mM Mg2+, Ca2+ or Mn2+ significantly protected heat-induced inactivation of MGMT activity (50-60% vs. 97% inactivation). Protective effect of Ca2+ on the MGMT activity was concentration-dependent up to 3 mM, and the thermal protection was effective up to 45 degrees C. In order to investigate Ca2+ binding site in rMGMT protein, truncated GST-rMGMT proteins containing N-terminal 39 amino acids (GST-rMGMT39), 70 amino acids (GST-rMGMT70) and full-length protein (GST-rMGMT) were prepared. Radiolabeled calcium ion [45Ca2+] was bound only to the GST-rMGMT70 and GST-rMGMT, but not to the GST-rMGMT39, indicating that divalent cations could bind the residues between 40th and 70th of the rMGMT protein. Calcium binding was not observed in the site-directed mutant rMGMT proteins (rMGMT(D42A) and rMGMT(E45A)), confirmed by autoradiography using [45Ca2+] after nondenaturing gel electrophoresis; however, the above two mutants had the same catalytic activity as well as proteolytic sensitivity as the wild MGMT protein. Analysis by equilibrium dialysis revealed stoichiometric binding of one molecule of Ca2+ to one molecule of the protein. Since circular dichroism (CD) spectra indicated no discernible difference before and after Ca2+ binding, the above results suggested that neutralization of two negative charges of Asp42 and Glu45 by Ca2+ resulted in thermal stabilization of the protein with minimum perturbation of its tertiary structure.     

The effect of divalent metal ions on the electrophoretic mobility of bovine prothrombin and bovine prothrombin fragment 1

Examination of metal ion-dependent effects on the electrophoretic mobility of bovine prothrombin and fragment 1 provides a useful and sensitive method for investigation of conformational processes in these proteins. Utilization of this method reveals a conformational change in bovine prothrombin and fragment 1 which occurs at low metal ion concentrations. Equilibrium dialysis studies indicate that the metal ion-induced shape change occurs concomitant with binding of a single calcium ion/molecule of prothrombin or fragment 1. Mixed metal electrophoretic mobility studies with Mg2+ and Ca2+ have demonstrated the "synergistic" effect for fragment 1 observed by others. Mixed metal equilibrium dialysis has provided experimental support for this observation and allows us to conclude that two tight Ca2+ sites are not affected by low Mg2+ concentrations and that the third Ca2+ site is also a tight site for Mg2+. Thus, at low Mg2+ concentrations and upon the addition of Ca2+, there are effectively three tight sites; consequently more Ca2+ will bind to the protein at lower total Ca2+ ion concentrations.     

Metal binding and metal-induced conformational change of frog bone gamma-carboxyglutamic acid-containing protein

Ca2+ or Cd2+ binding and the conformational change induced by the metal binding in two frog bone Gla-proteins (BGP, termed BGP-1 and BGP-2) were studied by equilibrium dialysis and CD measurement. By CD measurement in the far-ultraviolet region, the alpha-helix content of both apoBGPs was found to be 8%. Binding of both Ca2+ and Cd2+ was accompanied with a change in the CD spectrum, and the alpha-helix content increased to 15 and 25% for BGP-1 and BGP-2, respectively. CD measurement in the near-ultraviolet region indicated that the environment of aromatic amino acid residues in the protein molecule was changed by metal binding. Equilibrium dialysis experiments indicated that each of these two protein binds specifically 2 mol of Ca2+, and nonspecifically an additional 3-4 mol of Ca2+ in 0.02 M Tris-HCl/0.15 M NaCl (pH 7.4), at 4 degrees C. According to the two separate binding sites model, BGP-1 has 1 high-affinity Ca2+ binding site (Kd1 = 0.17 mM) and 1 low-affinity site (Kd2 = 0.29 mM), and BGP-2 contains 1 high-affinity site (Kd1 = 0.14 mM) and 1 low-affinity site (Kd2 = 0.67 mM). In addition, 2 Cd2+ bound to a high-affinity binding site on BGP-1 with Kd1 of 10.4 microM, and 1 Cd2+ bound to a low-affinity binding site with Kd2 of 41.5 microM. On the other hand, BGP-2 had three classes of binding sites and 1 Cd2+ bound to each binding site with Kd1 = 3.6 microM, Kd2 = 16.3 microM, Kd3 = 51.7 microM, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Heterologous overexpression of human NEFA and studies on the two EF-hand calcium-binding sites.

Human NEFA is an EF-hand, leucine zipper protein containing a signal sequence. To confirm the calcium binding capacity of NEFA, recombinant NEFA analogous to the mature protein and mutants with deletions in the EF-hand domain were expressed in Pichia pastoris and secreted into the culture medium at high yield. The calcium binding activity of each purified protein was measured by a modified equilibrium dialysis using the fluorescent Ca2+ indicator FURA-2 and atomic absorption spectroscopy. A stoichiometry of 2 mol Ca2+/mol NEFA was determined. The Ca2+ binding constants were resolved by intrinsic fluorescence spectroscopy. Fluorescence titration exhibited two classes of Ca2+ binding sites with Kd values of 0.08 microM and 0.2 microM. Circular dichroism (CD) spectroscopy showed an increase from 30 to 43% in the amount of alpha-helix in NEFA after addition of calcium ions. Limited proteolytic digestion indicated a Ca2+ dependent conformational change accompanied by an altered accessibility to the enzyme.     

Studies of ligand binding to arrestin

A striking homology is observed between the regions 70-83 and 361-374 of the sequence of bovine arrestin and the calcium-binding loops of calmodulin and troponin C. However, the predicted alpha-helices flanking the calcium-binding site in calmodulin and troponin C are not present in arrestin. Direct measurements therefore were made in order to assess whether arrestin can bind calcium. We found that arrestin does not bind Ca2+ at physiological ionic strength, as determined by equilibrium dialysis, gel filtration, and fluorescence spectroscopy. Rapid and quantitative precipitation of arrestin occurs with Tb3+. The precipitation is reversed by EDTA and blocked by Mg2+ but not by Ca2+. Prompted by several reports, we also investigated whether nucleotides bind to arrestin. Neither ATP nor GTP binds under the conditions tested. Binding of arrestin to photolyzed, phosphorylated rhodopsin also does not influence the binding of calcium or nucleotides.     

The role of Ca2+ binding in the self-aggregation of laminin-nidogen complexes

Laminin-nidogen complexes were found to aggregate in the presence of divalent cations in a manner dependent on ion concentration. This effect shows a selectivity for Ca2+, as half-maximal aggregation is achieved already at about 10 microM Ca2+, while Mg2+ induces aggregation at 10-fold higher ion concentrations and always to a lesser extent. When binding of Ca2+ to laminin-nidogen complexes was measured by equilibrium dialysis, a total of about 16 binding sites with dissociation constants in the range of 5-300 microM could be identified. At 50 microM Ca2+, where the aggregation is maximal, only two to three Ca2+ ions are bound to laminin-nidogen complexes, indicating that the aggregation reaction is induced by the binding of Ca2+ to a small number of sites and possibly to a single distinct site. Analysis of Ca2+ binding to various proteolytic fragments of laminin allowed the tentative localization of a high affinity binding site to a large fragment comprising two of the short arms connected by the central part of the laminin molecule.     

Resolution and calcium-binding properties of the two major isoforms of troponin C from crayfish

Crayfish tail muscle troponin C (TnC) has been fractionated into its five components and the Ca2+-binding properties of the two major isoforms (alpha and gamma) determined by equilibrium dialysis. alpha-TnC contains one Ca2+-binding site with a binding constant of 1 x 10(6) M-1 and one Ca2+ site with a binding constant of 1 x 10(4) M-1. In the complex of alpha-TnC with troponin I (TnI) or with TnI and troponin T (TnT), both sites bind Ca2+ with a single affinity constant of 2-4 x 10(6) M-1. gamma-TnC contains two Ca2+-binding sites with a binding constant of 2 x 10(4) M-1. In the gamma-TnC.TnI and gamma-TnC.TnI.TnT complexes, the binding constant of one of the sites is increased to 4-5 x 10(6) M-1, while Ca2+ binding to the second site is hardly affected (KCa = 4-7 x 10(4) M-1). In the presence of 10 mM MgCl2, the two Ca2+-binding sites of both TnC isoforms exhibit a 2-3-fold lower affinity. Assuming competition between Ca2+ and Mg2+ for these sites, their binding constants for Mg2+ were 120-230 M-1. In the absence of Ca2+, however, alpha-TnC and gamma-TnC bind 4-5 mol of Mg2+/mol with a binding constant of 1 x 10(3) M-1. These results suggest that the effect of Mg2+ on Ca2+ binding at the two Ca2+ sites is noncompetitive, i.e. Mg2+ does not bind directly to these sites (Ca2+-specific sites). Since the formation of the complex of crayfish TnI with alpha-TnC or gamma-TnC increases significantly the affinity of one of their two Ca2+-specific sites, I conclude that the binding of Ca2+ to only one site (regulatory Ca2+-specific site) controls the Ca2+-dependent interaction between crayfish TnCs and TnI.     

The vectorial specificity for calcium binding to the CaATPase of sarcoplasmic reticulum is controlled by phosphorylation, not by an E-E* conformational change.

The E-E* model for calcium pumping by the CaATPase of sarcoplasmic reticulum includes two distinct conformational states of the enzyme, E and E*. Exterior Ca2+ binds only to E and interior Ca2+ binds only to E*. Therefore, it is expected that there will be competition between the binding of calcium to the unphosphorylated enzyme from the two sides of the membrane. The equilibrium concentration of cECa2, the enzyme with Ca2+ bound at the exterior site, was measured at different Ca2+ concentrations with empty sarcoplasmic reticulum vesicles (SRV) and with SRV loaded with 40 mM Ca2+ by reaction with 0.5 mM [gamma-32P]ATP plus 20 mM EGTA for 13 ms (100 mM KCl, 5 mM MgSO4, 40 mM Mops/KOH, pH 7.0, 25 degrees C). The sigmoidal dependence on free exterior calcium concentration of the concentration of cECa2, measured as [32P]phosphoenzyme, is identical with empty and loaded SRV, within experimental error. The value of K0.5 is 2.8 microM, and the Hill coefficient is 2. This result shows that there is no competition between binding of Ca2+ to the outside and the inside of the membrane. This is consistent with a model in which the vectorial specificity for calcium binding is controlled by the chemical state of the enzyme, rather than a simple conformational change. It is concluded that there are not two interconverting forms of the free enzyme, E and E*, instead the vectorial specificity for binding and dissociation of Ca2+ is determined by the state of phosphorylation of the CaATPase.     

Calcium-linked self-association of human complement C1s.

The weight-average molecular weight of C1s, an activated serine protease subcomponent of human complement C1, has been measured by means of sedimentation equilibrium over a wide range of both protein and calcium ion concentrations. The combined data may be accounted for quantitatively by a simple model for Ca(2+)-dependent self-association of C1s to a dimer. According to this model, the monomer contains a single Ca2+ binding site with K approximately equal to 3 x 10(5) M-1, and the dimer contains three independent Ca binding sites, two having a Ca2+ affinity lower than that of the monomer (K approximately equal to 3 x 10(4) M-1). The third binding site in the dimer, which presumably lies at the interface between the two amino-terminal alpha domains, has a higher Ca2+ affinity (K approximately equal to 1 x 10(8) M-1) and provides the driving force for C1s dimerization in the presence of calcium.     

Purification, characterization and ion binding properties of human brain S100b protein

Human brain S100b (beta beta) protein was purified using zinc-dependent affinity chromatography on phenyl-Sepharose. The calcium- and zinc-binding properties of the protein were studied by flow dialysis technique and the protein conformation both in the metal-free form and in the presence of Ca2+ or Zn2+ was investigated with ultraviolet spectroscopy, sulfhydryl reactivity and interaction with a hydrophobic fluorescence probe 6-(p-toluidino)naphthalene-2-sulfonic acid (TNS). Flow dialysis measurements of Ca2+ binding to human brain S100b (beta beta) protein revealed six Ca2+-binding sites which we assumed to represent three for each beta monomer, characterized by the macroscopic association constants K1 = 0.44 X 10(5) M-1; K2 = 0.1 X 10(5) M-1 and K3 = 0.08 X 10(5) M-1. In the presence of 120 mM KCl, the affinity of the protein for calcium is drastically reduced. Zinc-binding studies on human S100b protein showed that the protein bound two zinc ions per beta monomer, with macroscopic constants K1 = 4.47 X 10(7) M-1 and K2 = 0.1 X 10(7) M-1. Most of the Zn2+-induced conformational changes occurred after the binding of two zinc ions per mole of S100b protein. These results differ significantly from those for bovine protein and cast doubt on the conservation of the S100 structure during evolution. When calcium binding was studied in the presence of zinc, we noted an increase in the affinity of the protein for calcium, K1 = 4.4 X 10(5) M-1; K2 = 0.57 X 10(5) M-1; K3 = 0.023 X 10(5) M-1. These results indicated that the Ca2+- and Zn2+-binding sites on S100b protein are different and suggest that Zn2+ may regulate Ca2+ binding by increasing the affinity of the protein for calcium.     

The binding of Ca2+ to solubilized band 3 protein of the human erythrocyte membrane

The binding of 45Ca2+ to isolated band 3 protein, the anion transport protein of the human erythrocyte membrane, was studied by equilibrium dialysis. The protein was solubilized and purified by either the nonionic detergent Ammonyx-L0 or acetic acid. Each preparation of band 3 protein showed a single high-affinity Ca2+ binding site and several Ca2+ binding sites of lower affinity. The association constant of the high-affinity site was 4-13 X 10(4)M-1; it was only moderately dependent on ionic strength. Mg2+ effectively competed with Ca2+ for the site. Anion exchange across the human erythrocyte membrane is inhibited by micromolar concentrations of intracellular Ca2+. Our results suggest that this inhibition is due to the binding of the cation to a single site on band 3 protein.     

The binding of calcium to a salivary phosphoprotein, protein A, common to human parotid and submandibular secretions.

The binding of Ca2+ to a previously described phosphoprotein from human parotid saliva, protein A [Bennick (1975) Biochem J. 145, 557-567] was studied by means of equilibrium dialysis. In 5 mM-Tris/HC1 buffer, pH7.5, protein A bound 664nmol of Ca/mg of protein. Km was determined to be 181 muM and the binding of Ca2+ to the protein was non-co-operative. The binding of Ca2+ apparently occurs to side-chain carboxyl groups in the protein, but protein phosphate is of minor if any importance in calcium binding. Hydrolysis of protein A by trypsin and collagenase or heating of the protein at 60 degrees or 100 degrees C did not affect Ca2+ binding. The Ca2+ binding decreases with increased concentration of the dialysis buffer and on the addition of SrCl2, or MgCl2 or MnCl2 to the dialysis buffer. Protein A does not aggregate in the presence of Ca2+, since the s20,w was identical when determined in the presence (1.30S) and absence (1.35S) of CaCl2. By use of a specific antiserum to protein A it was found that protein C [Bennick & Connell (1971) Biochem. J. 123, 455-464] and perhaps minor related components cross-reacted with protein A. No other salivary proteins showed immunological similarity. Proteins A and C were also present in submandibular saliva. The possible functions of protein A are discussed.     

Thermodynamics of cation binding to rabbit skeletal muscle calsequestrin. Evidence for distinct Ca(2+)- and Mg(2+)-binding sites

Ca2+ binding to rabbit skeletal calsequestrin was studied at physiological ionic strength by equilibrium flow dialysis, Hummel-Dryer gel filtration and microcalorimetry. 31 Ca(2+)-binding sites with a mean dissociation constant (KD) of 0.79 mM were titrated in the absence, and 23 sites with a KD of 0.88 mM in the presence of 3 mM Mg2+. No cooperativity was observed. For Mg2+ binding, the combination of gel filtration and microcalorimetry yielded a stoichiometry of 26 Mg2+/protein with a KD of 2mM. 1 mM Ca2+ decreased the stoichiometry to 20 Mg2+/protein. Binding of Ca2+ in the absence and presence of 3 mM Mg2+ was accompanied by a release of 2.0 and 2.7 H+/protein, respectively. Mg2+ binding did not lead to a significant proton release suggesting a qualitative difference in the Ca(2+)- and Mg(2+)-binding sites. After correction for proton release, the enthalpy change for Ca2+ binding was very low (-1.5 kJ/protein in the absence, and -15 kJ/protein in the presence of 3 mM Mg2+). The entropy change (+59 J/K.site in the absence and +56 J/K.site in the presence of Mg2+) was therefore virtually the sole driving force for Ca2+ binding. Mg2+ binding is slightly more exothermic (-12.6 kJ/protein), but as for Ca2+, the entropy change (+50 J/K.site) constituted the major driving force of the reaction. A fluorimetric study indicates that the conformation of tryptophan in Mg(2+)-saturated calsequestrin was clearly different from that in the Ca(2+)-saturated protein, but that the (Ca2+ + Mg2+)-saturated protein was not distinct from the Ca(2+)-saturated protein. Thus, in addition to the thermodynamic characterization of the Ca2+/calsequestrin interaction, our data indicate that Ca2+ and Mg2+ do not bind to the same sites on calsequestrin. The data also predict considerable proton fluxes upon Ca(2+)-Mg2+ exchange in vivo.     

Clathrin light chains are calcium-binding proteins

Clathrin light chains have been purified to near homogeneity. When analyzed by sodium dodecyl sulfate gel electrophoresis followed by silver stain for proteins, no bands corresponding to light chains were detected. As calmodulin and troponin C are known to behave in the same manner on silver staining, the possibility that clathrin light chains were Ca2+-binding proteins was investigated. Light chains fixed to nitrocellulose filters were found to bind 45Ca2+ in the presence of 5 mM Mg2+. The Ca2+-binding capacity of the light chains was further investigated, using gel filtration and equilibrium dialysis. The light chains were shown to bind, in the presence of 3 mM Mg2+, 1 mol of Ca2+ per mol of light chain with a Kd of 25-55 microM. Nitrocellulose binding and gel filtration studies showed that light chains present in triskelions are still capable of binding Ca2+, in this case with a calculated Kd of 45 microM.     

Properties of membrane-inserted protein kinase C

Protein kinase C (PKC) interacted with phospholipid vesicles in a calcium-dependent manner and produced two forms of membrane-associated PKC: a reversibly bound form and a membrane-inserted form. The two forms of PKC were isolated and compared with respect to enzyme stability, cofactor requirements, and phorbol ester binding ability. Membrane-inserted PKC was stable for several weeks in the presence of calcium chelators and could be rechromatographed on gel filtration columns in the presence of EGTA without dissociation of the enzyme from the membrane. The activity of membrane-inserted PKC was not significantly influenced by Ca2+, phospholipids, and/or PDBu. Partial dissociation of this PKC from phospholipid was achieved with Triton X-100, followed by dialysis to remove the detergent. The resulting free PKC appeared indistinguishable from original free PKC with respect to its cofactor requirements for activation (Ca2+, phospholipid, and phorbol esters), molecular weight, and phorbol 12,13-dibutyrate (PDBu) binding. The binding of PDBu to free and membrane-inserted PKC was measured under equilibrium conditions using gel filtration techniques. At 2.0 nM PDBu, free PKC bound PDBu with nearly 1:1 stoichiometry in the presence of Ca2+ and phospholipid. No PDBu binding to the free enzyme was observed in the absence of Ca2+. In contrast, membrane-inserted PKC bound PDBu in the presence or the absence of Ca2+; calcium did enhance the affinity of this interaction.(ABSTRACT TRUNCATED AT 250 WORDS)     

Ca2+ binding by Myxicola neurofilament proteins

Titrimetric, 45Ca dialysis, and autoradiographic methods were used to examine how axoplasmic proteins from the giant neuron of the marine annelid Myxicola infundibulum bind calcium. Following the autoradiographic method of Maruyama et al., the 150-160 kD neurofilament subunits were identified as prominent intracellular Ca-binding peptides. Using equilibrium dialysis, extracts of axoplasmic proteins (greater than 50% neurofilament subunits) were examined in 300 mM KCl at different concentrations of free Ca and Mg, and at different pH. Axoplasmic proteins showed a high affinity Ca binding site (K1/2 3-6 microM, capacity 3-7 mumole g-1 protein) at pH 6.8 or pH 7.5. Changing the Mg concentration from 0 to 5 mM had no effect on the Ca binding. Elevating the dialysis pH from 7.0 to 9.0 reduced the apparent number of binding sites for Ca. Using microelectrodes to record the free Ca, microtitrations of axoplasmic proteins were completed by adding small amounts of CaCl2 to 100 microliters volumes of protein solutions. In a medium containing ionic constituents closely resembling those of the Myxicola axon, a Ca binding capacity of 5.0 mumole g-1 protein and a K1/2 of approximately 1 microM were measured.