2.A.71 The Folate-Biopterin Transporter (FBT) Family
The FBT family includes functionally characterized members from protozoa, cyanobacteria (Synechocystis) and plants (Arabidopsis thaliana). Functionally characterized members of the family include FT1 (TC# 2.A.71.1.3), the major protozoan folate transporter, BT1 (2.A.71.1.2), the biopterin/folate transporter and AdoMetT1, the major S-adenosylmethionine uptake porter (2.A.71.1.5). There are at least 6 homologues of the FT1 transporter in Leishmania encoded by tandem genes. A related protein in Trypanosoma brucei, ESAGIO, shows weak folate/biopterin transport activity.
Proteins of the FBT family are reported to contain 627-631 aas (Leishmania species), 408-431 aas (A. thaliana) and 453-494 aas (cyanobacteria). All probably have 12 TMSs. They may function by H+ symport. The FBT family is a distant family within the major facilitator superfamily (MFS; TC #2.A.1).
The probable transport reaction catalyzed by characterized FBT family members is:
[folate, biopterin or AdoMet] (out) + nH+ (out) → [folate, biopterin or AdoMet] (in) + nH+ (in)
FT1 folate/biopterin transporter. This protein is nearly identical to the functionally characterized protein from L. infantum (2.A.71.1.5) which is a high affinity S-adenosyl methione uptake porter. Therefore the annotation shown above may be incorrect and this protein may also be an S-adenosyl methionine uptake porter (Dridi et al. 2010).
FT1 of Leishmania donovani
High affinity plasma membrane constitutively synthesized S-Adenosylmethionine (AdoMet) uptake transporter, AdoMetT1 of 706 aas. It also takes up the AdoMet analogue, Sinefungin (Dridi et al. 2010). This protein is nearly identical to 2.A.71.1.3 which may also be an AdoMet uptake transporter (by sequence similarity).
AdoMetT1 of Leishmania infantum
Folate/biopterin uptake carrier, Fbp. Transports 5-formyl tetrahydrofolate monoglutamate but not 5-formyltetrahydrofolate triglutamate. (Also transports methotrexate and aminopterin) (Klaus et al., 2005) (Nine homologues are present in Arabidopsis thaliana.) (Klaus et al., 2005). Mutational analyses suggest that the folate binding site involves the core helices 1, 4, 7 and 10 and is in the center of the membrane (Eudes et al., 2010).
Fbp of Arabidopsis thaliana
Folate/biopterin uptake carrier of 494 aas, Fbp (also transports methotrexate and aminopterin) (Klaus et al., 2005). Essential residues line the central cavity (based on the LacY structure) in TMSs 1, 4, 7 and 10 (Eudes et al. 2010).
Fbp (Srl0642) of Synechocystic sp. PCC6803 (BAA10362)
Folate transporter 3, FT3 of 537 aas and 14 putative TMSs.
FT3 of Nitrosospira lacus
Uncharacterized protein of 551 aas and 13 - 14 TMSs.
UP of Phytophthora parasitica