Structure and function of the M2 amphipathic helix in Influenza virus membrane scission

Influenza A is an enveloped, negative sense RNA virus which causes annual epidemics and major pandemics. Assembly and budding of new viral particles is a complex and multistep process, of which many aspects remain unclear despite many years of research. The Influenza virus M2 protein is a homotetrameric transmembrane protein, containing three domains: ecto domain, transmembrane domain and cytoplasmic tail (CT). In the final stage of budding it has been shown that M2 mediates membrane scission through an amphipathic helix (AH), which is formed by the first 17 amino acids of the protein's CT, however the exact mechanism by which membrane scission is triggered was not known.

Using a collection of biochemical and biophysical techniques we have investigated the structure of the M2 AH and assessed its function in viral assembly and budding. We have shown that the M2 AH is formed upon lipid binding and remains unstructured in solution. It preferentially binds to membranes with high positive membrane curvature, which is detected by sensing the associated lipid packing defects. There are many cationic residues in the polar face of the M2 AH which could interact with anionic lipid headgroups, however charged interactions do not significantly affect the M2 AH interactions with the membranes. When inserted into the membrane the M2 AH increases lipid ordering. The M2 AH also induces positive membrane curvature and mediates membrane scission, which is the last step of Influenza virus budding, allowing for release of newly formed virions in to the environment.

Membrane scission is a crucial step not only in viral budding but also in many biological processes, such as endocytosis. Many proteins have been associated with cellular membrane scission; however, the underlying molecular details are still not known. We have shown that AHs in some cellular peptides, such as Arf 1, Endophillin A3 and Epsin 1, which mediate membrane scission in biological processes, appear to work in a similar manner to the M2 AH. They represent a new protein motif that is capable of sensing curvature, inducing curvature and altering membrane fluidity, thereby mediating membrane scission and therefore belong to a novel group of AHs.