Part of the tripartite ATP-independent periplasmic (TRAP) transport system DctPQM involved in C4-dicarboxylates uptake. Binds C4-dicarboxylates such as fumarate, succinate, L-malate and D-malate. The complex comprises the extracytoplasmic solute receptor protein DctP, and the two transmembrane proteins DctQ and DctM. Deletion mutant is unable to transport succinate, and does not grow on D-malate, L-malate, succinate or fumarate as the sole carbon source under aerobic conditions in the dark. Belongs to the bacterial solute-binding protein 7 family.