Membrane fusion activity of the influenza virus hemagglutinin: interaction of HA2 N-terminal peptides with phospholipid vesicles

Abstract

We have investigated the interaction of a number of synthetic 20-residue peptides, corresponding to the HA2 N-terminus of the influenza virus hemagglutinin (X31 strain), with phospholipid vesicles and monolayers. Besides the wild-type sequence, two peptides were studied with mutations corresponding to those previously studied in entire HA's expressed in transfected cells [Gething et al., (1986) J. Cell. Biol. 102, 11-23]. These mutations comprised a single Glu replacement for Gly at the N-terminus ("El" mutant) or at position 4 ("E4") of the HA2 subunit and were shown to produce striking alterations in virus-induced hemolysis and syncytia formation, especially for E1. The X31 "wild-type" (wt) peptide and its E4 variant are shown here to have the capacity to insert into phosphatidylcholine (POPC) large unilamellar vesicle (LUV) membranes in a strictly pH-dependent manner, penetration being marginal at pH 7.4 and significant at pH 5.0. Bilayer insertion was evident from a shift in the intrinsic Trp fluorescence of the wt and E4 peptides and from the induction of calcein leakage from POPC LUV and correlated well with the peptides' ability at pH 5.0 to penetrate into POPC monolayers at initial surface pressures higher than 30 mN/m. By contrast, the E1 peptide was found, at pH 5.0, to bind less tightly to vesicles (assessed by a physical separation method) and to cause much less leakage of POPC LUV than the wt, even under conditions where the peptides were bound to approximately the same extent. Consistent with the correlation between leakage and penetration observed for the wt peptide at pH 5 versus 7, the E1 peptide, even at low pH, showed much less lipid-vesicle-induced shift of its Trp fluorescence than wt, caused a much slower rate of leakage of vesicle contents, and did not insert into POPC monolayers at surface pressures beyond 28.5 mN/m. Circular dichroism spectroscopy measurements of peptides in POPC SUV showed that the conformations of all three peptides are sensitive to pH, but only the wt and E4 peptides became predominantly alpha-helical at acid pH.

Intermediates in influenza virus PR/8 haemagglutinin-induced membrane fusion

“…the fusion peptide) and accessibility of these sites to the liposomes. Liposomes composed of egg lecithin/egg PE/NBD-PE (molar ratio 2/1/0"6) (PE, phosphatidylethanolamine; NBD, N-4-nitrobenzo-2-oxa-l,3-diazole) were prepared by freeze-thaw extrusion (Rafalski et al, 1991). Liposome binding was performed as described (Stegmann et al, 1987).…”

“…Since the substitutions are on the polar surface of the fusion peptide amphipathic helix , they are not likely to affect liposome binding or insertion. The observations of Wharton et al (1988) and Rafalski et al (1991) that a polar mutation in position 4 (E --. G) of a synthetic peptide of the above sequence did not affect its binding and insertion into lipid is consistent with this notion.…”

Low-pH induced conformational changes in viral fusion proteins: implications for the fusion mechanism

“…(iv) Synthetic peptides corresponding to the aminoterminal part of HAz interact with and destabilize lipid bilayers (Lear & Degrado, 1987;Wharton et al, 1988;Clague et al, 1991a;Rafalski et al, 1991).…”

Interaction of the bacterial protein toxin α‐haemolysin with model membranes: protein binding does not always lead to lytic activity

“…In particular, an increase in Trp fluorescence is expected upon interaction with the hydrophobic lipid matrix [13][14][15][16]. This is indeed the case.…”