Figure 5.

Models for hemagglutinin-lipid interactions in the viral envelope. (a) Regional model of hemagglutinin-lipid interaction, where hemagglutinin partitions into specific regions of the envelope that have a high cholesterol/phospholipid ratio and is excluded from other regions. (White) Cholesterol-enriched regions; (gray) cholesterol-depleted regions. (b) Local model of hemagglutinin-lipid interactions where the hemagglutinin is associated with local lipid domains that have a high cholesterol/phospholipid ratio. The diameter of local domains is <∼10 nm, the most common spacing between hemagglutinin trimers. In either case, depletion of cholesterol (arrows) causes phospholipids to exchange out of the hemagglutinin-containing nanodomains to maintain the cholesterol/phospholipid ratio in those domains. (Triangles) Hemagglutinin trimers. This model does not specify the distribution of neuraminidase tetramers.