Figure 3. Transporters involved in border control.

A, a filled circle signifies the presence of at least one homolog of the indicated protein family (FTR1, high-affinity iron permease; FEA/ISIP2a, Fe-assimilation proteins; Tf, transferrin; CTR, Cu assimilatory transport; ZIP, divalent metal transporter; NRAMP, divalent metal transporter; CDF, divalent metal transporter; Cu-ATPase, Cu-transporting subfamily of the P_1B-type ATPases; Zn/Cd-ATPase, Zn/Cd-transporting subfamily of the P_1B-type ATPases; HMA1, similar to Zn/Cd-subfamily of P_1B-type ATPases but with unknown function; CVL, Ccc1/VIT1-like. A schematic species tree is shown at the left. B, predicted membrane topologies and common substrates for each transporter family are shown. The C. reinhardtii protein names are given for the high-affinity Fe-transport complex involving FRE1, FOX1 and FTR1. Solid arrows represent the direction of metal ion transport either into the cytosol across the plasma or organelle membrane (left hand side of figure) or out of the cytosol across the intracellular membranes of organelles (right hand side of figure). Metal reductases (FRE1 for Fe reduction in C. reinhardtii) and an as yet unidentified reductase for Cu²⁺ reduction before transport by CTR are represented as dark red ovals Abbreviations for importers: FRE1, ferric reductase 1 ; FOX1, ferroxidase 1, MCO (multi-copper oxidase) family; FTR1, Fe transporter 1; FEA, Fe-assimilation protein; ISIP2a, iron starvation inducible protein 2a, first identified in Phaeodactylum tricornutum; ZIP, Zrt-, Irt-like proteins; CTR, Cu transporter; NRAMP (natural resistance-associated macrophage proteins). Abbreviations for exporters: CDF, cation diffusion facilitator, named MTP in plants; CVL, Ccc1/VIT1, Ca(II)-sensitive cross-complementer 1/vacuolar iron transporter 1.