The structural basis of modularity in ECF-type ABC transporters

Erkens, G. B. et al. Nat. Struct. Mol. Biol. 2011. 18 (7): 755-760.

Speaker: Sheng- huei Lin Chung (林張聖彙)                         Time: 15:10~16:00, Oct. 5, 2011

Commentator: Prof. Woei-Jer Chuang (莊偉哲 老師)         Place: Room 601

Abstract

Energy coupling factor (ECF) transporters regulate the uptake of micronutrients in bacteria and archaea¹.  They were identified as protein modules consisting of pairs of ABC ATPase domains (EcfA and EcfA’), a conserved transmembrane protein (EcfT) and a transmembrane substrate-capture protein (S-component).  S-components for different substrates shared little similarity in amino acid sequence but may associate with the same energizing module².  Thiamine-specific S component ThiT had an unusually high affinity for thiamine (KD = 122 ± 13pM) and bound the substrate with a 1:1 stoichiometry³.  This paper presents the crystal structure of ThiT from Lactococcus lactis at 2.0 Å resolution.  The thiamine-binding pocket interacts directly with the substrate by an intricate network of hydrogen bonds and aromatic interactions.  ThiT alone did not support thiamine transport, but coexpression of the energizing module (EcfAA’ T) and ThiT led to robust thiamine uptake.  The conserved AxxxA motif on the exposed face of helix 1 in S component mediates the interaction of ThiT with EcfT subunit and substitution of alanine to tryptophan individually led to complete loss of substrate transportation.  The authors propose the substrate translocation path within the interface of ThiT and EcfT.  Taken together, this study provides us molecular basis information about high-affinity binding and the mechanism of transport by ECF transporters.

References

1. Thomas Eitinger, et al. Canonical and ECF-type ATP-binding cassette importers in prokaryotes: diversity in modular organization and cellular functions. FEMS Microbiol Rev. 2011. 35, 3-67.
2. Josy ter Beek, et al. Quaternary Structure and Functional Unit of Energy Coupling Factor (ECF)-type Transporters. J. Biol. Chem. 2011. 286(7), 5471-5475.
3. Guus B. Erkens and Dirk Jan Slotboom. Biochemical Characterization of ThiT from Lactococcus lactis: A Thiamin Transporter with Picomolar Substrate Binding Affinity. Biochemistry. 2010. 49, 3203–3212.