Database: UniProt
Entry: BM2_INBLE
Original site: BM2_INBLE 
ID   BM2_INBLE               Reviewed;         109 AA.
AC   P03493;
DT   21-JUL-1986, integrated into UniProtKB/Swiss-Prot.
DT   10-JAN-2006, sequence version 2.
DT   02-DEC-2020, entry version 82.
DE   RecName: Full=Matrix protein 2;
DE   AltName: Full=BM2;
GN   Name=M;
OS   Influenza B virus (strain B/Lee/1940).
OC   Viruses; Riboviria; Orthornavirae; Negarnaviricota; Polyploviricotina;
OC   Insthoviricetes; Articulavirales; Orthomyxoviridae; Betainfluenzavirus.
OX   NCBI_TaxID=518987;
OH   NCBI_TaxID=9606; Homo sapiens (Human).
RN   [1]
RX   PubMed=6278729; DOI=10.1016/0042-6822(82)90150-7;
RA   Briedis D.J., Lamb R.A., Choppin P.W.;
RT   "Sequence of RNA segment 7 of the influenza B virus genome: partial amino
RT   acid homology between the membrane proteins (M1) of influenza A and B
RT   viruses and conservation of a second open reading frame.";
RL   Virology 116:581-588(1982).
RN   [2]
RX   PubMed=12620792; DOI=10.1016/s0042-6822(02)00083-1;
RA   Paterson R.G., Takeda M., Ohigashi Y., Pinto L.H., Lamb R.A.;
RT   "Influenza B virus BM2 protein is an oligomeric integral membrane protein
RT   expressed at the cell surface.";
RL   Virology 306:7-17(2003).
RN   [3]
RX   PubMed=12852861; DOI=10.1016/s1534-5807(03)00190-4;
RA   Mould J.A., Paterson R.G., Takeda M., Ohigashi Y., Venkataraman P.,
RA   Lamb R.A., Pinto L.H.;
RT   "Influenza B virus BM2 protein has ion channel activity that conducts
RT   protons across membranes.";
RL   Dev. Cell 5:175-184(2003).
RN   [4]
RX   PubMed=19956205; DOI=10.1038/nsmb1209-1207;
RA   Cross T.A.;
RT   "Flu BM2 structure and function.";
RL   Nat. Struct. Mol. Biol. 16:1207-1209(2009).
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:0000269|PubMed:19956205}.
CC   -!- SUBCELLULAR LOCATION: Virion membrane {ECO:0000305}; Single-pass type
CC       III membrane protein {ECO:0000305}. Host cell membrane
CC       {ECO:0000269|PubMed:12620792}; Single-pass type III membrane protein
CC       {ECO:0000269|PubMed:12620792}. Note=Transported to the plasma membrane
CC       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 P03489) 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; J02094; -; NOT_ANNOTATED_CDS; Genomic_RNA.
DR   PIR; A04086; MFIV2.
DR   SMR; P03493; -.
DR   TCDB; 1.A.58.1.1; the type b influenza virus matrix protein 2 (bm2-c) family.
DR   Proteomes; UP000008158; Genome.
DR   Proteomes; UP000119685; 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   1: Evidence at protein level;
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_0000078898"
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   COILED          58..81
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:0000305"
FT   SITE            23
FT                   /note="Seems to be involved in pH gating"
FT                   /evidence="ECO:0000305"
FT   MUTAGEN         19
FT                   /note="H->C: Complete loss of ion channel activity."
FT                   /evidence="ECO:0000269|PubMed:12852861"
FT   MUTAGEN         23
FT                   /note="W->C: Partial loss of ion channel activity."
FT                   /evidence="ECO:0000269|PubMed:12852861"
SQ   SEQUENCE   109 AA;  12522 MW;  13535F33D86120B3 CRC64;
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