2.A.88.3.2
Thiamin transporter, ThiT (Thiamin ECF transporter S component) (Rodionov et al. 2002; Rodionov et al., 2009). High affinity thiamin transporter ThiT (Ka=120 pM). Other substrates include TPP, TMP and pyrithiamin with nM binding constants at 1:1 stoichiometry (protein:ligand). Binding depends on a tryptophan-rich loop between TMSs 5 and 6 (Erkens and Slotboom, 2010). Erkens et al. (2011) presented the crystal structure of the thiamine-specific S-component of the ECF-type ABC transporter, ThiT from Lactococcus lactis at 2.0 Å. Extensive protein-substrate interactions explain its high binding affinity for thiamine (K_d ~ 10^-10 M). ThiT has a fold similar to that of the riboflavin-specific S-component RibU, with which it shares only 14% sequence identity. Two alanines in a conserved motif (AxxxA) located on the membrane-embedded surface of the S-components mediate the interaction with the energizing module. A general transport mechanism for ECF transporters has been proposed (Erkens et al., 2011).  Substrate binding induces conformational changes in ThiT (Majsnerowska et al. 2013).