contributed equally to this work During in¯uenza virus infection, viral ribonucleo proteins (vRNPs) are replicated in the nucleus and must be exported to the cytoplasm before assembling into mature viral particles. Nuclear export is mediated by the cellular protein Crm1 and putatively by the viral protein NEP/NS2. Proteolytic cleavage of NEP de®nes an N-terminal domain which mediates RanGTP-dependent binding to Crm1 and a Cterminal domain which binds to the viral matrix protein M1. The 2.6 A Ê crystal structure of the C-terminal domain reveals an amphipathic helical hairpin which dimerizes as a four-helix bundle. The NEP±M1 interaction involves two critical epitopes: an exposed tryptophan (Trp78) surrounded by a cluster of glutamate residues on NEP, and the basic nuclear localization signal (NLS) of M1. Implications for vRNP export are discussed.

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