The proton-coupled folate transporter (PCFF/SLC46A1) mediates intestinal folate uptake at acidic pH. Some loss of folic acid (FA) transport Mutations in PCFT from hereditary folate malabsorption (HFM) patients cluster in R113. thereby suggesting a functional role for this residue. Herein, unlike non-conservative substitutions, an R113H mutant displayed 80-fold increase in the FA transport Km while retaining parental Vmax, hence indicating a major fall in folate substrate affinity. Furthermore, consistent with the preservation of 9% of parental transport activity, R113H transfectants displayed a substantial decrease in the FA growth requirement relative to mock transfectants. Homology modeling based on the crystal structures of the Escherichia coli transporter homologues EmrD and glycerol-3-phosphate transporter revealed that the R113H rotamer properly protrudes into the cytoplasmic face of the minor cleft normally occupied by R113. These findings constitute the first demonstration that a basic amino acid at position 113 is required for folate substrate binding. (C) 2009 Elsevier Inc. All rights reserved.