Oligopeptide permeases (Opp) have been identified in numerous gram-negative and -positive bacteria. These transport systems belong to the superfamily of highly conserved ATP-binding cassette transporters. Typically, Opp importers comprise a complex of five proteins. The oligopeptide-binding protein OppA is responsible for the capture of peptides from the external medium. Two integral highly hydrophobic membrane spanning proteins, OppB and OppC, form a channel through the membrane used for peptide translocation. This N-terminal domain appears to be the first TM domain of the molecule [1].

1: Membrane topology of the integral membrane components, OppB and OppC, of the oligopeptide permease of Salmonella typhimurium. Pearce SR, Mimmack ML, Gallagher MP, Gileadi U, Hyde SC, Higgins CF; Mol Microbiol. 1992;6:47-57. PMID:1738314