Association of melittin with the isolated myosin light chains

Modulation of response to braconid wasp venom by adipokinetic hormone in Drosophila melanogaster

The minute wasp Habrobracon hebetor venom (HH venom) is a potent cocktail of toxins that paralyzes the victim's muscles and suppresses humoral and cellular immunity. This study examined the effect of HH venom on specific biochemical, physiological, and ultrastructural characteristics of the thoracic and nervous (CNS) tissues of Drosophila melanogaster under in vitro conditions. Venom treatment modulated the activities of superoxide dismutase (SOD) and catalase (CAT), endogenous Drome-AKH level, and affected the relative viability of the cells. Additionally, it reduced the expression of genes related to the immune system in the CNS, including Keap1, Relish, Nox, Eiger, Gadd45, and Domeless, as well as in the thoracic muscles, except for Nox. Besides, venom treatment led to deteriorative changes in the ultrastructure of muscle cells, particularly affecting the mitochondria. When venom and Drosophila melanogaster-adipokinetic hormone (Drome-AKH) were applied together, the effects of the venom alone were often modulated. The harmful effect of the venom on SOD activity was relatively reduced and the activity returned to a level similar to that of the control. In the CNS, the simultaneous application of venom and hormones abolished the suppression of previously reported immune-related genes (except for Gadd45), whereas in the muscles, this was only true for Eiger. Additionally, Drome-AKH restored cell structure to a level comparable to that of the control and lessened the harmful effects of HH venom on muscle mitochondria. These findings suggest a general body response of D. melanogaster to HH venom and a partial defensive role of Drome-AKH in this process.

Insect Body Defence Reactions against Bee Venom: Do Adipokinetic Hormones Play a Role?

Bees originally developed their stinging apparatus and venom against members of their own species from other hives or against predatory insects. Nevertheless, the biological and biochemical response of arthropods to bee venom is not well studied. Thus, in this study, the physiological responses of a model insect species (American cockroach, Periplaneta americana) to honeybee venom were investigated. Bee venom toxins elicited severe stress (LD₅₀ = 1.063 uL venom) resulting in a significant increase in adipokinetic hormones (AKHs) in the cockroach central nervous system and haemolymph. Venom treatment induced a large destruction of muscle cell ultrastructure, especially myofibrils and sarcomeres. Interestingly, co-application of venom with cockroach Peram-CAH-II AKH eliminated this effect. Envenomation modulated the levels of carbohydrates, lipids, and proteins in the haemolymph and the activity of digestive amylases, lipases, and proteases in the midgut. Bee venom significantly reduced vitellogenin levels in females. Dopamine and glutathione (GSH and GSSG) insignificantly increased after venom treatment. However, dopamine levels significantly increased after Peram-CAH-II application and after co-application with bee venom, while GSH and GSSG levels immediately increased after co-application. The results suggest a general reaction of the cockroach body to bee venom and at least a partial involvement of AKHs.

N-methyl-D-aspartate receptor subunits are non-myosin targets of myosin regulatory light chain

Excitatory synapses contain multiple members of the myosin superfamily of molecular motors for which functions have not been assigned. In this study we characterized the molecular determinants of myosin regulatory light chain (RLC) binding to two major subunits of the N-methyl-d-aspartate receptor (NR). Myosin RLC bound to NR subunits in a manner that could be distinguished from the interaction of RLC with the neck region of non-muscle myosin II-B (NMII-B) heavy chain; NR-RLC interactions did not require the addition of magnesium, were maintained in the absence of the fourth EF-hand domain of the light chain, and were sensitive to RLC phosphorylation. Equilibrium fluorescence spectroscopy experiments indicate that the affinity of myosin RLC for NR1 is high (30 nm) in the context of the isolated light chain. Binding was not favored in the context of a recombinant NMII-B subfragment one, indicating that if the RLC is already bound to NMII-B it is unlikely to form a bridge between two binding partners. We report that sequence similarity in the "GXXXR" portion of the incomplete IQ2 motif found in NMII heavy chain isoforms likely contributes to recognition of NR2A as a non-myosin target of the RLC. Using site-directed mutagenesis to disrupt NR2A-RLC binding in intact cells, we find that RLC interactions facilitate trafficking of NR1/NR2A receptors to the cell membrane. We suggest that myosin RLC can adopt target-dependent conformations and that a role for this light chain in protein trafficking may be independent of the myosin II complex.

Impact of hapten presentation on antibody binding at lipid membrane interfaces

We report the effects of ligand presentation on the binding of aqueous proteins to solid supported lipid bilayers. Specifically, we show that the equilibrium dissociation constant can be strongly affected by ligand lipophilicity and linker length/structure. The apparent equilibrium dissociation constants (K(D)) were compared for two model systems, biotin/anti-biotin and 2,4-dinitrophenyl (DNP)/anti-DNP, in bulk solution and at model membrane surfaces. The binding constants in solution were obtained from fluorescence anisotropy measurements. The surface binding constants were determined by microfluidic techniques in conjunction with total internal reflection fluorescence microscopy. The results showed that the bulk solution equilibrium dissociation constants for anti-biotin and anti-DNP were almost identical, K(D)(bulk) = 1.7 +/- 0.2 nM vs. 2.9 +/- 0.1 nM. By contrast, the dissociation constant for anti-biotin antibody was three orders of magnitude tighter than for anti-DNP at a lipid membrane interface, K(D) = 3.6 +/- 1.1 nM vs. 2.0 +/- 0.2 microM. We postulate that the pronounced difference in surface binding constants for these two similar antibodies is due to differences in the ligands' relative lipophilicity, i.e., the more hydrophobic DNP molecules had a stronger interaction with the lipid bilayers, rendering them less available to incoming anti-DNP antibodies compared with the biotin/anti-biotin system. However, when membrane-bound biotin ligands were well screened by a poly(ethylene glycol) (PEG) polymer brush, the K(D) value for the anti-biotin antibody could also be weakened by three orders of magnitude, 2.4 +/- 1.1 microM. On the other hand, the dissociation constant for anti-DNP antibodies at a lipid interface could be significantly enhanced when DNP haptens were tethered to the end of very long hydrophilic PEG lipopolymers (K(D) = 21 +/- 10 nM) rather than presented on short lipid-conjugated tethers. These results demonstrate that ligand presentation strongly influences protein interactions with membrane-bound ligands.

Similarities of cow mammary gland cytosolic 21-kDa protein, the substrate for protein kinase C, to the 20-kDa myosin light chain from smooth muscle

In the cytosol of cow mammary gland, several proteins are phosphorylated in the presence of the protein kinase C (PKC) cofactors 1-oleoyl-2-acetyl-sn-glycerol (OAG), phosphatidylserine (PS) and Ca2+. Of the substrates, the 21-kDa protein is inferred to be a 20-kDa regulatory myosin light chain (MLC20) from smooth muscle because of its molecular mass, its distribution in the cytosol, its association with melittin and sphingosine (the PKC modulators), and phosphorylation by PKC as well as by Ca2+/calmodulin-dependent myosin light chain kinase (MLCK). The present study was undertaken to examine whether the 21-kDa protein could be identified as MLC20, by adding cow uterine MLC20 to the reaction mixture containing cytosol with or without the PKC cofactors and/or calmodulin. In the absence of MLC20, the 21-kDa protein was phosphorylated when the PKC cofactors and calmodulin were added to the reaction mixture. Phosphorylation of the 21-kDa protein was inhibited by melittin or sphingosine, and the inhibition was reversed by PS, but not by calmodulin. When MLC20 was included in the reaction mixture, it was phosphorylated in the presence of the PKC cofactors, and the phosphorylated MLC20 band overlapped that of the 21-kDa protein. The indistinguishably overlapped band of the two proteins was inhibited by melittin and by sphingosine, and their inhibition was reversed by PS, not by calmodulin. It is suggested that the 21-kDa protein is the smooth muscle MLC20 and also that the 21-kDa MLC20 is phosphorylated by PKC, but not by MLCK.

Melittin induces both time-dependent aggregation and inhibition of Na,K-ATPase from duck salt glands however these two processes appear to occur independently

Using cupric phenanthroline as a cross-linking agent, we have shown that melittin induced time-dependent aggregations of Na,K-ATPase in microsomal fractions and in preparations of purified Na,K-ATPase from duck salt glands. Incubation of melittin with these preparations also led to the progressive loss of Na,K-ATPase activity. At melittin/protein molar ratio of 5:1, we did not observe inhibition of Na,K-ATPase in the microsomal fraction but the process of enzyme aggregation occurred. At higher melittin/protein molar ratios (10:1 and 30:1), the inhibition of the enzyme and its aggregation proceeded simultaneously but the rates of these processes and maximal values achieved were different. At a melittin/protein ratio of 30:1, Na,K-ATPase inhibition may be described as a biexponential curve with the values for pseudo-first order rate constants being 2.7 and 0.15 min(-1). However, the aggregation may be presented by a monoexponential curve with a pseudo-first order rate constant of 0.15 min(-1). In purified preparations of Na,K-ATPase, the maximal aggregation (about 90%) was achieved at a melittin/protein molar ratio of 2:1, and a further increase in the melittin/protein ratio increased the rate of aggregation but did not affect the value of maximal aggregation. The results show that melittin induced both aggregation and inhibition of Na,K-ATPase but these two processes proceeded independently.

Oligomerization of smooth muscle myosin light chain kinase and its modifications by melittin and calmodulin

The catalytic activity of smooth muscle myosin light chain kinase (MLCKase) requires the presence of calcium and calmodulin [CM; J. T. Stull et al. (1993) Molecular and Cellular Biochemistry, Vols. 127/128, pp. 229-237] and can also be modified through its own oligomerization [E. B. Babiychuk et al. (1995) Biochemistry, Vol. 34, pp. 6366-6372]. In the present report we demonstrate that melittin, one of the most potent CM antagonists, interacted reversibly with the MLCKase apoenzyme with affinities comparable to those of CM and influenced the oligomeric state of the kinase. At low melittin to kinase ratios the kinase formed insoluble oligomers (aggregates) while at higher melittin concentrations it existed predominantly as soluble oligomers revealed by cross-linking as octamers and hexamers. The kinase alone exhibited similar biphasic solubility within a 5-30 microM range and its solubility was strongly influenced by the ionic strength of the medium. Melittin was also found to promote both the aggregation of the purified 24-kDa C-terminal fragment of the kinase and its analogue telokin, as well as of myosin light chains, but had no effect on the solubility of bovine serum albumin, caldesmon, or calmodulin. These data and our cross-linkage experiments indicate that the insoluble kinase oligomers arose via melittin-induced aggregation of the kinase dimers in which the relokin-like domain played a main role. The soluble oligomers, in turn, were formed after saturation of the kinase with melittin, which resulted in a weakening of the interaction between the protomers with an increase of the long-range order within the oligomers. This interpretation is consistent with the observed effects of melittin on MLCKase catalytic and autocatalytic activities. At concentrations of melittin required to produce soluble oligomers, the binding of the kinase to myosin filaments was considerably enhanced. A plausible mechanism for the formation of the soluble oligomers and aggregates is suggested and its relation to the possible MLCKase assemblies discussed in terms of a model.

Induction by melittin of protein phosphorylation in bovine mammary gland and suppression of the phosphorylation by phosphatidylserine

The protein kinase C (PKC) inhibitor sphingosine induces phosphorylation of an 18-kDa protein in Jurkat T cells and of a 19-kDa protein in bovine mammary gland, and suppresses phosphorylation of substrate proteins for PKC. Melittin, a toxic amphiphilic peptide from bee venom known to inhibit PKC activity, was examined to determine whether it, like sphingosine, induced phosphorylation of the 19-kDa protein. Melittin inhibited PKC activity in both cytosolic and total particulate fractions of bovine mammary gland, with IC50 values (concentrations causing 50% inhibition) of 5-7 microM. Melittin suppressed phosphorylation of such PKC substrate proteins as a 91-kDa protein in cytosol and a 36-kDa protein in the particulate fraction. Besides the suppression, melittin induced phosphorylation of cytosolic 105-kDa, 94-kDa, 27-kDa, 24-kDa and 19-kDa and particulate 110-kDa, 53-kDa and 43-kDa proteins, which were not clearly observed in the absence of melittin. The induction could be detected at a 10 microM concentration. Phosphorylation of these proteins was reversed by excess addition of the PKC cofactor phosphatidylserine, but not by other cofactors such as 1-oleoyl-2-acetyl-sn-glycerol or Ca2+. PKC inhibitors other than melittin [1-(5-isoquinolinylsulfonyl)-2-methyl-piperazine, gossypol, palmitoylcarnitine and adriamycin] had little effect on the induction, except at a high concentration (500 microM) of adriamycin. These results suggest that melittin, like sphingosine, has both suppressive and stimulatory effects on protein phosphorylation in bovine mammary gland.

Effects of melittin on lipid-protein interactions in sarcoplasmic reticulum membranes

To investigate the physical mechanism by which melittin inhibits Ca-adenosine triphosphatase (ATPase) activity in sarcoplasmic reticulum (SR) membranes, we have used electron paramagnetic resonance spectroscopy to probe the effect of melittin on lipid-protein interactions in SR. Previous studies have shown that melittin substantially restricts the rotational mobility of the Ca-ATPase but only slightly decreases the average lipid hydrocarbon chain fluidity in SR. Therefore, in the present study, we ask whether melittin has a preferential effect on Ca-ATPase boundary lipids, i.e., the annular shell of motionally restricted lipid that surrounds the protein. Paramagnetic derivatives of stearic acid and phosphatidylcholine, spin-labeled at C-14, were incorporated into SR membranes. The electronic paramagnetic resonance spectra of these probes contained two components, corresponding to motionally restricted and motionally fluid lipids, that were analyzed by spectral subtraction. The addition of increasing amounts of melittin, to the level of 10 mol melittin/mol Ca-ATPase, progressively increased the fraction of restricted lipids and increased the hyperfine splitting of both components in the composite spectra, indicating that melittin decreases the hydrocarbon chain rotational mobility for both the fluid and restricted populations of lipids. No further effects were observed above a level of 10 mol melittin/mol Ca-ATPase. In the spectra from control and melittin-containing samples, the fraction of restricted lipids decreased significantly with increasing temperature. The effect of melittin was similar to that of decreased temperature, i.e., each spectrum obtained in the presence of melittin (10:1) was nearly identical to the spectrum obtained without melittin at a temperature approximately 5 degrees C lower. The results suggest that the principal effect of melittin on SR membranes is to induce protein aggregation and this in turn, augmented by direct binding of melittin to the lipid, is responsible for the observed decreases in lipid mobility. Protein aggregation is concluded to be the main cause of inactivation of the Ca-ATPase by melittin, with possible modulation also by the decrease in mobility of the boundary layer lipids.

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.

Effects of melittin on molecular dynamics and Ca-ATPase activity in sarcoplasmic reticulum membranes: time-resolved optical anisotropy

We have studied the effect of melittin, a basic membrane-binding peptide, on Ca-ATPase activity and on protein and lipid dynamics in skeletal sarcoplasmic reticulum (SR), using time-resolved phosphorescence and fluorescence spectroscopy. Melittin completely inhibits Ca-ATPase activity, with half-maximal inhibition at 9 +/- 1 mol of melittin bound to the membrane per mole of ATPase (0.1 mol of melittin per mole of lipid). The time-resolved phosphorescence anisotropy (TPA) decay of the Ca-ATPase labeled with erythrosin isothiocyanate (ERITC) shows that melittin restricts microsecond protein rotational motion. At 25 degrees C in the absence of melittin, the TPA is characterized by three decay components, corresponding to a rapid segmental motion (correlation time phi 1 = 2-3 microseconds), the uniaxial rotation of monomers or dimers (phi 2 = 16-22 microseconds), and the uniaxial rotation of larger oligomers (phi 3 = 90-140 microseconds). The effect of melittin is primarily to decrease the fraction of the more mobile monomer/dimer species (A2) while increasing the fractions of the larger oligomer (A3) and very large aggregates (A infinity). Time-resolved fluorescence anisotropy of the lipid-soluble probe diphenylhexatriene (DPH) shows only a slight increase in the lipid hydrocarbon chain effective order parameter, corresponding to an increase in lipid viscosity that is too small to account for the large decrease in protein mobility or inhibition of Ca-ATPase activity. Thus the inhibitory effect of melittin correlates with its capacity to aggregate the Ca-ATPase and is consistent with previously reported inhibition of this enzyme under conditions that increase protein-protein interactions.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium-regulated interactions of human alpha-lactalbumin with bee venom melittin

Affinity chromatography, fluorescence and circular dichroism spectroscopy methods have been used to study the interaction of melittin, a 26-residue peptide from bee venom, with Ca2(+)-binding alpha-lactalbumin from human milk. It has been revealed that melittin binds to the apo- and acidic states of alpha-lactalbumin while the presence of Ca2+ makes the interaction essentially weaker. The association constant for the complex of melittin with apo-alpha-lactalbumin determined from spectropolarimetric melittin-titration data is 2 X 10(7) M-1. The complexation of alpha-lactalbumin with melittin decreases its affinity to Ca2+ by three orders of magnitude. The interaction of apo-alpha-lactalbumin with melittin causes some changes in the environment of its aromatic amino acid residues and drastically alters the conformation of melittin, increasing its alpha-helical content but leaving its single tryptophan residue accessible to water. In the case of the acidic state of alpha-lactalbumin the interaction does not induce an increase in alpha-helical content of melittin.

Acyl carrier protein interacts with melittin

Acyl carrier protein (ACP) from Escherichia coli has been shown to form complexes with melittin, a cationic peptide from bee venom. ACP is a small (Mr 8847), acidic, Ca2(+)-binding protein, which possesses some characteristics resembling those of regulatory Ca2(+)-binding proteins including interaction with melittin. Complexing between melittin and ACP which occurred both in the presence and absence of Ca2+ was evident by chemical cross-linking the two peptides, fluorescence changes (including anisotropy measurements), and inhibition by melittin of the activity of a nonaggregated fatty acid synthetase from Euglena. Also, anti-Apis mellifera antibodies which contained antibodies against melittin specifically inhibited the same enzyme system activity relative to non-immune IgG.

Determination of dityrosine, phosphotyrosine, phosphothreonine, and phosphoserine by high-performance liquid chromatography

4'-Dimethylaminoazobenzene-4-sulfonyl chloride is a chromophoric reagent commonly used to detect amino acids at picomole levels. This article describes a single-column reverse-phase high-performance liquid chromatography system which allows the resolution and analysis of the dabsyl chloride derivatives of several modified amino acids. Highly derivatized C18 (22 and 31%) columns from Phenomenex are run at pH 8.1 to separate dityrosine, the phosphorylated amino acids (o-phosphoserine, o-phosphothreonine, and o-phosphotyrosine), and the 17 other amino acids normally present in protein hydrolysates. In order to gain additional sensitivity and to verify the presence of dityrosine, the dityrosine and lysine peaks are collected and run at pH 4.1 on the same columns. Our experience indicates that, with the described setup, the lower limit for accurate and reproducible detection is near 1 pmol. This method has been applied to the analysis of dityrosine in uv-irradiated calmodulin and cardiac troponin C and to the detection of phosphorylation sites in several polypeptides.

Interactive binding between the substrate and allosteric sites of carbamoyl-phosphate synthetase

The interaction between Escherichia coli carbamoyl-phosphate synthetase (CPS) and a fluorescent analogue of an allosteric effector molecule, 1,N6-ethenoadenosine 5'-monophosphate (epsilon-AMP), has been detected by using fluorescence techniques and kinetic measurements. From fluorescence anisotropy titrations, it was found that epsilon-AMP binds to a single site on CPS with Kd = 0.033 mM. The nucleotide had a small activating effect on the rate of synthesis of carbamoyl phosphate but had no effect on the Km for ATP. To test whether epsilon-AMP binds to an allosteric site, allosteric effectors (UMP, IMP, and CMP), known to bind at the UMP/IMP site, were added to solutions containing the epsilon-AMP-CPS complex. With addition of these effector molecules, a progressive decrease of the fluorescence anisotropy was observed, indicating that bound epsilon-AMP was displaced by the allosteric effectors examined. From these titrations, the dissociation constants for UMP, IMP, CMP, ribose 5-phosphate, 2-deoxyribose 5-phosphate, and orthophosphate were determined. When MgATP, a substrate, was employed as a titrant, the observed decrease in anisotropy was consistent with the formation of a ternary complex (epsilon-AMP-CPS-MgATP). The effect of ATP binding, monitored at the allosteric site, was magnesium dependent, and free magnesium in solution was required to obtain a hyperbolic binding isotherm. Solvent accessibility of epsilon-AMP in binary (epsilon-AMP-CPS) and ternary (epsilon-AMP-CPS-MgATP) complexes was determined from acrylamide quenching, showing that the base of epsilon-AMP is well shielded from the solvent even in the presence of MgATP.(ABSTRACT TRUNCATED AT 250 WORDS)