MEMBRANE-FUSION INDUCED BY 11-MER ANIONIC AND CATIONIC PEPTIDES - A STRUCTURE-FUNCTION STUDY

We recently demonstrated that an amphipathic net-negatively charged pe ptide consisting of 11 amino acids (WAE 11) strongly promotes fusion o f large unilamellar liposomes (LUV) when anchored to a liposomal membr ane [Pecheur, E. I., Hoekstra, D., Sainte-Marie, J., Maurin, L., Bienv enue, A., and Philippot, J. R. (1997) Biochemistry 36, 3773-3781]. To elucidate a potential relationship between peptide structure and its f usogenic properties and to test the hypothesis that specific structura l motifs are a prerequisite for WAE-induced fusion, three 11-mer WAE-p eptide analogues (WAK, WAE(Pro), and WAS) were synthesized and investi gated for their structure and fusion activity. Structural analysis of the synthetic peptides by infrared attenuated total reflection spectro scopy reveals a distinct propensity of each peptide toward a helical s tructure after their anchorage to a liposomal surface, emphasizing the importance of anchorage on conveying a secondary structure, thereby c onferring fusogenicity to these peptides. However, whereas WAE and WAK peptides displayed an essentially nonleaky fusion process, WAS-and WA E(Pro)-induced fusion was; accompanied by substantial leakage. It appe ars that peptide helicity as such is not a sufficient condition to con vey optimal fusion properties to these 11-mer peptides. Studies of cha nges in the intrinsic Trp fluorescence and iodide quenching experiment s were carried out and revealed the absence of migration of the Trp re sidue of WAS and WAE(Pro) to a hydrophobic environment, upon their int eraction with the target membranes. These results do not support the p enetration of both peptides as their mode of membrane interaction and destabilization but rather suggest their folding along the vesicle sur face, posing them as surface-seeking helixes. This is in striking cont rast to the behavior observed for WAE and WAK, for which at least part ial penetration of the Trp residue was demonstrated. These results ind icate that subtle differences in the primary sequence of a fusogenic p eptide could induce dramatic changes in the way the peptide interacts with a bilayer, culminating in equally drastic changes in their functi onal properties, The data also reveal a certain degree of sequence spe cificity in WAE-induced fusion.