Model of interaction between a cardiotoxin and dimyristoylphosphatidic acid bilayers determined by solid-state 31P NMR spectroscopy.

The interaction of cardiotoxin IIa, a small basic protein extracted from Naja mossambica mossambica venom, with dimyristoylphosphatidic acid (DMPA) membranes has been investigated by solid-state 31P nuclear magnetic resonance spectroscopy. Both the spectral lineshapes and transverse relaxation time values have been measured as a function of temperature for different lipid-to-protein molar ratios. The results indicate that the interaction of cardiotoxin with DMPA gives rise to the complete disappearance of the bilayer structure at a lipid-to-protein molar ratio of 5:1. However, a coexistence of the lamellar and isotropic phases is observed at higher lipid contents. In addition, the number of phospholipids interacting with cardiotoxin increases from about 5 at room temperature to approximately 15 at temperatures above the phase transition of the pure lipid. The isotropic structure appears to be a hydrophobic complex similar to an inverted micellar phase that can be extracted by a hydrophobic solvent. At a lipid-to-protein molar ratio of 40:1, the isotropic structure disappears at high temperature to give rise to a second anisotropic phase, which is most likely associated with the incorporation of the hydrophobic complex inside the bilayer.