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.