id: Q23424 gene_symbol: fzo-1 product_type: PROTEIN status: IN_PROGRESS taxon: id: NCBITaxon:6239 label: Caenorhabditis elegans description: FZO-1 is the sole C. elegans mitofusin ortholog, a transmembrane GTPase of the TRAFAC-class dynamin-like superfamily that localizes to the mitochondrial outer membrane and mediates outer membrane fusion. FZO-1 functions in a conserved pathway with EAT-3 (OPA1 ortholog) which mediates inner membrane fusion; both are required for complete mitochondrial fusion. The balance between fusion (FZO-1, EAT-3) and fission (DRP-1) determines mitochondrial morphology. Loss of fzo-1 causes constitutive mitochondrial fragmentation, disrupts cristae organization, activates the mitochondrial unfolded protein response (UPRmt), and alters stress physiology. FZO-1 interacts with the BCL-2 homolog CED-9, which can promote mitochondrial fusion by interacting with FZO-1. FZO-1 is dispensable for apoptosis activation but loss of fusion affects mitochondrial dynamics during development and stress responses. existing_annotations: - term: id: GO:0003924 label: GTPase activity evidence_type: IBA original_reference_id: GO_REF:0000033 review: summary: FZO-1 is a transmembrane GTPase of the TRAFAC-class dynamin-like superfamily. UniProt assigns EC 3.6.5.- and specifies GTP hydrolysis activity catalyzing GTP + H2O = GDP + phosphate + H(+). The protein contains a conserved Dynamin-type G domain (residues 97-352) with characteristic G1-G5 motifs and GTP-binding regions. GTPase activity is essential for mitofusin-mediated membrane fusion, powering conformational changes needed for membrane tethering and fusion. action: ACCEPT reason: Core molecular function of FZO-1. The dynamin-like GTPase activity is well-established for mitofusins across species, with conserved domain architecture including the P-loop NTPase domain. UniProt documents GTP binding sites and the catalytic reaction. IBA annotation is appropriately supported by phylogenetic inference from characterized orthologs including yeast Fzo1 (SGD:S000000383) and human MFN2 (UniProtKB:Q8IWA4). supported_by: - reference_id: UniProtKB:Q23424 supporting_text: Belongs to the TRAFAC class dynamin-like GTPase superfamily. Dynamin/Fzo/YdjA family. Mitofusin subfamily. - reference_id: UniProtKB:Q23424 supporting_text: Reaction=GTP + H2O = GDP + phosphate + H(+); Xref=Rhea:RHEA:19669 - reference_id: file:worm/fzo-1/fzo-1-deep-research-falcon.md supporting_text: FZO-1 is a single-pass transmembrane protein with an N-terminal GTPase domain located in the cytoplasm. GTP hydrolysis by the conserved P-loop NTPase domain powers conformational changes needed for membrane tethering/fusion. - term: id: GO:0008053 label: mitochondrial fusion evidence_type: IBA original_reference_id: GO_REF:0000033 review: summary: FZO-1 mediates mitochondrial outer membrane fusion as the sole C. elegans mitofusin ortholog. Loss of fzo-1 causes constitutive mitochondrial fragmentation due to unopposed fission. FZO-1 works in coordination with EAT-3 (OPA1 ortholog) which mediates inner membrane fusion; both are required for complete mitochondrial fusion. action: ACCEPT reason: This is the core biological process function of FZO-1. The IBA annotation is well-supported by phylogenetic inference from characterized mitofusins. Multiple primary literature sources in C. elegans confirm this function directly, including RNAi studies showing fragmented mitochondria in fzo-1 mutants. PMID:18722182 describes fzo-1 as a "profusion gene" required for mitochondrial fusion. PMID:19327994 shows "in a fzo-1 mutant, in which mitochondrial fission occurs but mitochondrial fusion is restricted." supported_by: - reference_id: PMID:18722182 supporting_text: profusion genes fzo-1 and eat-3 or the profission gene drp-1 are not required for apoptosis activation in C. elegans - reference_id: PMID:19327994 supporting_text: in a fzo-1 mutant, in which mitochondrial fission occurs but mitochondrial fusion is restricted - term: id: GO:0005741 label: mitochondrial outer membrane evidence_type: IBA original_reference_id: GO_REF:0000033 review: summary: FZO-1 is an integral mitochondrial outer membrane protein with two transmembrane helices (residues 618-638 and 641-661). The N-terminal GTPase domain and C-terminal region are cytoplasmic, with a short intermembrane space segment. This localization is essential for its function in mediating outer membrane fusion. action: ACCEPT reason: Well-supported cellular component annotation. UniProt documents the transmembrane topology with two helical transmembrane regions and cytoplasmic domains. The IBA annotation is supported by orthology to characterized mitofusins. This is consistent with its function in outer membrane fusion. supported_by: - reference_id: UniProtKB:Q23424 supporting_text: 'SUBCELLULAR LOCATION: Mitochondrion outer membrane {ECO:0000250|UniProtKB:Q8IWA4}; Multi-pass membrane protein' - term: id: GO:0051646 label: mitochondrion localization evidence_type: IBA original_reference_id: GO_REF:0000033 review: summary: Mitofusins including FZO-1 have been implicated in regulating mitochondrial distribution and localization through their role in fusion dynamics. However, the primary function of FZO-1 is membrane fusion itself, not the positioning or transport of mitochondria within cells. action: KEEP_AS_NON_CORE reason: While mitofusins can affect mitochondrial distribution as a consequence of fusion/fission balance, this is not a primary function of FZO-1. The core function is membrane fusion. Mitochondrion localization is more directly regulated by motor proteins and cytoskeletal interactions. The IBA annotation is based on phylogenetic inference, but for FZO-1 specifically, the primary evidence supports fusion rather than localization/transport. This annotation should be kept but marked as non-core since it may represent a secondary consequence of fusion activity rather than a primary function. supported_by: - reference_id: PMID:21248201 supporting_text: Fission, fusion, and cytoskeletal attachment control the connectivity and distribution of mitochondria. - term: id: GO:0000166 label: nucleotide binding evidence_type: IEA original_reference_id: GO_REF:0000043 review: summary: FZO-1 binds GTP through its conserved Dynamin-type G domain. This is a general parent term that encompasses GTP binding, which is the specific nucleotide bound by FZO-1. action: ACCEPT reason: This IEA annotation from UniProtKB keyword mapping is correct but redundant with the more specific GTP binding annotation. The protein does bind nucleotides (specifically GTP) via its G1-G5 motifs and GTP-binding regions documented in UniProt. Accept as a broader term that is implied by the more specific GTP binding function. supported_by: - reference_id: UniProtKB:Q23424 supporting_text: GTP-binding; Hydrolase; Membrane; Mitochondrion; Mitochondrion outer membrane; Nucleotide-binding - term: id: GO:0003924 label: GTPase activity evidence_type: IEA original_reference_id: GO_REF:0000120 review: summary: Duplicate annotation of GTPase activity from InterPro domain mapping. FZO-1 contains the Fzo/mitofusin HR2 domain (IPR006884) and Mitofusin family domain (IPR027094) which are associated with GTPase activity. action: ACCEPT reason: This IEA annotation from InterPro/UniRule mapping is consistent with the IBA annotation for the same term. The evidence from domain analysis supports the GTPase activity. Duplicate annotations with different evidence codes are acceptable and reflect independent lines of evidence. supported_by: - reference_id: UniProtKB:Q23424 supporting_text: InterPro; IPR006884; Fzo/mitofusin_HR2 - term: id: GO:0005525 label: GTP binding evidence_type: IEA original_reference_id: GO_REF:0000120 review: summary: FZO-1 binds GTP through its Dynamin-type G domain containing conserved G1-G5 motifs. Three specific GTP-binding sites are documented in UniProt at positions 110-115, 270-273, and 317. action: ACCEPT reason: Correct molecular function annotation. GTP binding is essential for the GTPase activity and fusion function of FZO-1. The IEA annotation from InterPro/keyword mapping is well-supported by the documented domain architecture and binding sites in UniProt. supported_by: - reference_id: UniProtKB:Q23424 supporting_text: DOMAIN 97..352 /note="Dynamin-type G" /evidence="ECO:0000255|PROSITE-ProRule:PRU01055" - term: id: GO:0005741 label: mitochondrial outer membrane evidence_type: IEA original_reference_id: GO_REF:0000120 review: summary: Duplicate annotation of mitochondrial outer membrane localization from InterPro/UniRule mapping based on the Fzo/mitofusin domain. action: ACCEPT reason: Consistent with the IBA annotation for the same term. The IEA evidence from domain analysis independently supports the outer membrane localization. Both annotations are valid and reflect different evidence sources. supported_by: - reference_id: UniProtKB:Q23424 supporting_text: 'SUBCELLULAR LOCATION: Mitochondrion outer membrane {ECO:0000250|UniProtKB:Q8IWA4}; Multi-pass membrane protein' - term: id: GO:0007005 label: mitochondrion organization evidence_type: IEA original_reference_id: GO_REF:0000117 review: summary: FZO-1 is required for proper mitochondrial organization through its role in membrane fusion. Loss of fzo-1 causes fragmented mitochondria and disrupted cristae organization. action: ACCEPT reason: This is a high-level biological process term that accurately captures FZO-1's role. Mitochondrial fusion is a key aspect of mitochondrion organization, and fzo-1 mutants show clear defects in mitochondrial network organization. The IEA annotation from ARBA machine learning is consistent with experimental evidence. This term is a parent of mitochondrial fusion (GO:0008053), so the annotation is logically consistent. supported_by: - reference_id: UniProtKB:Q23424 supporting_text: RNAi-mediated knockdown results in fragmented mitochondria, most likely due to lack of fusion activity - term: id: GO:0008053 label: mitochondrial fusion evidence_type: IEA original_reference_id: GO_REF:0000002 review: summary: Duplicate annotation of mitochondrial fusion from InterPro domain mapping based on the Fzo/mitofusin_HR2 domain (IPR006884). action: ACCEPT reason: Consistent with IBA and IMP annotations for the same term. The IEA evidence from InterPro domain analysis independently supports the fusion function. Multiple evidence codes for the same term are acceptable and reflect independent lines of evidence. supported_by: - reference_id: UniProtKB:Q23424 supporting_text: Mediates mitochondrial fusion (PubMed:18722182, PubMed:19327994, PubMed:21248201, PubMed:25190516, PubMed:33734301) - term: id: GO:0016020 label: membrane evidence_type: IEA original_reference_id: GO_REF:0000002 review: summary: FZO-1 is a membrane protein localized to the mitochondrial outer membrane. This is a very general cellular component term. action: ACCEPT reason: While this is a very general term, it is technically correct. FZO-1 is an integral membrane protein with two transmembrane helices. The more specific annotation to mitochondrial outer membrane (GO:0005741) is also present. IEA annotations to general terms are acceptable when more specific terms are also annotated. supported_by: - reference_id: UniProtKB:Q23424 supporting_text: TRANSMEM 618..638 /note="Helical; Name=1" /evidence="ECO:0000255" - term: id: GO:0016787 label: hydrolase activity evidence_type: IEA original_reference_id: GO_REF:0000043 review: summary: FZO-1 has GTPase activity, which is a type of hydrolase activity (hydrolyzes GTP to GDP + phosphate). This is a general parent term. action: ACCEPT reason: Correct but very general annotation. GTPase activity is a child term of hydrolase activity, so this annotation is implied by the more specific GTPase activity annotations. The IEA from UniProtKB keyword mapping is technically correct. supported_by: - reference_id: UniProtKB:Q23424 supporting_text: 'CATALYTIC ACTIVITY: Reaction=GTP + H2O = GDP + phosphate + H(+); Xref=Rhea:RHEA:19669' - term: id: GO:0008053 label: mitochondrial fusion evidence_type: IMP original_reference_id: PMID:18722182 review: summary: This IMP annotation is based on Breckenridge et al. (2008) which studied the roles of mitochondrial dynamics genes in apoptosis. The paper describes fzo-1 as a "profusion gene" and demonstrates that loss of fzo-1 leads to fragmented mitochondria, confirming its role in mitochondrial fusion. action: ACCEPT reason: This is the most direct experimental evidence for FZO-1's role in mitochondrial fusion in C. elegans. PMID:18722182 explicitly identifies fzo-1 as a profusion gene. The paper shows that "profusion genes fzo-1 and eat-3 or the profission gene drp-1 are not required for apoptosis activation in C. elegans," but confirms their established roles in mitochondrial dynamics. Additional evidence from PMID:19327994 describes "a fzo-1 mutant, in which mitochondrial fission occurs but mitochondrial fusion is restricted." supported_by: - reference_id: PMID:18722182 supporting_text: The dynamin family of GTPases regulate mitochondrial fission and fusion processes and have been implicated in controlling the release of caspase activators from mitochondria during apoptosis. - reference_id: PMID:18722182 supporting_text: Here we report that profusion genes fzo-1 and eat-3 or the profission gene drp-1 are not required for apoptosis activation in C. elegans. - reference_id: UniProtKB:Q23424 supporting_text: Mediates mitochondrial fusion (PubMed:18722182, PubMed:19327994, PubMed:21248201, PubMed:25190516, PubMed:33734301) references: - id: GO_REF:0000002 title: Gene Ontology annotation through association of InterPro records with GO terms findings: [] - id: GO_REF:0000033 title: Annotation inferences using phylogenetic trees findings: - statement: Phylogenetic analysis supports FZO-1 as ortholog of yeast Fzo1 and mammalian MFN1/MFN2 with conserved function in mitochondrial outer membrane fusion - id: GO_REF:0000043 title: Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword mapping findings: [] - id: GO_REF:0000117 title: Electronic Gene Ontology annotations created by ARBA machine learning models findings: [] - id: GO_REF:0000120 title: Combined Automated Annotation using Multiple IEA Methods findings: [] - id: PMID:18722182 title: Caenorhabditis elegans drp-1 and fis-2 regulate distinct cell-death execution pathways downstream of ced-3 and independent of ced-9. findings: - statement: fzo-1 is identified as a profusion gene in C. elegans supporting_text: profusion genes fzo-1 and eat-3 or the profission gene drp-1 are not required for apoptosis activation in C. elegans - statement: profusion genes fzo-1 and eat-3 are not required for apoptosis activation supporting_text: Here we report that profusion genes fzo-1 and eat-3 or the profission gene drp-1 are not required for apoptosis activation in C. elegans. - statement: fzo-1 is dispensable for normal apoptotic processes during embryonic development supporting_text: Our findings demonstrate that mitochondria dynamics do not regulate apoptosis activation in C. elegans - id: PMID:19327994 title: Bcl-2 proteins EGL-1 and CED-9 do not regulate mitochondrial fission or fusion in Caenorhabditis elegans. findings: - statement: In fzo-1 mutant, mitochondrial fission occurs but fusion is restricted supporting_text: in a fzo-1 mutant, in which mitochondrial fission occurs but mitochondrial fusion is restricted - statement: Loss of ced-9 does not affect mitochondrial morphology in fzo-1 mutant supporting_text: loss of ced-9 did not affect the mitochondrial morphology observed in a drp-1 mutant, in which mitochondrial fusion occurs but mitochondrial fission is defective, or in a fzo-1 mutant - statement: Confirms fzo-1 role in mitochondrial fusion supporting_text: in a fzo-1 mutant, in which mitochondrial fission occurs but mitochondrial fusion is restricted - id: PMID:21248201 title: A novel mitochondrial outer membrane protein, MOMA-1, that affects cristae morphology in Caenorhabditis elegans. findings: - statement: fzo-1 was used in epistatic analysis of mitochondrial fission supporting_text: we knocked down mitochondrial fusion genes (fzo-1 and eat-3) in these backgrounds and determined their epistatic relationship - statement: Study confirms fzo-1 promotes fusion opposite to fission mutants supporting_text: moma-1 mutants that were grown on drp-1 RNAi gave rise to mitochondria that were indistinguishable from wild-type animals grown on drp-1 RNAi - id: PMID:21949250 title: A molecular switch that governs mitochondrial fusion and fission mediated by the BCL2-like protein CED-9 of Caenorhabditis elegans. findings: - statement: CED-9 physically interacts with FZO-1 supporting_text: The BCL2-like protein CED-9 of Caenorhabditis elegans has previously been shown to promote mitochondrial fusion by physically interacting with the mitochondrial fusion protein FZO-1 - statement: CED-9 promotes mitochondrial fusion by interacting with FZO-1 supporting_text: CED-9 of Caenorhabditis elegans has previously been shown to promote mitochondrial fusion by physically interacting with the mitochondrial fusion protein FZO-1 - statement: EGL-1 shifts CED-9 from profusion to profission supporting_text: EGL-1 converts CED-9 into a mitochondrial receptor for DRP-1, thereby shifting its activity from profusion to profission - id: PMID:33734301 title: Autophagy facilitates mitochondrial rebuilding after acute heat stress via a DRP-1-dependent process. findings: - statement: fzo-1 depletion causes fragmented mitochondria and interacts with drp-1 supporting_text: "The depletion of fzo-1 results in the accumulation of fragmented\ \ mitochondria in the epidermis of animals grown at 20\xB0C and partially restores\ \ some tubular mitochondria in the drp-1 mutant animals" core_functions: - description: Mitochondrial outer membrane fusion GTPase. FZO-1 is the sole C. elegans mitofusin that mediates tethering and fusion of mitochondrial outer membranes. GTP hydrolysis by the dynamin-like GTPase domain powers conformational changes required for membrane fusion. molecular_function: id: GO:0003924 label: GTPase activity directly_involved_in: - id: GO:0008053 label: mitochondrial fusion locations: - id: GO:0005741 label: mitochondrial outer membrane proposed_new_terms: [] suggested_questions: - question: What is the specific mechanism by which CED-9 regulates FZO-1 activity, and how does EGL-1 modulate this interaction? PMID:21949250 reports that CED-9 interacts with FZO-1 to promote fusion, and EGL-1 shifts CED-9 activity toward profission by enhancing DRP-1 recruitment. The molecular details of how CED-9 binding activates FZO-1 are not fully understood. - question: Does FZO-1 directly interact with SLC-25A46, or does SLC-25A46 regulate fusion through indirect mechanisms? Recent work (Obinata 2024) shows SLC-25A46 acts upstream of FZO-1 to promote fusion and FZO-1 overexpression can rescue slc-25a46 defects. The nature of this functional relationship needs clarification. suggested_experiments: - description: Direct biochemical characterization of FZO-1 GTPase activity using purified protein and GTP hydrolysis assays. Goal is to confirm GTPase activity experimentally in C. elegans FZO-1 (currently inferred by similarity to mammalian MFN2). - description: Live imaging of mitochondrial fusion events in C. elegans to capture FZO-1-dependent outer membrane fusion. Goal is to provide direct visual evidence of FZO-1-mediated fusion events rather than inferring from steady-state morphology. tags: - caeel-mitophagy