Caught in a TRAP: substrate-binding proteins in secondary transport.

Substrate-binding protein (SBP)-dependent secondary transporters are ubiquitous in prokaryotes yet poorly characterised. Recently, the structures of over 10 prokaryotic SBPs have been solved, which we compare here to consider their impact on our understanding of transporter function and evolution. Seven structures are from tripartite ATP-independent periplasmic (TRAP) transporters of the DctP-type, which have similar overall structures distinct from SBPs used by ATP-binding cassette (ABC) transporters, despite recognising a range of substrates. A defining feature of substrate recognition in the DctP-TRAP SBPs is the formation of a salt bridge between a highly conserved arginine and a carboxylate group in the substrate, suggesting that these transporters might have evolved specifically for uptake of diverse organic acids. Remarkably, two of the DctP-TRAP SBPs are clearly dimers and the potential impact of this on transporter function will be discussed. Other SBPs used in secondary transporters are structurally similar to ABC SBPs, demonstrating that multiple families of SBPs have evolved to function with secondary transporters.