Antiporter that mediates the import of reduced folates or a subset of cyclic dinucleotides, driven by the export of organic anions (PubMed:10787414, PubMed:15337749, PubMed:16115875, PubMed:22554803, PubMed:31126740, PubMed:31511694, PubMed:32276275, PubMed:7826387, PubMed:9041240). Mechanistically, acts as a secondary active transporter, which exports intracellular organic anions down their concentration gradients to facilitate the uptake of its substrates (PubMed:22554803, PubMed:31126740, PubMed:31511694). Has high affinity for N5-methyltetrahydrofolate, the predominant circulating form of folate (PubMed:10787414, PubMed:14609557, PubMed:22554803). Also able to mediate the import of antifolate drug methotrexate (PubMed:22554803, PubMed:7615551, PubMed:7641195, PubMed:9767079). Also acts as an importer of immunoreactive cyclic dinucleotides, such as cyclic GMP-AMP (2′-3′-cGAMP), an immune messenger produced in response to DNA virus in the cytosol, and its linkage isomer 3′-3′-cGAMP, thus playing a role in triggering larger immune responses (PubMed:31126740, PubMed:31511694, PubMed:36745868). 5-amino-4-imidazolecarboxamide riboside (AICAR), when phosphorylated to AICAR monophosphate, can serve as an organic anion for antiporter activity (PubMed:22554803).
