Part of the tripartite ATP-independent periplasmic (TRAP) transport system DctPQM involved in C4-dicarboxylates uptake. 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 TRAP transporter large permease family.