MODEL OF INTERACTION BETWEEN A CARDIOTOXIN AND DIMYRISTOYL PHOSPHATIDIC-ACID BILAYERS DETERMINED BY SOLID-STATE P-31 NMR-SPECTROSCOPY

The interaction of cardiotoxin Ila, a small basic protein extracted fr om Naja mossambica mossambica venom, with dimyristoylphosphatidic acid (DMPA) membranes has been investigated by solid-state P-31 nuclear ma gnetic resonance spectroscopy. Both the spectral lineshapes and transv erse relaxation time values have been measured as a function of temper ature for different lipid-to-protein molar ratios. The results indicat e that the interaction of cardiotoxin with DMPA gives rise to the comp lete disappearance of the bilayer structure at a lipid-to-protein mola r ratio of 5:1. However, a coexistence of the lamellar and isotropic p hases 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 t ransition of the pure lipid. The isotropic structure appears to be a h ydrophobic complex similar to an inverted micellar phase that can be e xtracted by a hydrophobic solvent. At a lipid-to-protein molar ratio o f 40:1, the isotropic structure disappears at high temperature to give rise to a second anisotropic phase, which is most likely associated w ith the incorporation of the hydrophobic complex inside the bilayer.