The Tripartite Tricarboxylate Transporters (TTT) : The neglected family of high-affinity uptake systems

Bacteria inhabiting complex environments must possess efficient ways of acquiring nutrients, often present in low concentrations. In this context, Solute Binding Protein (SBP) dependent transporters make use of a periplasmic binding protein to bind substrates with high affinity, and deliver them to their transmembrane counterparts. There are three known families of SBP dependent transporters: The ABC (ATP-Binding Cassette), the TRAP (TRipartite ATP-independent Periplasmic transporters) and the TTT (Tripartite Tricarboxylate Transporter). The latter are very poorly characterized and only a few types of substrates for TTT transporters are currently known. This thesis first presents a review of the TTT transporters. Our database searches reveal a massive overrepresentation of TTT SBPs among α and β-proteobacteria, and highlight the presence of 434 TTT SBPs in the bacterium Rhodoplanes sp. Z2-YC6860, the biggest gene family representation described in a bacterial genome to date. We subsequently focus on the characterization of the TTT family in Rhodopseudomonas palustris, a model soil non-sulfur purple bacterium. The TTT family in R. palustris is formed by two complete tripartite systems, plus five orphan SBPs with no obvious membrane counterparts. Two of the SBP were characterized biochemically and physiologically. One of the orphan SBPs (AdpC) is described to bind to dicarboxylic acids ranging from six to nine carbons in length with low µM affinity, and is more expressed under low concentration of adipate. A second SBP also had its binding properties and crystal structure characterised. In summary, this study presents a characterization of the Tripartite Tricarboxylate Transporter family in bacteria, through a synergistic multidisciplinary approach.