PMID: 26342690

Authors:

Fischer M, Hopkins AP, Severi E, Hawkhead J, Bawdon D, Watts AG, Hubbard RE, Thomas GH

Title:

Tripartite ATP-independent periplasmic (TRAP) transporters use an arginine-mediated selectivity filter for high affinity substrate binding.

Journal:

J Biol Chem. 2015 Sep 5. pii: jbc.M115.656603.

Abstract:

Tripartite ATP-independent periplasmic (TRAP) transporters are secondary transporters that have evolved an obligate dependence on a substrate binding protein (SBP) to confer unidirectional transport. Different members of the DctP-family of TRAP SBPs have binding sites that recognise a diverse range of organic acid ligands, but appear to only share a common electrostatic interaction between a conserved arginine and a carboxylate group in the ligand. We investigated the significance of this interaction using the sialic acid-specific SBP, SiaP, from the Haemophilus influenzae virulence-related SiaPQM TRAP transporter. Using in vitro, in vivo and structural methods applied to SiaP, we demonstrate that the coordination of the acidic ligand moiety of sialic acid by the conserved arginine (R147) is essential for the function of the transporter as a high-affinity scavenging system. However, at higher substrate concentration the transporter can function in the absence of R147 suggesting that this bi-molecular interaction in not involved in further stages of the transport cycle. As well as being required for high affinity binding, we also demonstrate that the R147 is a strong selectivity filter for carboxylate-containing substrates in TRAP transporters, by engineering the SBP to recognise a non-carboxylate containing substrate, sialylamide, through water-mediated interactions. Together these data provide biochemical and structural support for the hypothesis that TRAP transporters function predominantly as high-affinity transporters for carboxylate-containing substrates.