Topological analysis of DctQ, the small integral membrane protein of the C4-dicarboxylate TRAP transporter of Rhodobacter capsulatus

Tripartite ATP-independent periplasmic ('TRAP') transporters are a novel gr oup of bacterial and archaeal secondary solute uptake systems which possess a periplasmic binding protein, but which are unrelated to ATP-binding cass ette (ABC) systems. In addition to the binding protein, TRAP transporters c ontain two integral membrane proteins or domains, one of which is 40-50 kDa with 12 predicted transmembrane (TM) helices, thought to be the solute imp ort protein, while the other is 20-30 kDa and of unknown function. Using a series of plasmid-encoded beta -lactamase fusions, we have determined the t opology of DctQ, the smaller integral membrane protein from the high-affini ty C4-dicarboxylate transporter of Rhodobacter capsulatus, which to date is the most extensively characterised TRAP transporter. DctQ was predicted by several topology prediction programmes to have four TM helices with the N- and C-termini located in the cytoplasm. The levels of ampicillin resistanc e conferred by the fusions when expressed in Escherichia coli were found to correlate with this predicted topology. The data have provided a topologic al model which can be used to test hypotheses concerning the function of th e different regions of DctQ and which can be applied to other members of th e DctQ family. (C) 2001 Federation of European Microbiological Societies. P ublished by Elsevier Science B.V. All rights reserved.