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

Microbiology (Reading). 1999 Dec:145 ( Pt 12):3431-3445. doi: 10.1099/00221287-145-12-3431.


Tripartite ATP-independent periplasmic transporters (TRAP-T) represent a novel 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 C4-dicarboxylates [Forward, J. A., Behrendt, M. C., Wyborn, N. R., Cross, R. & Kelly, D. J. (1997). J Bacteriol 179, 5482-5493]. It consists of three essential proteins, DctP, a periplasmic C4-dicarboxylate-binding receptor, and two integral membrane proteins, DctM and DctQ, which probably span the membrane 12 and 4 times, respectively. Homologues of DctM, DctP and DctQ were identified 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 all present day TRAP-T systems probably evolved from a single ancestral transporter with minimal shuffling of constituents between systems. Homologous TRAP-T constituents exhibit decreasing degrees of sequence identity in the order DctM > DctP > DctQ. DctM appears to belong to a large superfamily of transporters, 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 function in conjunction with auxiliary proteins that modify the transport process by providing (i) high-affinity solute reception, (ii) altered energy coupling and (iii) additional yet to be defined functions.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Archaea / genetics*
  • Archaea / metabolism
  • Bacteria / genetics*
  • Bacteria / metabolism
  • Bacterial Proteins*
  • Biological Transport, Active
  • Carrier Proteins / chemistry
  • Carrier Proteins / genetics*
  • Carrier Proteins / metabolism*
  • Dicarboxylic Acid Transporters*
  • Evolution, Molecular
  • Ion Transport
  • Membrane Proteins / chemistry
  • Membrane Proteins / genetics
  • Membrane Proteins / metabolism
  • Membrane Transport Proteins*
  • Molecular Sequence Data
  • Periplasmic Proteins*
  • Phylogeny*


  • Bacterial Proteins
  • Carrier Proteins
  • DctM protein, Rhodobacter capsulatus
  • DctQ protein, Rhodobacter capsulatus
  • Dicarboxylic Acid Transporters
  • Membrane Proteins
  • Membrane Transport Proteins
  • Periplasmic Proteins
  • DctP protein, Rhodobacter capsulatus