Sideroflexins enable mitochondrial transport and metabolism of neutral amino acids

Mitochondria contribute to compartmentalized metabolism in eukaryotic cells, supporting key enzymatic reactions for cell function and energy homeostasis. This compartmentalization necessitates regulated metabolite transport across mitochondrial membranes. While many transport proteins have been identified, several mitochondrial transporters remain poorly characterized. Among these are sideroflexins, an evolutionarily conserved family of mitochondrial inner membrane proteins with unclear function. Using CRISPR/Cas9-mediated candidate transporter knockouts coupled with assessment of mitochondrial membrane permeability via a swelling assay, we identify SFXN1, previously implicated in mitochondrial serine transport and iron homeostasis, as an enabler of mitochondrial transport of multiple neutral amino acids, including proline, glycine, threonine, taurine, hypotaurine, {beta}-alanine, and {gamma}-aminobutyric acid (GABA). We further show that SFXN paralogues exhibit substrate-dependent functional overlap, with SFXN2 and SFXN3 partially rescuing loss of SFXN1 function in glycine-related phenotypes, while SFXN2 and SFXN5 partially rescue SFXN1-dependent changes in enabling GABA transport and metabolism. Altogether, these data establish sideroflexins as key regulators of mitochondrial amino acid transport and metabolism.