Plays critical roles in virus replication, from virus entry and uncoating to assembly and budding of the virus particle. M1 binding to ribonucleocapsids (RNPs) in nucleus seems to inhibit viral transcription. Interaction of viral NEP with M1-RNP is thought to promote nuclear export of the complex, which is targeted to the virion assembly site at the apical plasma membrane in polarized epithelial cells. Interactions with NA and HA may bring M1, a non-raft-associated protein, into lipid rafts. Forms a continuous shell on the inner side of the lipid bilayer in virion, where it binds the RNP. During virus entry into cell, the M2 ion channel acidifies the internal virion core, inducing M1 dissociation from the RNP. M1-free RNPs are transported to the nucleus, where viral transcription and replication can take place. Determines the virion's shape: spherical or filamentous. Clinical isolates of influenza are characterized by the presence of significant proportion of filamentous virions, whereas after multiple passage on eggs or cell culture, virions have only spherical morphology. Filamentous virions are thought to be important to infect neighboring cells, and spherical virions more suited to spread through aerosol between hosts organisms. Homodimer and homomultimer. Interacts with NEP. Binds ribonucleocapsid by both interacting with genomic RNA and NP protein. May interact with HA and NA. Cannot bind NP without genomic RNA. Only the first 9 residues are shared by the 2 isoforms. SC35 was derived from A/Seal/Massachussetts/1/80 (H7N7) by serial passages in chicken embryo cells, thereby acquiring a multibasic cleavage site in its hemagglutinin (HA) and becoming 100% lethal for chickens. SC35 was then passaged 11 times in mouse lung, yielding the mouse-adapted variant SC35M. Most abundant protein in virion. When expressed alone can form virus-like particles in transfected cells. Belongs to the influenza viruses Matrix protein M1 family.