Action of Taiwan cobra cardiotoxin on membranes: binding modes of a beta-sheet polypeptide with phosphatidylcholine bilayers

The interaction of Taiwan cobra cardiotoxin (CTX A3), a basic polypeptide consisting of three-fingered loops and five-strand beta-sheet structure, with zwitterionic dipalmitoylphosphatidylcholine (DPPC) has been studied by 31P and 2H NMR to understand the binding modes of CTX in membrane bilayers. The results, in conjunction with DPH fluorescence anisotropy and differential scanning calorimetry studies, show that CTX may penetrate and lyse the bilayers into small aggregates at a lipid/protein molar ratio of about 20 in the ripple Pbeta' phase. Elevating the temperature to that of the liquid crystalline Lalpha phase leads to the fusion of the small aggregates into larger ones as evidenced by the change of the isotropic signal into a magnetically aligned 31P signal with a marked reduction in the chemical shift anisotropy. 2H NMR study on deuterium-labeled DPPC in the head group and fatty acyl region as a function of temperature and CTX concentration reveals a molecular model that CTX undergoes a redistribution between penetrating and peripheral binding states depending on the temperature studied. In addition, both the conformational and dynamic states of the phosphocholine head group of DPPC bilayers are significantly perturbed in the presence of CTX. Structural consideration of the CTX molecule indicates that the penetration binding mode of CTX with the DPPC bilayer may involve a novel membrane-binding motif identified recently in the three-fingered loops of P-type CTX. CTX can only bind to DPPC membrane peripherally in the Lalpha phase due to the mismatch of their hydrophobic lengths.