The TIM22 complex mediates the import of sideroflexins and is required for efficient mitochondrial one-carbon metabolism

Abstract

Acylglycerol Kinase (AGK) is a mitochondrial lipid kinase that contributes to protein biogenesis as a subunit of the TIM22 complex at the inner mitochondrial membrane. Mutations in AGK cause Sengers syndrome, an autosomal recessive condition characterized by congenital cataracts, hypertrophic cardiomyopathy, skeletal myopathy and lactic acidosis. We mapped the proteomic changes in Sengers patient fibroblasts and AGKKO cell lines to understand the effects of AGK dysfunction on mitochondria. This uncovered downregulation of a number of proteins at the inner mitochondrial membrane, including many SLC25 carrier family proteins, which are predicted substrates of the complex. We also observed downregulation of SFXN proteins, which contain five transmembrane domains, and show that they represent a novel class of TIM22 complex substrate. Perturbed biogenesis of SFXN proteins in cells lacking AGK reduces the proliferative capabilities of these cells in the absence of exogenous serine, suggesting that dysregulation of one carbon metabolism is a molecular feature in the biology of Sengers syndrome.

Interplay between Mitochondrial Protein Import and Respiratory Complexes Assembly in Neuronal Health and Degeneration

“…In this report, authors observed that the yeast mitochondrial pyruvate carrier, which has an odd number of transmembrane segments and a matrix-facing N-terminus, was imported specifically via the TIM22 complex. Similarly, it has been recently reported that human sideroflexins, a class of IMM proteins that contain five transmembrane domains and that do not belong to the SLC25 family, are imported via TIM22 [ 111 ]. Therefore, we can assume that the TIM22 substrate spectrum is less intransigent and contains proteins with paired and non-paired transmembrane domains.…”

“…Then, ND3, ND6, and ND4L are added to this intermediate. At least two of these assembly factors, namely TMEM126B and NDUFAF1, have been recently reported to be imported in a TIM22-dependent manner [ 111 ].…”

“…The remaining 12 subunits (NDUFA5, NDUFS5, NDUFC2, NDUFB10, NDUFB6, NDUFB9, NDUFB3, NDUFA3, NDUFA8, NDUFA13, NDUFB1, NDUFB4) [ 177 ] and three CI assembly factors (TMEM126B, FOXRED1, and TIMMDC1) [ 181 ] do not contain a cleavable MTS and are imported into the organelle as a result of uncharacterised internal signals. Notwithstanding, it has been recently demonstrated that NDUFB10, TIMMDC1, and TMEM126B are imported via TIM22 [ 111 ], possibly suggesting a similar import route for this class of proteins that requires further investigation. This pathway is also used by NDUFA11, a supernumerary subunit of CI that is conspicuously located at the interface between CI and CIII in SCs.…”

“…However, while cellular respiration was reduced in the patient cells, no evident defect in respiratory complexes or SC assembly was found. In a different report, where TIM22 assembly and activity was impaired by the removal of AGK, a mild CI assembly defect and respiration impairment was observed [ 111 ]. In contrast to the compound heterozygous variant, this observation suggests a direct involvement of the TIM22 complex in the import of CI subunits and/or factors required for CI assembly that requires further investigation.…”

Molecular Insights into Mitochondrial Protein Translocation and Human Disease

“…Recently, subunits of the mitochondrial pyruvate carrier (MPC) were identified as unconventional TIM22 cargo proteins containing two or three predicted transmembrane spans [ 169 , 170 ]. In addition, sideroflexins (SFXN), which contain five transmembrane segments, were also recently described as cargos of this complex [ 171 ]. The human TIM22 complex is a complex of 440-kDa comprised of TIM22, TIM29, TIM10B, and the lipid kinase AGK [ 165 , 166 , 167 , 168 ].…”