Association of influenza virus proteins with membrane rafts

Abstract

Assembly and budding of influenza virus proceeds in the viral budozone, a domain in the plasma membrane with characteristics of cholesterol/sphingolipid-rich membrane rafts. The viral transmembrane glycoproteins hemagglutinin (HA) and neuraminidase (NA) are intrinsically targeted to these domains, while M2 is seemingly targeted to the edge of the budozone. Virus assembly is orchestrated by the matrix protein M1, binding to all viral components and the membrane. Budding progresses by protein- and lipid-mediated membrane bending and particle scission probably mediated by M2. Here, we summarize the experimental evidence for this model with emphasis on the raft-targeting features of HA, NA, and M2 and review the functional importance of raft domains for viral protein transport, assembly and budding, environmental stability, and membrane fusion.

Figure 1

The membrane-associated proteins of influenza virus and their raft association. (a) Primary amino acid sequence of hemagglutinin (HA, processed into HA₁ and HA₂, blue), neuraminidase (NA, green), M2 (purple), and M1 (red, with amphiphilic helix in black). Transmembrane regions (TMR) in gray, S-acylations in HA and M2 indicated by zigzag line, signal peptide of HA in white, and fusion peptide in HA in cyan. (b) Topology of HA, NA, M2, and M1 in the membrane, raft localization indicated. Raft-targeting features: (1) hydrophobic amino acids in the outer part of the HA-TMR; (2) S-acylation of HA; (3) outer part of the NA-TMR; (4) S-acylation and cholesterol binding of the amphiphilic helix of M2. M1 according to the structure model of Shishkov et al. [1], membrane-interactive regions in red. Only one monomer of the trimeric HA and the tetrameric NA and M2 is shown for clarity.

Figure 2

Schematic representation of influenza virus budding. (a) Formation of the budozone, a coalesced raft domain, in the plasma membrane. HA (blue) and NA (not shown) are targeted to rafts; M2 (purple) might be positioned at the edge of the budozone. M1 (red) binds to membranes and all the viral proteins including the viral RNPs (not shown) and clusters the viral components. The cytoskeleton (cyan) is involved in establishment of the budozone. (b) Formation of curvature for budding. Interactions involved: M1 acts as a scaffold from beneath, the cytoskeleton provides outward pushing, the line tension at the domain boundary leads to bending, and the amphiphilic helix of M2 acts as a wedge. See text for details.