mppA (Escherichia coli K12): Murein tripeptide (L-ala-gamma-D-glutamyl-meso-DAP) transporter subunit; Essential for the uptake of the murein peptide L-alanyl- gamma-D-glutamyl-meso-diaminopimelate. Also transports some alpha- linked peptides such as Pro-Phe-Lys with low affinity. The transport is effected by the oligopeptide permease system; Belongs to the bacterial solute-binding protein 5 family.