SLC25A45 is required for mitochondrial uptake of methylated amino acids and de novo carnitine biosynthesis

Methylated amino acids accumulate upon the degradation of methylated proteins and are implicated in diverse metabolic and signaling pathways. Disturbed methylated amino acid homeostasis is associated with cardiovascular disease and renal failure. Mitochondria are core processing hubs in conventional amino acid metabolism, but how they interact with methylated amino acids is unclear. Here, we reveal that the orphan mitochondrial solute carrier 25A45 (SLC25A45) is required for the mitochondrial uptake of methylated amino acids. SLC25A45 binds with dimethylarginine and trimethyllysine but has no affinity for unmethylated arginine and lysine. A non-synonymous mutation of human SLC25A45 (R285C) stabilizes the carrier by limiting its proteolytic degradation and associates with altered methylated amino acids in human plasma. Metabolic tracing of trimethyllysine in cancer cells demonstrates that SLC25A45 drives the biosynthesis of the key amino acid derivative, carnitine. SLC25A45 is therefore an essential mediator of compartmentalized methylated amino acid metabolism.