J. Biol. Chem., Vol. 268, Issue 3, 1716-1722, Jan, 1993
Influenza hemagglutinin-mediated membrane fusion does not involve inverted phase lipid intermediates
T Stegmann
Abteilung Biophysikalische Chemie, Universitat Basel, Switzerland.
Intermediate lipid structures such as inverted micelles and interlamellar
attachments, which can form near liquid crystalline lamellar (L alpha) to
inverted hexagonal (HII) phase boundaries, are thought to play a role in
membrane fusion. To investigate whether these structures are also involved
in influenza hemagglutinin-mediated membrane fusion, measurement of fusion
under conditions where such structures could not form was attempted. It was
found that the fusion of influenza virus with liposomal membranes
containing phosphatidylcholine and gangliosides, which cannot form HII
phases, was only slightly slower than fusion with liposomes that also
contained the HII competent phosphatidylethanolamine. Furthermore, the
virus fused efficiently with liposomes consisting either of pure saturated
phosphatidylcholines or phosphatidylcholine/ganglioside mixtures, even when
the liposomal membranes were in the gel (L beta') phase and thus far from L
alpha/HII transitions. Isolated hemagglutinin, reconstituted into
dimyristoylphosphatidylcholine membranes, induced fusion with liposomes
composed of dimyristoylphosphatidylcholine and gangliosides at temperatures
below the L beta' to L alpha phase transition temperature of
dimyristoylphosphatidylcholine. This latter finding excluded the
possibility that the viral lipids alone could have formed inverted phase
intermediates, thus enabling them to fuse with liposomes that do not
contain lipids capable of forming inverted phases. Therefore, it is
concluded that structures resembling intermediates in L alpha/HII
transitions are most likely not involved in influenza
hemagglutinin-mediated fusion.