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1.A.58.1.1
The Matrix protein BM2 (Pielak and Chou, 2010).  The solution structure for the channel domain of 33 aas is known (PDB# 2KIK) (Wang et al. 2009).  The channel transports H+ and K+ (Hyser and Estes 2015). Like M2, it is a tetrameric pore that acidifies the virion after endocytosis and it has a HxxxW motif (residues 19 - 23) in the single TMS responsible for proton selectivity and gating. This motif is within a 14 aa sequence with 35% identity and 86% similarity with M2 (1.A.19.1.1), both within the C-terminal part of the single TMS, suggesting homology. It also has a second histidine in a WxxxH motif involving the same W. The solvent-accessible His27 facilitates proton conduction of the channel by increasing the proton dissociation rates of His19 (Williams et al. 2017).  

Accession Number:P03493
Protein Name:Matrix protein 2 aka BM2
Length:109
Molecular Weight:12522.00
Species:Influenza B virus (strain B/Lee/1940) [107412]
Number of TMSs:1
Location1 / Topology2 / Orientation3: Virion membrane1 / Single-pass type III membrane protein2
Substrate H+, K+

Cross database links:

Pfam: PF04772   

Gene Ontology

GO:0020002 C:host cell plasma membrane
GO:0016021 C:integral to membrane
GO:0055036 C:virion membrane
GO:0005216 F:ion channel activity
GO:0015992 P:proton transport

References (3)

[1] “Sequence of RNA segment 7 of the influenza B virus genome: partial amino acid homology between the membrane proteins (M1) of influenza A and B viruses and conservation of a second open reading frame.”  Briedis D.J.et.al.   6278729
[2] “Influenza B virus BM2 protein is an oligomeric integral membrane protein expressed at the cell surface.”  Paterson R.G.et.al.   12620792
[3] “Influenza B virus BM2 protein has ion channel activity that conducts protons across membranes.”  Mould J.A.et.al.   12852861

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Predict TMSs (Predict number of transmembrane segments)
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FASTA formatted sequence
1:	MLEPLQILSI CSFILSALHF MAWTIGHLNQ IKRGVNLKIQ IRNPNKEAIN REVSILRHNY 
61:	QKEIQAKETM KKILSDNMEV LGDHIVVEGL STDEIIKMGE TVLEVEELQ