MTIF2 encodes mitochondrial translation initiation factor 2 (mtIF2, IF-2mt), a nuclear-encoded translational GTPase of the IF-2 family that is imported into the mitochondrial matrix via an N-terminal presequence. mtIF2 binds the small (28S) mitoribosomal subunit and the 55S mitoribosome and, in a GTP-dependent reaction, places formylated methionyl-tRNAMet on the start codon of mitochondrial mRNAs. Because mitochondria use a single tRNAMet for initiation and elongation, mtIF2 selects the formylated form, preferring fMet-tRNA over Met-tRNA. A 37-residue insertion absent from bacterial IF2 takes over the role of the missing initiation factor IF1, closes the decoding centre and stabilises leaderless mRNAs on the initiating ribosome. Most mitochondrial mRNAs are leaderless and are initiated on intact 55S monosomes in an mtIF2-dependent manner. Knockout of MTIF2 in human cells abolishes mitochondrial protein synthesis and lowers the levels of mtDNA-encoded OXPHOS subunits.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0001731 formation of translation preinitiation complex | IDA PMID:36629253 Translation initiation of leaderless and polycistronic trans... | MODIFY | Summary: Reconstituted initiation assays and knockout cells showed that mtIF2 is required for fMet-tRNA binding to mitoribosomes and formation of the mitochondrial initiation complex. Reason: The finding is sound, but GO:0001731 formation of translation preinitiation complex is defined as part of cytoplasmic translational initiation and results in a cytosolic translation preinitiation complex. The mitochondrial process, mitochondrial translational initiation, is the correct term. Proposed replacements: mitochondrial translational initiation Supporting Evidence: PMID:36629253 Omission of mtIF2 severely inhibited [3H]-fMet-tRNAfMet binding to monosomes, showing that mtIF2 is crucial for the formation of the mitochondrial initiation complex. |
| GO:0003723 RNA binding | HDA PMID:22658674 Insights into RNA biology from an atlas of mammalian mRNA-bi... | MARK AS OVER ANNOTATED | Summary: MTIF2 was detected in an mRNA interactome capture screen. Reason: mtIF2 contacts initiator tRNA, mRNA and rRNA only within the initiating mitoribosome. That role is captured by translation initiation factor activity; a generic RNA-binding call from a high-throughput crosslinking screen adds no independent function. Supporting Evidence: PMID:30089917 this insertion stabilizes the binding of leaderless mRNAs and induces conformational changes in the rRNA nucleotides involved in decoding |
| GO:0003743 translation initiation factor activity | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT inference of translation initiation factor activity for the IF-2 family. Reason: Core activity, supported by direct experiments on human mtIF2 and on the bovine orthologue. Supporting Evidence: PMID:36629253 Furthermore, we show that mtIF2 is indispensable for mitochondrial protein synthesis. |
| GO:0003743 translation initiation factor activity | IDA PMID:36629253 Translation initiation of leaderless and polycistronic trans... | ACCEPT | Summary: Human mtIF2 was required in vitro for initiator tRNA binding to mitoribosomes, and its knockout in HEK293T cells abolished mitochondrial translation, which was restored by re-expression. Reason: Direct evidence for the core activity of the human protein. Supporting Evidence: PMID:36629253 Omission of mtIF2 severely inhibited [3H]-fMet-tRNAfMet binding to monosomes, showing that mtIF2 is crucial for the formation of the mitochondrial initiation complex. PMID:36629253 The results of the [35S]-methionine/cysteine metabolic labeling indicated a near complete loss of mitochondrial translation in KO clones (Figure 3A and B), which was restored upon expression of mtIF2. |
| GO:0003743 translation initiation factor activity | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro IF-2 signature mapping. Reason: Correct; consistent with experimental evidence. Supporting Evidence: PMID:36629253 Furthermore, we show that mtIF2 is indispensable for mitochondrial protein synthesis. |
| GO:0003924 GTPase activity | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro G-domain mapping of GTPase activity. Reason: mtIF2 is a translational GTPase; the initiation-complex structure identified mtIF2 features that regulate its GTPase activity, and GTP hydrolysis by the bovine factor supports initiation complex formation. Supporting Evidence: PMID:30089917 We identify unique features of mtIF2 that are required for specific recognition of fMet-tRNAMet and regulation of its GTPase activity. |
| GO:0005525 GTP binding | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro G-domain mapping of GTP binding. Reason: GTP binding is part of the mechanism; initiator tRNA binding by IF-2mt is GTP-dependent. Supporting Evidence: PMID:8621449 All of these forms are active in promoting the GTP-dependent binding of formyl-Met-tRNA to the small subunit of either E. coli or bovine mitochondrial ribosomes. |
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: PAINT inference of cytoplasmic localisation from a node of the IF-2/eIF5B family that includes cytosolic eIF5B and bacterial IF2. Reason: Mitochondria are formally part of the cytoplasm in GO, so the statement is not false, but mtIF2 is confined to mitochondria. The is_active_in cytoplasm assertion reflects the cytosolic and bacterial members of the broad family node and is uninformative for the mitochondrial subfamily, which separately carries the correct mitochondrion IBA. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: COMPARTMENT OR COMPLEX MISMATCH GRANULARITY MISMATCH Sources checked: PANTHER:PTN001897651 SUPPORTS SOURCE BUT NOT TARGET Broad IF-2/eIF5B family node whose descendants include cytosolic eIF5B (O60841) and bacterial IF2 (P0A705); cytoplasm fits those members, but for the mitochondrial IF-2 subfamily it only restates mitochondrion at a coarser level. Supporting Evidence: file:human/MTIF2/MTIF2-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion. |
| GO:0005739 mitochondrion | HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an... | ACCEPT | Summary: Detected in the high-confidence human mitochondrial proteome. Reason: Correct. Supporting Evidence: file:human/MTIF2/MTIF2-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion. |
| GO:0005739 mitochondrion | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT inference of mitochondrial localisation for the mitochondrial IF-2 subfamily. Reason: Correct. Supporting Evidence: file:human/MTIF2/MTIF2-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion. |
| GO:0005739 mitochondrion | IDA GO_REF:0000052 | ACCEPT | Summary: Human Protein Atlas immunofluorescence localisation to mitochondria. Reason: Correct. Supporting Evidence: file:human/MTIF2/MTIF2-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion. |
| GO:0005739 mitochondrion | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular location mapping. Reason: Correct. Supporting Evidence: file:human/MTIF2/MTIF2-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion. |
| GO:0005759 mitochondrial matrix | IC PMID:36629253 Translation initiation of leaderless and polycistronic trans... | ACCEPT | Summary: Matrix localisation inferred by curator from the initiation factor activity on mitoribosomes. Reason: Mitoribosomes and mitochondrial initiation complexes are in the matrix. Supporting Evidence: PMID:36629253 Omission of mtIF2 severely inhibited [3H]-fMet-tRNAfMet binding to monosomes, showing that mtIF2 is crucial for the formation of the mitochondrial initiation complex. |
| GO:0005759 mitochondrial matrix | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA prediction of mitochondrial matrix. Reason: Correct compartment. Supporting Evidence: PMID:36629253 Omission of mtIF2 severely inhibited [3H]-fMet-tRNAfMet binding to monosomes |
| GO:0006413 translational initiation | IEA GO_REF:0000120 | MODIFY | Summary: Automated assignment of translational initiation. Reason: Correct but generic; mtIF2 acts only in mitochondrial translation. Proposed replacements: mitochondrial translational initiation Supporting Evidence: PMID:36629253 Furthermore, we show that mtIF2 is indispensable for mitochondrial protein synthesis. |
| GO:0008135 translation factor activity, RNA binding | ISS GO_REF:0000024 | MODIFY | Summary: Transfer from bovine IF-2mt of the general term translation factor activity, RNA binding. Reason: The general term is correct, but the specific activity, translation initiation factor activity, is established for both the bovine and the human factor. Proposed replacements: translation initiation factor activity Supporting Evidence: PMID:15477394 In direct binding experiments, bovine IF-2(mt) has a 25-fold greater affinity for mitochondrial fMet-tRNA than Met-tRNA |
| GO:0008135 translation factor activity, RNA binding | TAS PMID:7829522 Cloning and sequence analysis of the human mitochondrial tra... | MODIFY | Summary: Statement from the paper reporting the cloning of human IF-2mt cDNA. Reason: The cloning paper describes the human orthologue of the initiation factor; the specific activity term translation initiation factor activity is more informative than the general translation factor term. Proposed replacements: translation initiation factor activity Supporting Evidence: PMID:7829522 Overall, human IF-2mt has 30-40% identity to the corresponding prokaryotic factors. |
| GO:0032790 ribosome disassembly | ISS GO_REF:0000024 | UNDECIDED | Summary: Ribosome disassembly (ISS), inferred from an experimentally annotated mitochondrial IF-2; the source abstract (PMID:12095986) attributes subunit dissociation to IF3(mt), and the full text is not available. Reason: The row is inferred by sequence similarity from a mitochondrial IF-2 that carries the term as a direct-assay annotation from the study that identified IF3(mt) and examined initiation complex formation on mitochondrial ribosomes in the presence of IF2(mt) (PMID:12095986). The abstract of that study attributes the shift toward subunit dissociation to IF3(mt), but only the abstract is available, so whether the full text shows a contribution of IF2(mt) cannot be checked. An experimentally based annotation is not treated as a mis-attribution between the two factors on the strength of an abstract, and the row is left undecided rather than removed. Supporting Evidence: PMID:12095986 IF3(mt) shifts the equilibrium between the 55 S mitochondrial ribosome and its subunits toward subunit dissociation. |
| GO:0043024 ribosomal small subunit binding | ISS GO_REF:0000024 | MODIFY | Summary: Transfer of ribosomal small subunit binding from bovine IF-2mt. Reason: Bovine IF-2mt binds the 28S small subunit of the mitochondrial ribosome with nanomolar affinity. The child term mitochondrial small ribosomal subunit binding states this precisely. Proposed replacements: mitochondrial small ribosomal subunit binding Supporting Evidence: PMID:15935986 The binding of wild-type IF-2(mt) was strong (K(d) approximately 10-20 nM) and was not affected by fMet-tRNA. |
| GO:0070124 mitochondrial translational initiation | IBA GO_REF:0000033 | ACCEPT | Summary: PAINT inference of mitochondrial translational initiation. Reason: Core process. Supporting Evidence: PMID:36629253 Furthermore, we show that mtIF2 is indispensable for mitochondrial protein synthesis. |
| GO:0070124 mitochondrial translational initiation | ISS GO_REF:0000024 | ACCEPT | Summary: Transfer from bovine IF-2mt. Reason: Core process; supported directly for the human protein. Supporting Evidence: PMID:36629253 Omission of mtIF2 severely inhibited [3H]-fMet-tRNAfMet binding to monosomes, showing that mtIF2 is crucial for the formation of the mitochondrial initiation complex. |
| GO:0070124 mitochondrial translational initiation | TAS PMID:7829522 Cloning and sequence analysis of the human mitochondrial tra... | ACCEPT | Summary: Statement in the human IF-2mt cDNA cloning paper. Reason: Correct core process. Supporting Evidence: PMID:7829522 Complete cDNAs encoding human mitochondrial translational initiation factor 2 |
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Download this section (compressed HTML)Q: The bovine IF-2mt (P46198) IDA annotations to ribosome disassembly, small subunit binding and mitochondrial translational initiation cite PMID:12095986, a paper whose abstract attributes subunit dissociation to IF3mt. Were these annotations intended for bovine or human IF3mt?
Q: Does mtIF2, which lacks the N-terminal domains of bacterial IF2, contribute to joining of the 39S subunit on the minority of transcripts initiated on the 28S subunit?
Experiment: Single-molecule FRET or stopped-flow subunit-joining assays with human 28S and 39S subunits, leadered and leaderless mRNAs, and wild-type or insertion-domain-deleted mtIF2, to test whether mtIF2 has a subunit-association activity.
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