Tripartite ATP-independent periplasmic transporters (TRAP transporters) are a large family of solute transporters found in bacteria and archaea, but not in eukaryotes, that appear to be specific for the uptake of organic acids or related molecules containing a carboxylate or sulfonate group. They are unique in that they utilize a substrate binding protein (SBP) in combination with a secondary transporter.

TRAP transporters were discovered in the laboratory of Prof. David J. Kelly at the University of Sheffield, UK. His group were working on the mechanism used by the photosynthetic bacterium Rhodobacter capsulatus to take up certain dicarboxylic acids. They characterised a binding protein component (DctP) of a transporter that recognized these compounds, which they assumed would form part of a typical ABC transporter, but when they sequenced the genes surrounding dctP they found two other genes encoding integral membrane proteins, dctQ and dctM, but no genes encoding components of an ABC transporter. They further showed that uptake of the same dicarboxylates was independent of ATP and that uptake required an electrochemical ion gradient, making this a unique binding protein-dependent secondary transporter.

Since these early studies, it has become clear that TRAP transporters are present in many bacteria and archaea, with many bacterial having multiple TRAP transporters, some having over 20 different systems.

To date, most substrates for TRAP transporters contain a common feature which is that they are organic acids. This includes C4-dicarboxylates such as succinate, malate and fumarate,keto-acids such as pyruvate and alpha-ketobutyrate and the sugar acid, N-acetyl neuraminic acid (or sialic acid). Other substrates include the compatible solute ectoine and hydroxyectoine and pyroglutamate.

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