Both Fusion Peptide and Trans-Membrane Domain of HIV gp41 Individually Reduce the Activation Barriers for the Fusion Process

Abstract

Poly(ethylene glycol)- (PEG-) mediated fusion of 25 nm vesicles was examined in the presence of the HIV gp41 fusion peptide (FP) and trans-membrane domain (TMD) at temperatures between 17°C and 37°C. Membrane lipid composition was Dioleoylphosphatidylcholine (DOPC), dioleoylphosphatidylethanolamine (DOPE), bovine brain sphingomyelin (SM) and Cholesterol (CH) (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), yielding 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 in control and peptide-containing vesicles implied that the nature of the barrier between states for all systems changed with temperature (i.e., activation enthalpy and entropy varied with temperature). In control vesicles, CM occurred earlier in the process at higher temperatures such that fusion shifts from a 4-state to a 3-state model above (≥27°C). Mainly FP but also TMD enhanced the rate of initial intermediate formation. Above about 22°C, this resulted from a large increase in activation entropy overcoming an unfavorable large increase in activation enthalpy, suggesting that the peptides reduced exposure of water to hydrocarbon in the transition state relative to control vesicles. Both peptides enhanced the rate of final pore formation to such an extent that the fusion process followed a 3-state (single intermediate) model even at low temperature (170C). This effect was greatest for TMD and was also largely an entropic effect. Supported by NIGMS grant 32707 to BRL.