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. B/Memphis/12/97 is an avirulent strain in mice. Influenza B virus genome RNA segment 7 encodes the M1 and BM2 (AC Q80DN6) proteins. Normal translation produces the M1 protein. The M1 termination codon overlaps the BM2 initiation codon in an overlapping stop-start pentanucleotide 5'-UAAUG-3'. Termination of M1 translation triggers reinitiation on the BM2 AUG in the +2 open reading frame. 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.