Bacterial high affinity transport systems are involved in active transport of solutes across the cytoplasmic membrane. The protein components of these traffic systems include one or two transmembrane protein components, one or two membrane-associated ATP-binding proteins (ABC transporters; see <PDOC00185>) and a high affinity periplasmic solute-binding protein. The later are thought to bind the substrate in the vicinity of the inner membrane, and to transfer it to a complex of inner membrane proteins for concentration into the cytoplasm.

In gram-positive bacteria which are surrounded by a single membrane and have therefore no periplasmic region the equivalent proteins are bound to the membrane via an N-terminal lipid anchor. These homolog proteins do not play an integral role in the transport process per se, but probably serve as receptors to trigger or initiate translocation of the solute throught the membrane by binding to external sites of the integral membrane proteins of the efflux system.

In addition at least some solute-binding proteins function in the initiation of sensory transduction pathways.

On the basis of sequence similarities, the vast majority of these solute-binding proteins can be grouped [1] into eight families of clusters, which generally correlate with the nature of the solute bound.

Family 5 currently includes the following proteins:

• Periplasmic oligopeptide-binding proteins (gene oppA) of Gram-negative bacteria and homologous lipoproteins in Gram-positive bacteria (genes oppA, amiA or appA).
• Periplasmic dipeptide-binding proteins of Escherichia coli (gene dppA) and of Bacillus subtilis (gene dppE).
• Periplasmic murein peptide-binding protein of Escherichia coli (gene mppA).
• Periplasmic peptide-binding proteins sapA of Escherichia coli, Salmonella typhimurium and Haemophilus influenzae.
• Periplasmic nickel-binding protein (gene nikA) of Escherichia coli.
• Heme-binding lipoprotein (gene hbpA or dppA) from Haemophilus influenzae.
• Lipoprotein xP55 from Streptomyces lividans.
• Haemophilus influenzae hypothetical protein HI0213.
• Hypothethical protein y4tO from Rhizobium strain NGR234 symbiotic plasmid.
• Hypothethical protein y4wM from Rhizobium strain NGR234 symbiotic plasmid.

The signature pattern is located near the N-terminus of the mature proteins.