The tripartite ATP-independent periplasmic (TRAP) transporters of bacteriaand archaea

Until recently, extracytoplasmic solute receptor (ESR)-dependent uptake sys tems were invariably found to possess a conserved ATP-binding protein (the ATP-binding cassette protein or ABC protein), which couples ATP hydrolysis to the translocation of the solute across the cytoplasmic membrane. While i t is clear that this class of ABC transporter is ubiquitous in prokaryotes, it is now firmly established that other, unrelated types of membrane trans port systems exist which also have ESR components. These systems have been designated tripartite ATP-independent periplasmic (TRAP) transporters, and they form a distinct class of ESR-dependent secondary transporters where th e driving force for solute accumulation is an electrochemical ion gradient and not ATP hydrolysis. Currently, the most well characterised TRAP transpo rter at the functional and molecular level is the high-affinity C4-dicarbox ylate transport (Dct) system from Rhodobacter capsulatus. This consists of three proteins: an ESR (DctP) and small (DctQ) and large (DctM) integral me mbrane proteins. The characteristics of this system are discussed in detail . Homologues of the R. capsulatus DctPQM proteins arc present in a diverse range of prokaryotes, both bacteria and archaea, but not in eukaryotes. The deduced structures and possible functions of these homologous systems are described. In addition to the DctP family, other types of ESRs can be assoc iated with TRAP transporters. A conserved family of immunogenic extracytopl asmic proteins is shown to be invariably associated with TRAP systems that contain a large DctQM fusion protein. All of the currently known archaeal s ystems are of this type. It is concluded that TRAP transporters are a wides pread and ancient type of solute uptake system that transport a potentially diverse range of solutes and most likely evolved by the addition of auxili ary proteins to a single secondary transporter. (C) 2001 Federation of Euro pean Microbiological Societies. Published by Elsevier Science BN. All right s reserved.