The tripartite ATP-independent periplasmic transporter (TRAP) family of inner membrane transporters catalyse the transport of a wide range of solutes, all of which are organic acids. TRAP transporters are comprised of three domains: a soluble substrate binding protein (SBP), a small membrane-spanning domain and a large membrane-spanning domain. The SBP binds solute in the periplasm, then delivers it to the membrane domains, which form a secondary transporter complex. To date, there are no structures of the membrane component of any TRAP transporter, and as such, the proposed transport cycle is yet to be experimentally validated.
Here, we show the expression and purification of two TRAP membrane transporters. Biophysical, crystallisation and preliminary diffraction data are also presented. Both of these transporters are from pathogenic bacteria and are predicted to have specificity for sialic acid. These data are a promising step toward the first structure of a TRAP membrane component, which would greatly aid the understanding of this unique family of transporters.