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. Disruption decreases growth on C4-dicarboxylates succinate, fumarate and L-malate as sole carbon sources, but does not affect growth on sialic acid. Belongs to the bacterial solute-binding protein 7 family. Was proposed to function as a mannose-sensitive hemagglutinin and a virulence factor, and to be involved in the utilization of sialic acid (PubMed:21873407). However, it was later shown to be probably part of a C4-dicarboxylate TRAP transporter (PubMed:21980116, PubMed:22556361).