S. Gunther-ausborn et al., Role of hemagglutinin surface density in the initial stages of influenza virus fusion: Lack of evidence for cooperativity, J VIROLOGY, 74(6), 2000, pp. 2714-2720
Membrane fusion mediated by influenza virus hemagglutinin (HA) is believed
to proceed via the cooperative action of multiple HA trimers. To determine
the minimal number of HA trimers required to trigger fusion, and to assess
the importance of cooperativity between these HA trimers, we have generated
virosomes containing coreconstituted HAs derived from two strains of virus
with different pH dependencies for fusion, X-47 (optimal fusion at pH 5.1;
threshold at pH 5.6) and A/Shangdong (optimal fusion at pH 5.6; threshold
at pn 6.0), and measured fusion of these virosomes with erythrocyte ghosts
by a fluorescence lipid mixing assay. Virosomes with different X-47-to-A/Sh
angdong HA ratios, at a constant HA-to-lipid ratio, showed comparable ghost
-binding activities, and the low-pH-induced conformational change of A/Shan
gdong HA did not affect the fusion activity of X-47 HA. The initial rate of
fusion of these virosomes at pH 5.7 increased directly proportional to the
surface density of A/Shangdong HA, and a single A/Shangdong trimer per vir
osome appeared to suffice to induce fusion. The reciprocal of the lag time
before the onset of fusion was directly proportional to the surface density
of fusion-competent HA. These results support the notion that there is no
cooperativity between HA trimers during influenza virus fusion.