Mitochondrial amino-acid transporter that mediates transport of serine into mitochondria (PubMed:30442778). Involved in mitochondrial iron homeostasis by regulating heme biosynthesis (PubMed:30570704). L-serine(in) = L-serine(out) Widely expressed, highest levels in kidney, liver, and pancreas. Belongs to the sideroflexin family.