Amino acid transporter importing serine, an essential substrate of the mitochondrial branch of the one-carbon pathway, into mitochondria. Mitochondrial serine is then converted to glycine and formate, which exits to the cytosol where it is used to generate the charged folates that serve as one-carbon donors. May also transport other amino acids including alanine and cysteine. L-serine(in) = L-serine(out) L-alanine(in) = L-alanine(out) L-cysteine(in) = L-cysteine(out) Belongs to the sideroflexin family.