Line-Tension Controlled Mechanism for Influenza Fusion

Abstract

Our molecular simulations reveal that wild-type influenza fusion peptides are able to stabilize a highly fusogenic pre-fusion structure, i.e. a peptide bundle formed by four or more trans-membrane arranged fusion peptides. We rationalize that the lipid rim around such bundle has a non-vanishing rim energy (line-tension), which is essential to (i) stabilize the initial contact point between the fusing bilayers, i.e. the stalk, and (ii) drive its subsequent evolution. Such line-tension controlled fusion event does not proceed along the hypothesized standard stalk-hemifusion pathway. In modeled influenza fusion, single point mutations in the influenza fusion peptide either completely inhibit fusion (mutants G1V and W14A) or, intriguingly, specifically arrest fusion at a hemifusion state (mutant G1S). Our simulations demonstrate that, within a line-tension controlled fusion mechanism, these known point mutations either completely inhibit fusion by impairing the peptide’s ability to stabilize the required peptide bundle (G1V and W14A) or stabilize a persistent bundle that leads to a kinetically trapped hemifusion state (G1S). In addition, our results further suggest that the recently discovered leaky fusion mutant G13A, which is known to facilitate a pronounced leakage of the target membrane prior to lipid mixing, reduces the membrane integrity by forming a ‘super’ bundle. Our simulations offer a new interpretation for a number of experimentally observed features of the fusion reaction mediated by the prototypical fusion protein, influenza hemagglutinin, and might bring new insights into mechanisms of other viral fusion reactions.

Expansion of the fusion stalk and its implication for biological membrane fusion

“…Leakage pores as pores in general, however, are expected to become larger and longer-lived, and thus more detectable, when the membrane(s) are under tension. Alternatively, our simulations (20) suggest that leakage pores can become metastable in the presence of amphiphilic fusion peptides, which line such a pore and thereby hinder "closing" of the elongated stalk (SI Appendix, Fig. S13).…”

“…3 shows that pulling the probes toward each other induced a leakage pore in the direct stalk vicinity. Because of the unfavorable excess free energy of a pore's rim (line tension), the stalk rapidly encircles the pore (stalk elongation) resulting in an alternative formation of an HD (20,24).…”

“…The leakage pores observed in fusion simulations with tensionless membranes (8,9,20) are typically less than 2 nm in diameter with lifetimes of only up to hundreds of nanoseconds. Notably, we have observed similar small, shortlived pores in our simulations of SNARE-mediated fusion (26, 51) (SI Appendix, Fig.…”

“…How the stalk transitions (expands) into the fusion pore remains controversial (7-9). Different pathways have been proposed based on experimental observations (3, 5, 10-13), molecular simulations (8,9,(14)(15)(16)(17)(18)(19)(20), continuum elastic models (15,21,22), and self-consistent field theory (23,24).…”

“…Here, even single point mutations can selectively trap the fusion reaction in a hemifused state (25). Hemagglutinin therefore very likely plays an active, essential role in the subsequent evolution of hemifusion intermediates (20). In contrast, it remains unclear if SNARE molecules play a role therein.…”