Until recently, FRα and RFT1 were the only proteins that had been shown to be involved in the cellular uptake of folate. Based on the cellular expression of these two proteins in the retina, our studies suggested that FRα mediates the entry of folate from blood into RPE, RFT1 mediates the efflux of folate into subretinal space, and FRα mediates the uptake of folate into the different cells of the retina.
3 7 Our studies had also postulated a role for a putative H
+-coupled folate transporter that is functionally coupled to FRα.
14 As we envisaged, this putative transporter would mediate the transport of folate that is internalized into endosomes bound to FRα and subsequently released from the receptor after acidification from the acidic endosomal compartment into the cytoplasm. Originally, we believed that RFT1 might mediate this transport.
19 Some investigators have reported that RFT1 expression can result in two distinct transport activities, one that has a neutral pH optimum and a second that has optimal activity at the acidic pH.
20 21 Tissue-specific modulators are thought to be responsible for the two distinct activities. However, since the expression of RFT1 is limited to the RPE in the retina whereas FRα is ubiquitously expressed, it is unlikely that RFT1 could be the endosomal H
+-coupled transporter. The expression pattern of the newly identified PCFT/HCP1 mirrors that of FRα. It is expressed robustly in all cell types in the retina. In addition, the transporter functions robustly at an acidic pH. Though the presence of an electrochemical proton gradient has not been demonstrated across the plasma membrane of any retinal cell type, it is common knowledge that endosomal pH is acidic; thus, the function of PCFT/HCP1 as an H
+-coupled folate transporter is ideally suited for its role as a transporter mediating the transport of folate or its derivatives from the endosomal compartment into the cytoplasm. It is very likely that this new transporter is the one that works in conjunction with FRα in receptor-mediated endocytosis of folates and its derivatives. Accordingly, we predict that PCFT/HCP1 may colocalize with FRα in the plasma membrane to enable the two proteins to work together during receptor-mediated endocytosis. It is also interesting to note that PCFT/HCP1, like FRα, has similar affinity for the oxidized and reduced forms of folate (∼1–10 μM), whereas RFT1 has higher affinity for the reduced form compared to the oxidized form (1–2 μM vs. 50–200 μM, respectively
2 4 ). Thus, in the retina, FRα and PCFT/HCP1 can mediate the entry of both forms of folate, oxidized as well as reduced, from the blood into the retinal pigment epithelial cell at the outer blood–retinal barrier. Inside the cell, folate is reduced to MTF, which now becomes a good substrate for RFT1. RFT1, which is expressed only in the retinal pigment cells of the retina, mediates the extrusion of the reduced folate into the subretinal space. Because FRα and PCFT/HCP1 can interact with the reduced folate and are expressed in all cell types of the retina, the combined action of the two proteins would mediate the entry of MTF from the subretinal space into the cytoplasm of all retinal cells. This represents the first report on the expression and the functional significance of PCFT/HCP1 in retinal folate transport.