METTL17

METTL17 is a mitochondria-localized SAM-binding methyltransferase that functions as an assembly factor for the mitoribosomal small subunit, coupling iron-sulfur cluster availability to mitochondrial translation. It localizes to mitochondria via an N-terminal targeting sequence, directly binds 12S mt-rRNA and small subunit proteins, and installs m4C840 and m5C842 modifications on 12S mt-rRNA; loss of METTL17 destabilizes 12S mt-rRNA, impairs mitochondrial translation of OXPHOS-encoding mRNAs, and disrupts cellular bioenergetics (PMID:31487196, PMID:38377789). METTL17 harbors an [Fe4S4]²⁺ cluster essential for its stability and acts as an Fe-S checkpoint during late mitoribosome assembly, licensing translation of Fe-S cluster-rich OXPHOS complexes only when iron-sulfur cofactors are replete—overexpression of METTL17 rescues bioenergetic defects in frataxin-deficient cells (PMID:38199006). METTL17 protein levels are regulated post-translationally by SIRT5-mediated desuccinylation at Lys274, which facilitates RNF126-dependent ubiquitination at K116 and subsequent degradation (PMID:42021405).

1. 2019 High

   Establishing METTL17 as a mitochondrial SAM-binding protein that modifies 12S mt-rRNA and is required for mitochondrial translation resolved its basic molecular function and cellular role.

   Evidence CRISPR knockout in mouse ESCs with subcellular fractionation, Co-IP, SAM-binding assays, mass spectrometry quantification of rRNA modifications, and mitochondrial translation assays

   PMID:31487196

   Open questions at the time

   • Whether METTL17 is the direct catalytic writer of m4C840/m5C842 or acts indirectly through recruitment of another enzyme
   • Structural basis of METTL17 binding to 12S rRNA and the mitoribosomal small subunit
   • Regulatory mechanisms controlling METTL17 protein levels or activity
2. 2023 Low

   Direct biochemical testing of METTL17 methyltransferase activity on its putative 12S rRNA target attempted to resolve whether METTL17 is a bona fide catalytic methyltransferase at this site.

   Evidence MALDI-TOF mass spectrometry of rRNA methylation on the METTL17-interacting region

   PMID:38234605

   Open questions at the time

   • No clear positive in vitro reconstitution of catalytic activity was reported, leaving the direct writer question unresolved
   • Single biochemical approach without orthogonal confirmation
   • Absence of mutant controls for SAM-binding domain
3. 2024 High

   Discovery of an [Fe4S4]²⁺ cluster in METTL17 and its role as a late-assembly checkpoint on the mitoribosomal small subunit revealed how mitochondrial translation is coupled to iron-sulfur cofactor availability.

   Evidence Cryo-EM structure, Fe-S cluster biochemistry, site-directed mutagenesis, quantitative proteomics in FXN-deficient cells, and overexpression rescue experiments

   PMID:38199006

   Open questions at the time

   • Whether the Fe-S cluster is sensed by a specific signaling pathway or its loss is the direct destabilizing event
   • Whether METTL17 catalytic activity versus its structural presence on the mitoribosome is the critical checkpoint function
   • In vivo validation of Fe-S checkpoint in animal models of Friedreich ataxia
4. 2024 Medium

   Linking METTL17 depletion to ferroptosis sensitivity demonstrated that its role in mitochondrial RNA methylation and OXPHOS maintenance protects cells from lipid peroxidation and iron-dependent cell death.

   Evidence siRNA/shRNA knockdown in colorectal cancer cells, mitochondrial RNA methylation profiling, ROS/lipid peroxidation measurements, xenograft and AOM/DSS mouse models

   PMID:38377789

   Open questions at the time

   • Whether ferroptosis sensitization is a direct consequence of METTL17 loss or secondary to general OXPHOS collapse
   • Specificity of the broad methylation changes (m4C, m5C, m3C, m7G, m6A) to METTL17 writer activity versus indirect effects
   • No in vitro reconstitution of METTL17 catalytic activity on non-rRNA substrates
5. 2025 Medium

   Identification of RNF126-mediated ubiquitination and SIRT5-mediated desuccinylation revealed the post-translational regulatory circuit controlling METTL17 protein turnover.

   Evidence Mass spectrometry identification of ubiquitination (K116) and succinylation (K274) sites, reciprocal Co-IP, lentiviral knockdown/overexpression, and functional mitochondrial assays in glioma cells

   PMID:42021405

   Open questions at the time

   • Whether SIRT5-RNF126 regulation of METTL17 operates in non-cancer cell types
   • Structural basis for how K274 desuccinylation exposes K116 for ubiquitination
   • Physiological signals that activate this degradation pathway

• Whether METTL17 possesses direct catalytic methyltransferase activity on 12S mt-rRNA (versus serving as an assembly platform for another methyltransferase such as METTL15), and the full structural basis of its checkpoint function in mammalian cells, remain unresolved.
• In vitro reconstitution of METTL17 catalytic activity with purified components has not been achieved
• Cooperative versus sequential relationship between METTL17 and METTL15 in mammalian mitoribosome assembly
• Whether METTL17 has non-mitochondrial substrates or functions