TRAP transporters: an ancient family of extracytoplasmic solute-receptor-dependent secondary active transporters

Tripartite ATP-independent periplasmic transporters (TRAP-T) represent a no vel type of secondary active transporter that functions in conjunction with an extracytoplasmic solute-binding receptor. The best characterized TRAP-T family member is from Rhodobacter capsulatus and is specific for C-4-dicar boxylates [Forward, J. A., Behrendt, M. C., Wyborn, N. R., Cross, R. & Kell y, D. J. (1997). I Bacteriol 179, 5482-5493]. It consists of three essentia l proteins, DctP, a periplasmic C-4-dicarboxylate-binding receptor. and two integral membrane proteins, DctM and DctQ, which probably span the membran e 12 and 4 times, respectively. Homologues of DctM, DctP and DctQ were iden tified in all major bacterial subdivisions as well as in archaea. An orphan DctP homologue in the Gram-positive bacterium Bacillus subtilis may serve as a receptor for a two-component transcriptional regulatory system rather than as a constituent of a TRAP-T system. Phylogenetic data suggest that al l present day TRAP-T systems probably evolved from a single ancestral trans porter with minimal shuffling of constituents between systems. Homologous T RAP-T constituents exhibit decreasing degrees of sequence identity in the o rder DctM > DctP > DctQ. DctM appears to belong to a large superfamily of t ransporters, the ion transporter (IT) superfamily, one member of which can function by either protonmotive force- or ATP-dependent energization. It is proposed that IT superfamily members exhibit the unusual capacity to funct ion in conjunction with auxiliary proteins that modify the transport proces s by providing (i) high-affinity solute reception, (ii) altered energy coup ling and (iii) additional yet to be defined functions.