The TIM22 complex regulates mitochondrial one-carbon metabolism by mediating the import of Sideroflexins

Abstract

words)The 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 inAGK cause Sengers syndrome, an autosomal recessive condition characterized by congenital cataracts, hypertrophic cardiomyopathy, skeletal myopathy and lactic acidosis. We undertook proteomic profiling of Sengers patient fibroblasts and an AGKKO cell line to map the proteomic changes that ensue upon AGK dysfunction. This uncovered extensive remodelling of mitochondrial one-carbon metabolism enzymes and showed that inner membrane serine transporters, Sideroflexins (SFXNs), are novel substrates of the TIM22 complex. Deletion of SFXN1 recapitulates the remodelling of onecarbon metabolism observed in Sengers patient cells. Proliferation of cells lacking AGK is perturbed in the absence of exogenous serine and rescuable through addition of formate, highlighting the dysregulation of one carbon metabolism as a key molecular feature in the biology of Sengers syndrome.

Biogenesis of Mitochondrial Metabolite Carriers

“…However, recent work has identified the mitochondrial pyruvate carrier proteins with their divergent topology as substrates of the TIM22 pathway [ 31 , 32 ]. What is more, the unrelated sideroflexins also rely on the TIM22 carrier translocase for their biogenesis [ 38 , 39 ]. Of the MPC components, at least Mpc2 and Mpc3 have unpaired TM helices and none of the MPC proteins possess more than three TM segments [ 6 , 7 , 35 , 36 , 37 ], while the sideroflexins also have an uneven number of TM segments and yet another topology [ 8 , 39 ] ( Figure 2 ).…”

“…For the human TIM22 translocase a differential requirement of the auxiliary subunits for different substrate classes has been reported: AGK is required for efficient carrier import and dispensable for the import of Tim22 and related proteins [ 103 , 104 ], whereas the opposite is the case for Tim29 [ 92 ]. Interestingly, sideroflexins, like classical carriers, rely on AGK, while Tim29 is dispensable [ 38 ]. It will be very interesting to learn how TIM22 adapts its function to import this range of structurally distinct substrates.…”

“…In contrast, Mpc2/Mpc3 has only 3 TM segments, and Mpc1 has 2 or 3 TM segments [ 6 , 7 , 35 , 36 , 37 ]. The third unique family of metabolite carriers, the sideroflexins, has 5 TM domains with the N-terminus in the intermembrane space (IMS) [ 8 , 38 , 39 ]. Aside from metabolite carriers, the TIM22 pathway also imports the members of the Tim17 protein family including the translocase components Tim17, Tim22 and Tim23 ( green ).…”

“…Additionally, recent studies indicate that the sideroflexins, with five transmembrane segments and the N-terminus in the intermembrane space (IMS), also depend on the TIM22 carrier pathway for their biogenesis [ 38 , 39 ] ( Figure 2 ). Thus, the mitochondrial pyruvate carrier components and the sideroflexins with their unique topologies have challenged long-held views of the structural requirements for TIM22 substrates.…”

Sequence Features of Mitochondrial Transporter Protein Families

“…SLC56A3/SFXN3 and SLC56A2/SFXN2 can compensate for this function, as well as yeast and Drosophila homologues, indicating an ancestral function, while SLC56A4/SFXN4 and SLC56A5/SFXN5 were unable to do so, indicating a possible altered transport rate or substrate selectivity [54]. Regarding their mitochondrial targeting, sideroflexins do not show any canonical MTS [53], and instead are targeted to the mitochondria via the TIM22 import complex [55][56][57], similarly to SLC25 proteins. Very little is currently known about the structure and function of sideroflexin transporters.…”