Summary Many viruses utilize host ESCRT proteins for budding; however, influenza virus budding is thought to be ESCRT-independent. In this study we have found a role for the influenza virus M2 proton-selective ion channel protein in mediating virus budding. We observed that a highly conserved amphipathic helix located within the M2 cytoplasmic tail mediates a cholesterol-dependent alteration in membrane curvature. The 17 amino acid amphipathic helix is sufficient for budding into giant unilamellar vesicles, and mutation of this sequence inhibited budding of transfected M2 protein in vivo. We show that M2 localizes to the neck of budding virions and that mutation of the M2 amphipathic helix results in failure of the virus to undergo membrane scission and virion release. These data suggest that M2 mediates the final steps of budding for influenza viruses, bypassing the need for host ESCRT proteins.

The influenza A virus M2 protein (A/M2) is a homotetrameric pH-activated proton transporter/channel that mediates acidification of the interior of endosomally encapsulated virus. This 97-residue protein has a single transmembrane (TM) helix, which associates to form homotetramers that bind the anti-influenza drug amantadine. However, the minimal fragment required for assembly and proton transport in cellular membranes has not been defined. Therefore, the conductance properties of truncation mutants expressed in Xenopus oocytes were examined. A short fragment spanning residues 21-61, M2(21-61), was inserted into the cytoplasmic membrane and had specific, amantadine-sensitive proton transport activity indistinguishable from that of full-length A/M2; an epitope-tagged version of an even shorter fragment, M2(21-51)-FLAG, had specific activity within a factor of 2 of the full-length protein. Furthermore, synthetic fragments including a peptide spanning residues 22-46 were found to transport protons into liposomes in an amantadine-sensitive manner. In addition, the functionally important His-37 residue pKa values are highly perturbed in the tetrameric form of the protein, a property conserved in the TM peptide and full-length A/M2 in both micelles and bilayers. These data demonstrate that the determinants for folding, drug binding, and proton translocation are packaged in a remarkably small peptide that can now be studied with confidence.liposomes ͉ oocyte ͉ H ϩ ͉ intracellular pH ͉ Xenopus laevis

Influenza A virus buds from cells as spherical (ϳ100-nm diameter) and filamentous (ϳ100 nm ؋ 2 to 20 m) virions. Previous work has determined that the matrix protein (M1) confers the ability of the virus to form filaments; however, additional work has suggested that the influenza virus M2 integral membrane protein also plays a role in viral filament formation. In examining the role of the M2 protein in filament formation, we observed that the cytoplasmic tail of M2 contains several sites that are essential for filament formation. Additionally, whereas M2 is a nonraft protein, expression of other viral proteins in the context of influenza virus infection leads to the colocalization of M2 with sites of virus budding and lipid raft domains. We found that an amphipathic helix located within the M2 cytoplasmic tail is able to bind cholesterol, and we speculate that M2 cholesterol binding is essential for both filament formation and the stability of existing viral filaments.

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