A Role for the Proton-coupled Folate Transporter (PCFT-SLC46A1) in Folate Receptor-mediated Endocytosis

Recently, this laboratory identified a proton-coupled folate transporter (PCFT), with optimal activity at low pH. PCFT is critical to intestinal folate absorption and transport into the central nervous system because there are loss-of-function mutations in this gene in the autosomal recessive disorder, hereditary folate malabsorption. The current study addresses the role PCFT might play in another transport pathway, folate receptor (FR)-mediated endocytosis. FR alpha cDNA was transfected into novel PCFT+ and PCFT- HeLa sublines. FR alpha was shown to bind and trap folates in vesicles but with minimal export into the cytosol in PCFT- cells. Cotransfection of FR alpha and PCFT resulted in enhanced folate transport into cytosol as compared with transfection of FR alpha alone. Probenecid did not inhibit folate binding to FR, but inhibited PCFT- mediated transport at endosomal pH, and blocked FR alpha-mediated transport into the cytosol. FR alpha and PCFT co-localized to the endosomal compartment. These observations (i) indicate that PCFT plays a role in FR alpha-mediated endocytosis by serving as a route of export of folates from acidified endosomes and (ii) provide a functional role for PCFT in tissues in which it is expressed, such as the choroid plexus, where the extracellular milieu is at neutral pH.