Matrix protein 2 (M2) is a 97-amino acid transmembrane protein that forms a pH-activated, proton-selective ion channel essential for influenza A virus replication. The protein assembles as a homotetramer and functions in two critical stages of the viral life cycle: during viral entry, M2 acidifies the virion interior to facilitate genome release from the viral ribonucleoprotein complex, and during viral maturation, M2 maintains neutral pH in the trans-Golgi network to prevent premature conformational changes of hemagglutinin. The channel is selectively permeable to protons and is activated by low pH (~5.5-6.0). M2 is a validated antiviral drug target, though resistance to adamantane drugs is now widespread.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005216 monoatomic ion channel activity | IEA GO_REF:0000104 | MODIFY | Summary: This broad term accurately captures M2 function as an ion channel, though it is less specific than the proton channel activity. M2 forms a proton-selective channel that is well-characterized structurally and functionally. While accurate, more specific child terms like proton channel activity (GO:0015252) would be more informative. Reason: While technically correct, this term is too general for a well-characterized proton-selective channel. The M2 channel is highly selective for protons over other monovalent cations, with extensive structural and functional evidence supporting its specific proton conductance mechanism. Proposed replacements: proton channel activity Supporting Evidence: UniProt:A0A1S7IWC7 Forms a proton-selective ion channel that is necessary for the efficient release of the viral genome during virus entry |
| GO:0006811 monoatomic ion transport | IEA GO_REF:0000043 | MODIFY | Summary: This general biological process term is accurate but non-specific. M2 specifically mediates proton transport across membranes during viral entry and maturation. The more specific term proton transmembrane transport (GO:1902600) better captures the actual function. Reason: The term is too general for a protein with well-characterized proton-specific transport function. M2 exclusively transports protons, not other monoatomic ions, and this specificity is crucial for its viral functions. Proposed replacements: proton transmembrane transport Supporting Evidence: UniProt:A0A1S7IWC7 The influx of protons into virion interior is believed to disrupt interactions between the viral ribonucleoprotein (RNP), matrix protein 1 (M1), and lipid bilayers |
| GO:0015078 proton transmembrane transporter activity | IEA GO_REF:0000120 | MODIFY | Summary: This molecular function term accurately describes M2 activity. M2 is a well-characterized proton transporter that mediates pH-activated proton flux across membranes. However, the more specific child term proton channel activity (GO:0015252) would better reflect that M2 functions as a channel rather than a carrier or pump. Reason: While accurate, this parent term encompasses multiple mechanisms of proton transport including channels, pumps, and carriers. M2 specifically functions as a proton channel with passive, pH-gated transport, making the more specific proton channel activity term more appropriate. Proposed replacements: proton channel activity Supporting Evidence: UniProt:A0A1S7IWC7 Forms a proton-selective ion channel that is necessary for the efficient release of the viral genome during virus entry |
| GO:0015267 channel activity | IEA GO_REF:0000043 | MODIFY | Summary: This general channel activity term is accurate but lacks specificity. M2 is not just any channel but specifically a proton-selective channel with well-defined selectivity and gating properties. The annotation should reflect this specificity. Reason: Too general for a well-characterized proton channel. M2 has been extensively studied structurally and functionally as a proton-selective channel, and this specificity is essential for its viral functions. Proposed replacements: proton channel activity Supporting Evidence: UniProt:A0A1S7IWC7 Forms a proton-selective ion channel |
| GO:0016020 membrane | IEA GO_REF:0000104 | MODIFY | Summary: This very general cellular component term is accurate but uninformative. M2 localizes to specific membranes including the virion membrane and host cell plasma membrane, particularly at the apical surface of polarized epithelial cells. More specific terms are available and should be used. Reason: Too general for a protein with well-defined membrane localizations. M2 has specific localizations to virion membrane and host plasma membrane that are functionally important for viral replication. Proposed replacements: virion membrane host cell plasma membrane Supporting Evidence: UniProt:A0A1S7IWC7 SUBCELLULAR LOCATION: Virion membrane |
| GO:0020002 host cell plasma membrane | IEA GO_REF:0000120 | ACCEPT | Summary: This cellular component term accurately describes one of M2 key localizations. M2 is abundantly expressed at the apical plasma membrane of infected polarized epithelial cells, where it plays a role in preventing premature hemagglutinin activation and potentially in viral budding. Reason: Well-supported localization that is functionally important. M2 localizes to the host cell plasma membrane, particularly the apical surface, where it maintains pH homeostasis during viral protein trafficking and assembly. Supporting Evidence: UniProt:A0A1S7IWC7 Abundantly expressed at the apical plasma membrane in infected polarized epithelial cells, in close proximity to budding and assembled virions |
| GO:0033644 host cell membrane | IEA GO_REF:0000120 | REMOVE | Summary: This term is redundant with the more specific host cell plasma membrane (GO:0020002) annotation. While accurate, it adds no additional information and the more specific term should be preferred. Reason: Redundant with the more specific GO:0020002 (host cell plasma membrane) annotation. The specific plasma membrane localization is more informative than this general host cell membrane term. Supporting Evidence: UniProt:A0A1S7IWC7 Host apical cell membrane |
| GO:0034220 monoatomic ion transmembrane transport | IEA GO_REF:0000043 | MODIFY | Summary: This biological process term is accurate but too general. M2 specifically mediates proton transmembrane transport, not transport of other monoatomic ions. The specificity for protons is essential for M2 function and should be reflected in the annotation. Reason: While technically correct, this term is too broad for a proton-specific channel. M2 selectivity for protons over other ions is a defining characteristic that enables its viral functions. Proposed replacements: proton transmembrane transport Supporting Evidence: UniProt:A0A1S7IWC7 Acidification of the endosome triggers M2 ion channel activity. The influx of protons into virion interior |
| GO:0044423 virion component | IEA GO_REF:0000043 | MODIFY | Summary: This cellular component term is accurate but could be more specific. M2 is specifically a component of the virion membrane, not just a general virion component. The more specific virion membrane (GO:0055036) term would be more informative. Reason: While accurate, the more specific virion membrane localization better describes M2 incorporation into virions as a transmembrane protein in the viral envelope. Proposed replacements: virion membrane Supporting Evidence: file:9INFA/M2/M2-uniprot.txt Minor component of virions (only 16-20 molecules/virion). |
| GO:0044694 symbiont genome entry into host cell via pore formation in plasma membrane | IEA GO_REF:0000104 | REMOVE | Summary: This biological process term is misleading for M2 function. M2 does not form pores in the plasma membrane for genome entry. Instead, M2 acidifies the virion interior within endosomes after receptor-mediated endocytosis, facilitating genome release from the RNP complex. The virus enters via endocytosis, not plasma membrane pore formation. Reason: Incorrect mechanism. Influenza virus enters cells through receptor-mediated endocytosis, not via pore formation in the plasma membrane. M2 functions within endosomes after viral uptake to acidify the virion interior, not to create entry pores in the plasma membrane. Supporting Evidence: UniProt:A0A1S7IWC7 After attaching to the cell surface, the virion enters the cell by endocytosis. Acidification of the endosome triggers M2 ion channel activity |
| GO:0051259 protein complex oligomerization | IEA GO_REF:0000104 | ACCEPT | Summary: This biological process term accurately describes M2 assembly into homotetramers. M2 forms tetramers composed of two disulfide-linked dimers, and this oligomerization is essential for channel function. The tetrameric structure creates the central pore for proton conduction. Reason: Well-supported core function. M2 homotetramerization is essential for forming the functional proton channel. The tetrameric assembly has been confirmed by multiple structural studies and is required for channel activity. Supporting Evidence: UniProt:A0A1S7IWC7 Homotetramer; composed of two disulfide-linked dimers held together by non-covalent interactions |
| GO:0055036 virion membrane | IEA GO_REF:0000120 | ACCEPT | Summary: This cellular component term accurately describes M2 localization in mature virions. M2 is incorporated into the viral envelope as a minor but essential component (16-20 molecules per virion). This localization is critical for M2 function during viral entry. Reason: Core localization essential for viral entry function. M2 incorporation into the virion membrane enables its pH-activated channel activity during endosomal acidification to facilitate genome release. Supporting Evidence: file:9INFA/M2/M2-uniprot.txt Minor component of virions (only 16-20 molecules/virion). |
| GO:0140321 symbiont-mediated suppression of host autophagy | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: This biological process annotation appears to be based on limited evidence. While some studies suggest influenza proteins can modulate autophagy, this is not a well-established core function of M2. The primary functions of M2 are proton channel activity for viral entry and pH regulation during viral maturation. Reason: Potential secondary function but not well-established as a core M2 activity. The primary and well-characterized functions of M2 are proton channel activity during viral entry and pH maintenance during viral protein trafficking. Any autophagy modulation would be a secondary effect. Supporting Evidence: UniProt:A0A1S7IWC7 Inhibition of host autophagy by virus |
| GO:1902600 proton transmembrane transport | IEA GO_REF:0000120 | ACCEPT | Summary: This biological process term accurately and specifically describes M2 core function. M2 mediates pH-activated proton transport across membranes, which is essential for both viral entry (acidifying virion interior) and maturation (maintaining Golgi pH). This is the most appropriate process term for M2. Reason: Core function with appropriate specificity. This term correctly captures M2 primary activity as a proton-selective channel that mediates transmembrane proton flux during viral entry and maturation. Supporting Evidence: UniProt:A0A1S7IWC7 The influx of protons into virion interior is believed to disrupt interactions between the viral ribonucleoprotein (RNP), matrix protein 1 (M1), and lipid bilayers |
| GO:0046761 viral budding from plasma membrane | IC | NEW | Summary: PROPOSED NEW: M2 amphipathic helix induces membrane curvature and scission during viral budding. This critical function is well-documented but not in current GOA annotations. Supporting Evidence: file:9INFA/M2/M2-deep-research.md M2 localizes to the budding neck and its amphipathic helix inserts into the inner leaflet, deforming the membrane to pinch off the virion. This M2-mediated membrane scission is unique to influenza (an ESCRT-independent budding mechanism). |
| GO:0061906 autophagosome localization | IC | NEW | Summary: PROPOSED NEW: M2 interacts with LC3 to redirect autophagosomes to plasma membrane. This host manipulation function is well-documented but not in current GOA annotations. Supporting Evidence: file:9INFA/M2/M2-deep-research.md M2 interacts with LC3 (a marker on autophagosome membranes), and this interaction was shown to redistribute autophagosomes to the plasma membrane where virus buds. M2-mediated relocalization of autophagosome membranes may supply additional membrane for virion budding. |
| GO:0005515 protein binding | IC | NEW | Summary: PROPOSED NEW: M2 binds M1 matrix protein to recruit it for viral assembly. This essential interaction is well-documented but not in current GOA annotations. Proposed replacements: viral matrix protein binding Supporting Evidence: file:9INFA/M2/M2-deep-research.md M2 strongly interacts with M1, the matrix protein lining the inner virion shell. Fluorescence microscopy studies in infected cells show that M2 actively recruits M1 to the membrane, initiating assembly of virion buds. |
| GO:0099050 vesicle scission | IC | NEW | Summary: PROPOSED NEW: M2 amphipathic helix mediates membrane scission during viral budding. This membrane remodeling function is well-documented but not in current GOA annotations. Proposed replacements: viral membrane scission Supporting Evidence: file:9INFA/M2/M2-deep-research.md Following recruitment, M2 amphipathic helix in its cytoplasmic tail induces membrane curvature and scission during the final step of budding. |
| GO:1900227 positive regulation of NLRP3 inflammasome complex assembly | IC | NEW | Summary: PROPOSED NEW: M2 ion channel activity activates NLRP3 inflammasome. This host immune modulation is well-documented but not in current GOA annotations. Supporting Evidence: file:9INFA/M2/M2-deep-research.md The ion channel activity of M2 in the Golgi and endosomes can inadvertently activate the NLRP3 inflammasome, triggering pro-inflammatory cytokine responses. |
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