2.A.48 The Reduced Folate Carrier (RFC) Family
Proteins of the RFC family have been characterized only from animals, but they can also be formed in other eukaryotes such as slime molds (2.A.48.3.1) and Giardia (2.A.48.4.1). They have been sequenced from several mammals and from the worm, Caenorhabditis elegan, as well as the fly, Drosophila melanogaster. Humans have at least two RFC family paralogues, and C. elegans has three. All homologues exhibit a high degree of sequence similarity with each other. They are usually 500-600 amino acyl residues long and possess 12 putative transmembrane α-helical spanners (TMSs). Evidence for a 12 TMS topology has been published for a human RFC. RFCs take up folate, reduced folate, derivatives of reduced folate and the drug, methotrexate. Residues in the first TMS and in the region between TMSs 1, 2 and 11 appear to play a role in substrate recognition (Flintoff et al., 2003; Hou et al., 2005). The large cytoplasmic loop between TMSs 6 and 7 is required for stability and efficient transport. These proteins are distantly related to carriers of the major facilitator superfamily (MFS; TC #2.A.1). The reduced folate carrier (RFC) is cytotoxic to animal cells under conditions of severe folate deprivation (Ifergan et al., 2008).
Mammals possess at least three folate transporters: the RFC (KB = 100 nM; KM = 1 μM) described here as well as a lower affinity system and a higher affinity system. The RFC appears to transport reduced folate by an energy-dependent, pH-dependent, Na+-independent mechanism. Folate:H+ symport, folate:OH- antiport and folate:anion antiport mechanisms have been proposed. Intracellular anions are able to promote folate derivative uptake. A bidirectional anion antiport mechanism for RFC family members is favored. In support of this notion, RFC1 has been shown to catalyze efflux of thiamin pyrophosphate (TPP) (Zhao et al., 2001; Visentin et al., 2012).
The human thiamine transporter is a member of the RFC family. The transporter is highly specific for thiamine and is not inhibited by other organic cation. It transports thiamine by a Na+-independent pmf-dependent process. Folates are not substrates of this system.
The generalized transport reaction(s) catalyzed by the proteins of the RFC family is/are probably:
Folate derivative (out) + anion (in) ⇌ folate derivative (in) + anion (out).
Thiamine (out) + nH+ (out) ⇌ thiamine (in) + nH+ (in).