SMIM26 is a 95-amino-acid mitochondrial microprotein encoded by a transcript that was originally annotated as the long non-coding RNA LINC00493. It has a single predicted transmembrane helix near its N terminus and is anchored in the mitochondrial outer membrane, with the bulk of its soluble sequence reported to span the intermembrane space. SMIM26 has no catalytic or transport activity of its own; it acts as a tether that holds mitochondrial metabolite carriers of the two membranes in proximity. Reported partners include the outer-membrane porins VDAC1 and VDAC2, the inner-membrane ADP/ATP translocase SLC25A6 (ANT3), the 2-oxoglutarate/malate carrier SLC25A11, the serine transporters SFXN1 and SFXN2, and the TIM22-complex lipid kinase AGK. Loss of SMIM26 leaves cultured cells viable but strongly reduces respiratory chain activity; knockout mice die at early developmental stages and homozygous zebrafish mutants show reduced fitness, survival and respiration. A separate line of work places SMIM26 in a complex with the sideroflexin serine transporters and the mitoribosome, where it is proposed to support localized translation of the complex I subunit mt-ND5; whether SMIM26 also has an inner-membrane, matrix-facing pool that could account for direct mitoribosome contact is unresolved, because a single-pass outer-membrane protein spanning the intermembrane space cannot reach the matrix face of the inner membrane. The protein is phosphorylated at several residues of unassigned function, and its loss has been linked to progression of clear cell renal cell carcinoma.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005739 mitochondrion | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic assertion that SMIM26 acts in the mitochondrion, which matches every experimental study of this protein. Reason: Correct, though less informative than the mitochondrial outer membrane (GO:0005741) annotations this gene also carries. |
| GO:0005741 mitochondrial outer membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic transfer of the UniProt subcellular location, which is itself grounded in two experimental papers. Reason: Correct and specific primary location. |
| GO:0005515 protein binding | IPI PMID:37009826 LINC00493-encoded microprotein SMIM26 exerts anti-metastatic... | REMOVE | Summary: Bare protein binding carries no functional information. Reason: No more informative molecular function is supportable from this evidence, so the row is removed rather than rewritten. Here the interactions in question (AGK, SLC25A11) are already captured by the transmembrane transporter binding and kinase binding annotations, and mechanistically by the adaptor activity term. Removal does not imply the reported interaction is false. |
| GO:0005741 mitochondrial outer membrane | IDA PMID:41991342 The microprotein SMIM26 connects metabolite transporters of ... | ACCEPT | Summary: Direct localisation of SMIM26 to the outer mitochondrial membrane, with the protein reported to extend across the intermembrane space to contact an inner membrane carrier. Reason: Correct and specific primary location, agreeing with two earlier studies. Supporting Evidence: PMID:41991342 In biochemical and single-molecule tracking studies, we found that SMIM26 interacts with VDAC1/2 in the outer mitochondrial membrane and with SLC25A6 in the inner mitochondrial membrane. PMID:41991342 It spans the intermembrane space and is phosphorylated at distinct residues. |
| GO:0005743 mitochondrial inner membrane | IDA PMID:40578345 Microprotein SMIM26 drives oxidative metabolism via serine-r... | UNDECIDED | Summary: Inner-membrane localisation reported alongside interaction with the sideroflexin serine transporters and the mitoribosome. This sits awkwardly with the outer-membrane localisation reported by three other studies. Reason: GOA now carries both sides of a topological disagreement. Three studies place SMIM26 in the outer membrane (PMID:33386847, PMID:37009826, PMID:41991342), and PMID:41991342 explicitly has a single-pass outer-membrane anchor whose soluble portion spans the intermembrane space to reach the inner-membrane carrier SLC25A6. PMID:40578345 instead places it in the inner membrane, in a complex that includes the mitoribosome - which resides on the matrix face and is out of reach of an outer-membrane-anchored protein. The two readings are reconcilable only if a second, inner-membrane-integral pool of SMIM26 exists. This is an experimental annotation whose full text is not available here (the cached record is abstract-only), so it is neither removed nor demoted; the conflict is recorded instead. This annotation and the GO:0140978 mitoribosome-binding annotation stand or fall together. Supporting Evidence: PMID:40578345 SMIM26 interacts with mitochondrial serine transporters SFXN1/2 and the mitoribosome, forming a functional triad that facilitates translation of the complex I subunit mt-ND5. |
| GO:0030674 protein-macromolecule adaptor activity | IDA PMID:40578345 Microprotein SMIM26 drives oxidative metabolism via serine-r... | ACCEPT | Summary: SMIM26 is described as the bridging component of a transporter-ribosome assembly, i.e. it brings other macromolecules together rather than acting on them. Reason: Core molecular function. This is the one mechanistic description on which all three functional studies of SMIM26 converge, even though each reports a different partner set; SMIM26 has no catalytic or transport activity of its own. Supporting Evidence: PMID:40578345 These findings define SMIM26 as a critical integrator of one-carbon flux and complex I biogenesis and establish a paradigm for localized mitochondrial translation through transporter-ribosome interactions. |
| GO:0030674 protein-macromolecule adaptor activity | IDA PMID:41991342 The microprotein SMIM26 connects metabolite transporters of ... | ACCEPT | Summary: SMIM26 physically connects outer-membrane porins to an inner-membrane nucleotide carrier, the textbook shape of an adaptor activity. Reason: Core molecular function. This is the one mechanistic description on which all three functional studies of SMIM26 converge, even though each reports a different partner set; SMIM26 has no catalytic or transport activity of its own. Supporting Evidence: PMID:41991342 Our work suggests that SMIM26 coordinates metabolite transport through the inner and outer mitochondrial membranes and is essential for respiratory chain function in vivo. |
| GO:0044325 transmembrane transporter binding | IPI PMID:40578345 Microprotein SMIM26 drives oxidative metabolism via serine-r... | ACCEPT | Summary: Interaction with the mitochondrial serine transporter SFXN1 (UniProtKB:Q9H9B4). Reason: An informative binding term that names the actual mechanism, unlike bare protein binding. Four different carriers have now been reported as partners across three papers (SLC25A11, SFXN1, SLC25A6, VDAC1), which is consistent with a tether of low partner specificity rather than with any one paper being wrong. Supporting Evidence: PMID:40578345 SMIM26 interacts with mitochondrial serine transporters SFXN1/2 and the mitoribosome, forming a functional triad that facilitates translation of the complex I subunit mt-ND5. |
| GO:0044325 transmembrane transporter binding | IPI PMID:41991342 The microprotein SMIM26 connects metabolite transporters of ... | ACCEPT | Summary: Interaction with the inner-membrane ADP/ATP translocase SLC25A6/ANT3 (UniProtKB:P12236). Reason: An informative binding term that names the actual mechanism, unlike bare protein binding. Four different carriers have now been reported as partners across three papers (SLC25A11, SFXN1, SLC25A6, VDAC1), which is consistent with a tether of low partner specificity rather than with any one paper being wrong. Supporting Evidence: PMID:41991342 In biochemical and single-molecule tracking studies, we found that SMIM26 interacts with VDAC1/2 in the outer mitochondrial membrane and with SLC25A6 in the inner mitochondrial membrane. |
| GO:0044325 transmembrane transporter binding | IPI PMID:41991342 The microprotein SMIM26 connects metabolite transporters of ... | ACCEPT | Summary: Interaction with the outer-membrane porin VDAC1 (UniProtKB:P21796). Reason: An informative binding term that names the actual mechanism, unlike bare protein binding. Four different carriers have now been reported as partners across three papers (SLC25A11, SFXN1, SLC25A6, VDAC1), which is consistent with a tether of low partner specificity rather than with any one paper being wrong. Supporting Evidence: PMID:41991342 In biochemical and single-molecule tracking studies, we found that SMIM26 interacts with VDAC1/2 in the outer mitochondrial membrane and with SLC25A6 in the inner mitochondrial membrane. |
| GO:0140978 mitochondrial large ribosomal subunit binding | IDA PMID:40578345 Microprotein SMIM26 drives oxidative metabolism via serine-r... | UNDECIDED | Summary: Direct binding of SMIM26 to the mitoribosome, reported as part of a transporter-ribosome triad that supports mt-ND5 translation. Reason: This is the claim that is hardest to reconcile with the rest of the record. The mitochondrial large ribosomal subunit sits on the matrix face of the inner membrane, and a single-pass protein anchored in the outer membrane and spanning the intermembrane space - the topology PMID:41991342 reports - cannot reach it. Either a second, inner-membrane-integral and matrix-facing pool of SMIM26 exists, or the mitoribosome association is indirect (via SFXN1/2 or the co-translational insertion machinery) and the direct-binding reading is too strong. The cached record for PMID:40578345 is abstract-only, so the supporting experiments cannot be inspected and an experimental annotation is not overruled from an abstract. Marked UNDECIDED rather than removed; it stands or falls with the GO:0005743 inner-membrane annotation from the same paper. Supporting Evidence: PMID:40578345 SMIM26 interacts with mitochondrial serine transporters SFXN1/2 and the mitoribosome, forming a functional triad that facilitates translation of the complex I subunit mt-ND5. |
| GO:0005741 mitochondrial outer membrane | EXP PMID:33386847 Identification and analysis of short open reading frames (sO... | ACCEPT | Summary: Experimental localisation of the LINC00493-encoded peptide to mitochondria, in the paper that first demonstrated the ORF is translated. Reason: Correct and specific primary location. Supporting Evidence: PMID:33386847 While proteomic analysis revealed that the LINC00493 peptide interacts with many mitochondrial proteins, immunofluorescence assays showed that its peptide is mitochondrially localized. |
| GO:0019901 protein kinase binding | IPI PMID:37009826 LINC00493-encoded microprotein SMIM26 exerts anti-metastatic... | MODIFY | Summary: The interaction is with AGK (UniProtKB:Q53H12), which is a lipid kinase, not a protein kinase. Reason: The interaction itself is well supported and is one of the best-characterised SMIM26 contacts, but the term is the wrong one. AGK (acylglycerol kinase, EC 2.7.1.94/2.7.1.107) phosphorylates monoacylglycerol and diacylglycerol and is a subunit of the TIM22 inner-membrane translocase; it is not a protein kinase. The parent term GO:0019900 kinase binding states exactly what was shown. Proposed replacements: kinase binding Supporting Evidence: PMID:37009826 SMIM26, but not LINC00493, suppresses ccRCC growth and metastatic lung colonization by interacting with acylglycerol kinase (AGK) and glutathione transport regulator SLC25A11 via its N-terminus. |
| GO:0044325 transmembrane transporter binding | IPI PMID:37009826 LINC00493-encoded microprotein SMIM26 exerts anti-metastatic... | ACCEPT | Summary: Interaction with SLC25A11 (UniProtKB:Q02978), the inner-membrane 2-oxoglutarate/malate carrier. Reason: An informative binding term that names the actual mechanism, unlike bare protein binding. Four different carriers have now been reported as partners across three papers (SLC25A11, SFXN1, SLC25A6, VDAC1), which is consistent with a tether of low partner specificity rather than with any one paper being wrong. Supporting Evidence: PMID:37009826 SMIM26, but not LINC00493, suppresses ccRCC growth and metastatic lung colonization by interacting with acylglycerol kinase (AGK) and glutathione transport regulator SLC25A11 via its N-terminus. |
| GO:0005739 mitochondrion | IDA PMID:33386847 Identification and analysis of short open reading frames (sO... | ACCEPT | Summary: Immunofluorescence localisation of the LINC00493 peptide to mitochondria. Reason: Correct, though less informative than the mitochondrial outer membrane (GO:0005741) annotations this gene also carries. Supporting Evidence: PMID:33386847 While proteomic analysis revealed that the LINC00493 peptide interacts with many mitochondrial proteins, immunofluorescence assays showed that its peptide is mitochondrially localized. |
| GO:0005741 mitochondrial outer membrane | IDA PMID:37009826 LINC00493-encoded microprotein SMIM26 exerts anti-metastatic... | ACCEPT | Summary: Experimental localisation to the mitochondrial outer membrane in renal carcinoma cells. Reason: Correct and specific primary location. Supporting Evidence: PMID:37009826 Here, we show that LINC00493-encoded SMIM26, an understudied microprotein localized in mitochondria, is tendentiously downregulated in clear cell renal cell carcinoma (ccRCC) and correlated with poor overall survival. |
| GO:0005758 mitochondrial intermembrane space | IDA PMID:41991342 The microprotein SMIM26 connects metabolite transporters of ... | NEW | Summary: Proposed new annotation: the soluble portion of SMIM26 is reported to span the intermembrane space between its outer-membrane anchor and its inner-membrane partner. Reason: GOA records the outer and inner membranes but not the compartment the protein is said to traverse, which is the part of the topology that makes the bridging model work. Offered as a reviewer proposal, not a settled call - the topology rests on a single 2026 study and its full text is not available here. Supporting Evidence: PMID:41991342 It spans the intermembrane space and is phosphorylated at distinct residues. |
| GO:1990542 mitochondrial transmembrane transport | IMP PMID:41991342 The microprotein SMIM26 connects metabolite transporters of ... | NEW | Summary: Proposed new annotation: by holding outer- and inner-membrane carriers together, SMIM26 is proposed to coordinate metabolite movement across the mitochondrial envelope. Reason: GOA currently gives SMIM26 no biological process term at all, so the consequence of the adaptor activity is unrecorded. This is the process the bridging model predicts and that knockout cells, knockout mice and zebrafish mutants are consistent with. Offered as a reviewer proposal - the authors themselves phrase it as a suggestion, and no direct flux measurement across both membranes is reported in the abstract. Supporting Evidence: PMID:41991342 Our work suggests that SMIM26 coordinates metabolite transport through the inner and outer mitochondrial membranes and is essential for respiratory chain function in vivo. PMID:41991342 Knockout cells are viable, but respiratory chain activity is strongly reduced. |
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Download this section (compressed HTML)Q: Is there an inner-membrane-integral, matrix-facing pool of SMIM26? This is the single question that would reconcile the outer-membrane/intermembrane-space topology of PMID:41991342 with the inner-membrane localisation and direct mitoribosome binding reported in PMID:40578345, and it decides whether GO:0005743 and GO:0140978 should be retained.
Suggested experts: Gunter Meister, Lena Ho, David A Stroud
Q: Are the three reported partner sets - AGK plus SLC25A11, SFXN1/2 plus the mitoribosome, and VDAC1/2 plus SLC25A6 - alternative occupancies of one promiscuous tether, or genuinely distinct cell-type-specific complexes? The answer determines whether transmembrane transporter binding should be annotated once as a general property or separately per carrier.
Suggested experts: Gunter Meister, Qing-Yu He
Q: What is the function of the phosphorylation sites reported on SMIM26? PMID:41991342 states that the protein is phosphorylated at distinct residues but assigns no role to the modification.
Suggested experts: Gunter Meister, Andrea Pauli
Experiment: Determine SMIM26 topology directly by protease protection of intact mitochondria, mitoplasts and detergent-solubilised membranes, using antibodies against the N and C termini and against an internal epitope tag. Combine with alkaline carbonate extraction to distinguish integral from peripheral association, and with separation of outer membrane, intermembrane space, inner membrane and matrix fractions. A single-pass outer-membrane anchor with an intermembrane-space-facing C terminus predicts complete loss of the C-terminal epitope on mitoplasting.
Hypothesis: SMIM26 is anchored only in the outer membrane, and its association with the mitoribosome is indirect.
Type: subcellular fractionation and protease protection
Experiment: Repeat the mitoribosome co-purification in SFXN1/SFXN2 double-knockout cells and with recombinant SMIM26 and purified mitoribosomes in vitro. Direct binding should survive loss of the sideroflexins and should be reconstitutable from purified components; indirect association should not.
Hypothesis: The mitoribosome association reported for SMIM26 is bridged by SFXN1/2 rather than being a direct contact.
Type: co-immunoprecipitation in knockout background plus in vitro reconstitution
Experiment: Measure ADP/ATP exchange and serine import in mitochondria isolated from SMIM26 knockout cells and from knockout cells rescued with wild-type SMIM26 or with separation -of-function mutants that retain the outer-membrane anchor but lose the intermembrane-space region. If the tether coordinates transport, the anchor-only mutant should fail to restore flux despite correct localisation.
Hypothesis: The adaptor function of SMIM26 is required for coordinated metabolite flux, not merely correlated with it.
Type: isolated-mitochondria transport assay with structure-guided rescue
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