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 polypept ide consisting of three-fingered loops and five-strand beta-sheet stru cture, with zwitterionic dipaimitoylphosphatidylcholine (DPPC) has bee n studied by P-31 and H-2 NMR to understand the binding modes of CTX i n membrane bilayers. The results, in conjunction with DPH fluorescence anisotropy and differential scanning calorimetry studies, show that C TX may penetrate and lyse the bilayers into small aggregates at a lipi d/protein molar ratio of about 20 in the ripple P-beta' phase. Elevati ng the temperature to that of the liquid crystalline L-alpha phase lea ds to the fusion of the small aggregates into larger ones as evidenced by the change of the isotropic signal into a magnetically aligned P-3 1 Signal with a marked reduction in the chemical shift anisotropy. H-2 NMR study on deuterium-labeled DPPC in the head group and fatty acyl region as a function of temperature and CTX concentration reveals a mo lecular 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 phosphoch oline head group of DPPC bilayers are significantly perturbed in the p resence of CTX. Structural consideration of the CTX molecule indicates that the penetration binding mode of CTX with the DPPC bilayer may in volve a novel membrane-binding motif identified recently in the three- fingered loops of P-type CTX. CTX can only bind to DPPC membrane perip herally in the L-alpha phase due to the mismatch of their hydrophobic lengths.