2.A.71 The Folate-Biopterin Transporter (FBT) Family
The FBT family includes functionally characterized members only from protozoa, but homologues are found in blue-green bacteria (Synechocystis) and plants (Arabidopsis thaliana). The functionally characterized members of the family include FT1, the major folate transporter, and BT1, the biopterin/folate transporter. There are at least 6 homologues of the FT1 transporter in Leishmania carried in a tandem array, but the genetic data suggest that only FT1 has appreciable activity (S. Beverley, personal communication). A related protein in Trypanosoma brucei, ESAGIO, shows weak folate/biopterin transport activity.
Proteins of the FBT family are reported to contain 686 aas (T. brucei), 627-631 aas (Leishmania species), 408-431 aas (A. thaliana) and 453 or 494 aas (cyanobacteria). All probably have 12 transmembrane α-helical spanners (TMSs). They may function by H+symport. The FBT family is a distant family within the major facilitator superfamily (MFS; TC #2.A.1), most closely related to drug resistance permeases.
The probable transport reaction catalyzed by characterized FBT family members is:
[folate or biopterin] (out) + nH+ (out) → [folate or biopterin] (in) + nH+ (in).