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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). The membrane environment is an important factor influencing the conformation and hydration of BM2 (Zhang et al. 2020).  

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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FASTA formatted sequence
1:	MLEPLQILSI CSFILSALHF MAWTIGHLNQ IKRGVNLKIQ IRNPNKEAIN REVSILRHNY 
61:	QKEIQAKETM KKILSDNMEV LGDHIVVEGL STDEIIKMGE TVLEVEELQ