Part of the ABC transporter DppABCDF involved in dipeptide transport (PubMed:1702779, PubMed:7536291, PubMed:10537211). Binds dipeptides and accepts a wide range of side chains, including small neutral, bulky hydrophobic, and positively and negatively charged groups (PubMed:10537211). Tripeptides are poor substrates (PubMed:10537211). DppA alone controls the specificity of the Dpp transporter (PubMed:10537211). In addition, plays a role in chemotaxis toward peptides via interaction with the chemotaxis protein Tap (PubMed:3520334, PubMed:8563629). Binds heme. When a foreign outer membrane heme receptor is expressed in E.coli, DppABCDF can also transport heme and its precursor, 5-aminolevulinic acid (ALA), from the periplasm into the cytoplasm. Heme binding is inhibited by dipeptide. The complex is composed of two ATP-binding proteins (DppD and DppF), two transmembrane proteins (DppB and DppC) and a solute-binding protein (DppA). Expression is repressed by the presence of casamino acids. Consists of two distinct domains (I and II) connected by two strands that presumably function as a hinge (PubMed:8527431, PubMed:8563629). Undergoes conformational change upon substrate binding (PubMed:10537211). Inactivation of the gene results in the inability of a proline auxotroph to utilize Pro-Gly as a proline source (PubMed:1702779). Inactivation of the gene has no effect on iron-heme utilization, but the double mppA dppA mutant is unable to use heme as iron source (PubMed:16905647). Belongs to the bacterial solute-binding protein 5 family.