The mitochondrial carrier SFXN1 is critical for Complex III integrity and cellular metabolism

Mitochondrial carriers (MC) mediate the passage of small molecules across the inner mitochondrial membrane (IMM) enabling regulated crosstalk between compartmentalized reactions. Despite MCs representing the largest family of solute carriers in mammals, most have not been subjected to a comprehensive investigation, limiting our understanding of their metabolic contributions. Here, we functionally characterized SFXN1, a member of the non-canonical, sideroflexin MC family. We find that SFXN1, an integral membrane protein in the IMM with an uneven number of transmembrane domains, is a novel TIM22 substrate. SFXN1 deficiency specifically impairs Complex III (CIII) biogenesis, activity, and assembly, compromising coenzyme Q levels. This CIII dysfunction is independent of one-carbon metabolism, the known primary role for SFXN1 as a mitochondrial serine transporter. Instead, SFXN1 supports CIII function by participating in heme and central carbon metabolism. Our findings highlight the multiple ways that SFXN1-based amino acid transport impacts mitochondrial and cellular metabolic efficiency.