melitten and 1-anilino-8-naphthalenesulfonate

Named for the capacity to stimulate differentiation and maturation of T-cell precursors, avian thymic hormone (ATH) is nonetheless a beta-parvalbumin that is also expressed in the avian retina. With Ca(2+)- and Mg(2+)-binding constants in excess of 10(8) and 10(4) M(-1), respectively, both EF-hand motifs qualify as Ca(2+)/Mg(2+) sites. However, whereas addition of either apo- or Mg(2+)-bound ATH to 1,8-anilinonaphthalenesulfonic acid (ANS) causes a large increase in quantum yield and a pronounced blue shift, addition of the Ca(2+)-bound protein is without effect. These observations suggest that apo- and Mg(2+)-bound ATH adopt conformations distinct from the Ca(2+)-bound protein, exposing apolar surface for interaction with ANS. Differential scanning calorimetry (DSC) data imply that unfolding of apo-ATH is accompanied by diminished exposure of apolar surface, relative to Ca(2+)-free rat beta-PV, perhaps due to greater solvent-accessible apolar surface in the native form. The fluorescence and DSC results, considered together, may indicate that the AB and CD-EF domains of ATH are not tightly associated in the absence of bound Ca(2+). Consistent with this idea, sedimentation velocity data reveal that the apo- and Mg(2+)-bound forms of ATH show greater departures from spherical symmetry than the Ca(2+)-bound state. These findings suggest that a high-affinity binding signature does not require that the parvalbumin apo- and Ca(2+)-bound conformations be indistinguishable, as we have recently proposed. They also suggest that it is possible to engineer a Ca(2+)-dependent conformational change into a high-affinity EF-hand protein, furnishing a mechanism by which the protein could play a reverse Ca(2+) sensor role.

Topics: Anilino Naphthalenesulfonates; Animals; Calcium; Calorimetry, Differential Scanning; Circular Dichroism; Melitten; Parvalbumins; Protein Conformation; Rats; Spectrometry, Fluorescence

The thermodynamics of interaction of two model peptides melittin and mastoparan with bovine brain calmodulin (CAM) and a smaller CAM analogue, a calcium binding protein from Entamoeba histolytica (CaBP) in 10 mM MOPS buffer (pH 7.0) was examined using isothermal titration calorimetry (ITC). These data show that CAM binds to both the peptides and the enthalpy of binding is endothermic for melittin and exothermic for mastoparan at 25 degrees C. CaBP binds to the longer peptide melittin, but does not bind to mastoparan, the binding enthalpy being endothermic in nature. Concurrently, we also observe a larger increase in alpha-helicity upon the binding of melittin to CAM when compared to CaBP. The role of hydrophobic interactions in the binding process has also been examined using 8-anilino-1-naphthalene-sulphonic acid (ANS) binding monitored by ITC. These results have been employed to rationalize the energetic consequences of the binding reaction.

Topics: Amino Acid Sequence; Anilino Naphthalenesulfonates; Animals; Calcium-Binding Proteins; Calmodulin; Calorimetry; Circular Dichroism; Entamoeba histolytica; Escherichia coli; Hydrogen-Ion Concentration; Intercellular Signaling Peptides and Proteins; Melitten; Models, Molecular; Molecular Sequence Data; Peptides; Protein Binding; Protein Structure, Secondary; Recombinant Proteins; Sequence Homology, Amino Acid; Thermodynamics; Wasp Venoms

Whereas melittin at micromolar concentrations is unfolded under conditions of low salt at neutral pH, it transforms to a tetrameric alpha-helical structure under several conditions, such as high peptide concentration, high anion concentration, or alkaline pH. The anion- and pH-dependent stabilization of the tetrameric structure is similar to that of the molten globule state of several acid-denatured proteins, suggesting that tetrameric melittin might be a state similar to the molten globule state. To test this possibility, we studied the thermal unfolding of tetrameric melittin using far-UV CD and differential scanning calorimetry. The latter technique revealed a broad but distinct heat absorption peak. The heat absorption curves were consistent with the unfolding transition observed by CD and were explainable by a 2-state transition mechanism between the tetrameric alpha-helical state and the monomeric unfolded state. From the peptide or salt-concentration dependence of unfolding, the heat capacity change upon unfolding was estimated to be 5 kJ (mol of tetramer)-1 K-1 at 30 degrees C and decreased with increasing temperature. The observed change in heat capacity was much smaller than that predicted from the crystallographic structure (9.2 kJ (mol of tetramer)-1 K-1), suggesting that the hydrophobic residues of tetrameric melittin in solution are exposed in comparison with the crystallographic structure. However, the results also indicate that the structure is more ordered than that of a typical molten globule state. We consider that the conformation is intermediate between the molten globule state and the native state of globular proteins.

Topics: Anilino Naphthalenesulfonates; Calorimetry; Circular Dichroism; Crystallography; Cytochrome c Group; Fluorescent Dyes; Hot Temperature; Melitten; Models, Chemical; Protein Conformation; Protein Denaturation; Spectrometry, Fluorescence; Thermodynamics

Hemolytic delta-toxin from Staphylococcus aureus was soluble in either water, methanol or chloroform/methanol (2 : 1, v/v). The toxin spread readily from distilled water into films with pressures (pi) of 10 dynes/cm on water and 30 dynes/cm on 6 M urea; from chloroform/methanol it produced 40 dynes/cm pressure on distilled water. The toxin adsorbed barely from water (pi = 1 dyne/ cm) but it did rapidly from 6 M urea (pi = 35 dynes/cm). The protein films had unusually high surface potentials, which increased with the film pressure and decreased with increasing both pH and urea concentration in the aqueous phase. The fluorescence of 1-aniline 8-naphthalene sulfonate with delta-toxin was much greater than that with RNAase and dipalmitoyl phosphatidylcholine itself, indicating probably a marked lipid-binding character of the toxin. By circular dichroism the alpha-helix content of delta-toxin was 42% in water, 45% in methanol, 24% in 6 M urea. Infrared spectroscopy showed predominant alpha-helix in both 2H2O and deuterated chloroform/methanol as well as in films spread from either solvent on 2H2O. In spreading from 6 M [2H]urea, in which the major infrared absorption was that of [2H]urea with peaks at 1600 and 1480 cm(-1), the delta-toxin film showed prevalently non-alpha-helix structures with major peak intensities at 1633 cm(-1) > 1680 cm(-1), indicating the appearance of new beta-aggregated and beta-antiparallel pleated sheet structures in the film. The data prove that (1) high pressure protein films can consist of alpha-helix as well as non-alpha-helix structures and, differently from another cytolytic protein, melittin, delta-toxin does not resume the alpha-helix conformation in going into the film phase from the extended chain in 6 M urea; (2) conformational changes are important in the transport of proteins from aqueous to lipid or membrane phase; (3) delta-toxin is by far more versatile in structural dynamics and more surface active than alpha-toxin.

Topics: 1,2-Dipalmitoylphosphatidylcholine; Adsorption; Anilino Naphthalenesulfonates; Bacterial Toxins; Biochemistry; Cell Membrane; Chloroform; Circular Dichroism; Hydrogen-Ion Concentration; Ions; Melitten; Methanol; Microscopy, Fluorescence; Molecular Weight; Pressure; Protein Conformation; Protein Structure, Secondary; Ribonucleases; Spectrophotometry, Infrared; Surface Properties; Temperature; Time Factors; Type C Phospholipases; Urea; Water