Part of the tripartite ATP-independent periplasmic (TRAP) transport system DctPQM involved in C4-dicarboxylates uptake (By similarity). Required for the utilization of succinate, fumarate, L-malate and alpha-ketoglutarate (PubMed:29769716). Binds succinate and malate (PubMed:25540822). The complex comprises the extracytoplasmic solute receptor protein DctP, and the two transmembrane proteins DctQ and DctM. Belongs to the bacterial solute-binding protein 7 family.