Sideroflexins enable mitochondrial transport of polar neutral amino acids

Mitochondria contribute to compartmentalized metabolism in eukaryotic cells, supporting key enzymatic reactions for cell function and energy homeostasis. However, this compartmentalization necessitates regulated metabolite transport across mitochondrial membranes. Although many transport proteins have been identified, several mitochondrial amino acid transporters remain largely uncharacterized. Using CRISPR-Cas9-mediated candidate transporter knockouts coupled with assessment of metabolite transport via a mitochondrial swelling assay, we identify SFXN1, previously characterized for its role in mitochondrial serine transport, as a protein that mediates mitochondrial transport of a range of other polar neutral amino acids including proline, glycine, threonine, taurine, hypotaurine, {beta}-alanine, and {gamma}-aminobutyric acid (GABA). Furthermore, the SFXN1 paralogues SFXN2 and SFXN3 partially complement loss of SFXN1 to enable glycine transport, while SFXN2 and SFXN5 partially complement loss of SFXN1 to enable GABA transport. Altogether, these data suggest that sideroflexins facilitate the transport of polar neutral amino acids across the inner mitochondrial membrane.