Forms presumably a highly low-pH gated proton-selective channel. Trp-23 may function as a minimalistic gate that opens and closes the pore. When the environmental pH is lower than a threshold, the BM2 channel would be activated and selectively transport protons across the membrane from the extracellular side to the cytoplasmic side. Crucial for the uncoating process. When the virion is internalized into the endosome, the channel acidifies the virion's interior, promoting the dissociation of matrix protein 1 (M1) from the ribonucleoprotein (RNP) thus allowing the transport of the RNP from the virion into the cell's nucleus. Also plays a role in viral protein secretory pathway. Elevates the intravesicular pH of normally acidic compartments, such as trans-Golgi network, preventing newly formed hemagglutinin from premature switching to the fusion-active conformation (By similarity). Plays a crucial role in virion assembly. Expressed in the late phase of the infection (By similarity). Homotetramer. Transported to the plasma membrane through the trans Golgi network. Phosphorylated by host. Influenza B virus genome RNA segment 7 encodes the M1 (AC P13880) and BM2 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.