The tripartite ATP-independent periplasmic (TRAP) transporters are the best-studied family of substrate-binding protein (SBP)-dependent secondary transporters and are ubiquitous in prokaryotes, but absent from eukaryotes. They are comprised of an SBP of the DctP or TAXI families and two integral membrane proteins of unequal sizes that form the DctQ and DctM protein families, respectively. The SBP component has a structure comprised of two domains connected by a hinge that closes upon substrate binding. In DctP-TRAP transporters, substrate binding is mediated through a conserved and specific arginine/carboxylate interaction in the SBP. While the SBP component has now been relatively well characterized, the membrane components of TRAP transporters are still poorly understood both in terms of their structure and function. We review the expanding repertoire of substrates and physiological roles for experimentally characterized TRAP transporters in bacteria and discuss mechanistic aspects of these transporters using data primarily from the sialic acid-specific TRAP transporter SiaPQM from Haemophilus influenzae, which suggest that TRAP transporters are high-affinity, Na(+)-dependent unidirectional secondary transporters.
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