HA Fusion Peptide, but Not Two Biologically Inactive Mutants, Lowers Activation Barrier of the Pore Formation Step during PEG-mediated Fusion

Abstract

Poly(ethylene glycol)- (PEG-) mediated fusion of 25 nm vesicles was examined in the presence of hemagglutinin (HA) fusion peptide and its biologically inactive mutants (G1V, G1S and W14A) at different temperatures between 26°C and 43°C. Lipid composition was dioleoylphosphatidylcholine (DOPC), dioleoylphosphatidylethanolamine (DOPE), bovine brain sphingomyelin (SM) and Cholesterol (CH) in a ratio of 35:30:15:20. Lipid mixing (LM), content mixing (CM) and content leakage (L) time courses were fitted globally to 3-state or 4-state sequential models (Biophys. J., 2007, 92; 4012). From this we obtained estimates of rate constants for conversion between states as well as probabilities of the occurrence of LM, CM, or L in each state. Non-linear Arrhenius plots implied that the nature of the barrier between states changed with temperature such that activation enthalpy and entropy for all systems varied with temperature. Wild type (WT) fusion peptide enhanced the rate of pore formation by lowering the activation free energy (increased favorable activation entropy) as well as increased the extent of content mixing. G1S and W14A mutants decreased the rate of pore formation and extent of content mixing. A G1V mutant enhanced the rate of pore formation even more than did wild type HA peptide but made the vesicles so leaky that content mixing could not be observed. For both control vesicles and in the presence of WT peptide, the probability of CM increased for states early in the process, such that fusion became 3-state model at higher temperatures with WT. Fluorescence probes revealed that W14A increased the acyl chain packing at all the temperatures and G1V and W14A reduce bilayer packing at the water-membrane interface while WT peptide had little effect. Supported by NIGMS grant 32707 to BRL.