Part of a putrescine export transport system, does not play a role in resistance to antimicrobial peptides. Single mutation, decreased extracellular putrescine, in an sapBCDF operon deletion no change in resistance to antimicrobial peptide LL-37. Operon deletion has no effect on growth, but if the putrescine importer PuuP is also deleted then growth is slower. Belongs to the binding-protein-dependent transport system permease family. OppBC subfamily.