Matrix protein 1 (M1) is essential for the viral structure and lifecycle. The protein associates in multimeric complexes to form the nucleocapsid, a rigid proteinous shell beneath the lipid membrane. M1 stabilizes the 3D structure of the envelope. The shape (filamentous or spherical) of the virion is also determined by M1. During genome packaging, M1 interacts with nuclear export protein to promote export from the nucleus and associates with nucleoprotein (NP) to cover nuclear localization sites. During budding, M1 is recruited to and multimerizes at the plasma membrane along with M2, HA, and NA at the site of a nascent virion. The protein assembly process proceeds until the bilayer bends and a new viral particle is formed and released. During viral entry, M1 disassociates from NP after acidification of the virion core due to the activation of matrix protein 2 and thus allows the nuclear localization of the viral genome.