ID   BM2_INBSI               Reviewed;         109 AA.
AC   P08383;
DT   01-AUG-1988, integrated into UniProtKB/Swiss-Prot.
DT   10-JAN-2006, sequence version 2.
DT   23-FEB-2022, entry version 82.
DE   RecName: Full=Matrix protein 2;
DE   AltName: Full=BM2;
GN   Name=M;
OS   Influenza B virus (strain B/Singapore/222/1979).
OC   Viruses; Riboviria; Orthornavirae; Negarnaviricota; Polyploviricotina;
OC   Insthoviricetes; Articulavirales; Orthomyxoviridae; Betainfluenzavirus.
OX   NCBI_TaxID=107417;
OH   NCBI_TaxID=9606; Homo sapiens (Human).
RN   [1]
RP   NUCLEOTIDE SEQUENCE [GENOMIC RNA].
RX   PubMed=3788064; DOI=10.1016/0042-6822(86)90237-0;
RA   Hiebert S.W., Williams M.A., Lamb R.A.;
RT   "Nucleotide sequence of RNA segment 7 of influenza B/Singapore/222/79:
RT   maintenance of a second large open reading frame.";
RL   Virology 155:747-751(1986).
CC   -!- FUNCTION: Forms presumably a highly low-pH gated proton-selective
CC       channel. Trp-23 may function as a minimalistic gate that opens and
CC       closes the pore. When the environmental pH is lower than a threshold,
CC       the BM2 channel would be activated and selectively transport protons
CC       across the membrane from the extracellular side to the cytoplasmic
CC       side. Crucial for the uncoating process. When the virion is
CC       internalized into the endosome, the channel acidifies the virion's
CC       interior, promoting the dissociation of matrix protein 1 (M1) from the
CC       ribonucleoprotein (RNP) thus allowing the transport of the RNP from the
CC       virion into the cell's nucleus. Also plays a role in viral protein
CC       secretory pathway. Elevates the intravesicular pH of normally acidic
CC       compartments, such as trans-Golgi network, preventing newly formed
CC       hemagglutinin from premature switching to the fusion-active
CC       conformation (By similarity). Plays a crucial role in virion assembly.
CC       Expressed in the late phase of the infection (By similarity).
CC       {ECO:0000250}.
CC   -!- SUBUNIT: Homotetramer. {ECO:0000250}.
CC   -!- SUBCELLULAR LOCATION: Virion membrane {ECO:0000305}; Single-pass type
CC       III membrane protein {ECO:0000305}. Host cell membrane {ECO:0000250};
CC       Single-pass type III membrane protein {ECO:0000250}. Note=Transported
CC       to the plasma membrane through the trans Golgi network. {ECO:0000250}.
CC   -!- PTM: Phosphorylated by host.
CC   -!- MISCELLANEOUS: Influenza B virus genome RNA segment 7 encodes the M1
CC       (AC P06816) and BM2 proteins. Normal translation produces the M1
CC       protein. The M1 termination codon overlaps the BM2 initiation codon in
CC       an overlapping stop-start pentanucleotide 5'-UAAUG-3'. Termination of
CC       M1 translation triggers reinitiation on the BM2 AUG in the +2 open
CC       reading frame.
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DR   EMBL; M14909; AAA67101.1; -; Genomic_RNA.
DR   PIR; B25619; MFIVB2.
DR   SMR; P08383; -.
DR   Proteomes; UP000137758; Genome.
DR   GO; GO:0020002; C:host cell plasma membrane; IEA:UniProtKB-SubCell.
DR   GO; GO:0016021; C:integral component of membrane; IEA:UniProtKB-KW.
DR   GO; GO:0044385; C:integral to membrane of host cell; IEA:UniProtKB-KW.
DR   GO; GO:0055036; C:virion membrane; IEA:UniProtKB-SubCell.
DR   GO; GO:0005216; F:ion channel activity; IEA:UniProtKB-KW.
DR   GO; GO:0039707; P:pore formation by virus in membrane of host cell; IEA:UniProtKB-KW.
DR   GO; GO:0051259; P:protein complex oligomerization; IEA:UniProtKB-KW.
DR   GO; GO:1902600; P:proton transmembrane transport; IEA:InterPro.
DR   InterPro; IPR006859; Flu_B_M2.
DR   Pfam; PF04772; Flu_B_M2; 1.
PE   3: Inferred from homology;
KW   Coiled coil; Host cell membrane; Host membrane; Hydrogen ion transport;
KW   Ion channel; Ion transport; Membrane; Phosphoprotein; Reference proteome;
KW   Signal-anchor; Transmembrane; Transmembrane helix; Transport;
KW   Viral ion channel; Virion.
FT   CHAIN           1..109
FT                   /note="Matrix protein 2"
FT                   /id="PRO_0000078900"
FT   TOPO_DOM        1..4
FT                   /note="Virion surface"
FT                   /evidence="ECO:0000255"
FT   TRANSMEM        5..27
FT                   /note="Helical; Signal-anchor for type III membrane
FT                   protein"
FT                   /evidence="ECO:0000255"
FT   TOPO_DOM        28..109
FT                   /note="Intravirion"
FT                   /evidence="ECO:0000255"
FT   SITE            19
FT                   /note="Essential for channel activity, possibly by being
FT                   protonated during channel activation, and by forming the
FT                   channel gate and the selective filter"
FT                   /evidence="ECO:0000250"
FT   SITE            23
FT                   /note="Seems to be involved in pH gating"
FT                   /evidence="ECO:0000250"
SQ   SEQUENCE   109 AA;  12625 MW;  8B230F38F37F8978 CRC64;
     MLEPFQILSI CSFILSALHF MAWTIGHLNQ IKRGVNLKIR IRNPNKETIN REVSILRHSY
     QKEIQAKETM KEVLSDNMEI LSDHIVIEGL SAEEIIKMGE TVLEVEELH