Bacterial high affinity transport systems are involved in active transport of solutes across the cytoplasmic membrane. Most of the bacterial ABC (ATP-binding cassette) importers are composed of one or two transmembrane permease proteins, one or two nucleotide-binding proteins and a highly specific periplasmic solute-binding protein. In Gram-negative bacteria the solute-binding proteins are dissolved in the periplasm, while in archaea and Gram-positive bacteria, their solute-binding proteins are membrane-anchored lipoproteins [1, 2].

On the basis of sequence similarities, the vast majority of these solute-binding proteins can be grouped [3] into eight families or clusters, which generally correlate with the nature of the solute bound. This entry represents a domain found in the solute-binding protein family 5.

Family 5 members include:

• Periplasmic oligopeptide-binding proteins (oppA) of Gram-negative bacteria and homologous lipoproteins in Gram-positive bacteria (oppA, amiA or appA)
• Periplasmic dipeptide-binding proteins of Escherichia coli (dppA) and Bacillus subtilis (dppE)
• Periplasmic murein peptide-binding protein of E. coli (mppA)
• Periplasmic peptide-binding proteins sapA of E. coli, Salmonella typhimurium and Haemophilus influenzae
• Periplasmic nickel-binding protein (nikA) of E. coli
• Haem-binding lipoprotein (hbpA or dppA) from H. influenzae
• Lipoprotein xP55 from Streptomyces lividans
• Hypothetical proteins from H. influenzae (HI0213) and Rhizobium sp. (strain NGR234) symbiotic plasmid (y4tO and y4wM)
• HTH-type transcriptional regulator SgrR from E. coli. The solute-binding domain is localised in its C-terminal [4].

1. Sequence relationships between integral inner membrane proteins of binding protein-dependent transport systems: evolution by recurrent gene duplications. Protein Sci. 3, 325-44
2. Characterization of a Pseudomonas putida ABC transporter (AatJMQP) required for acidic amino acid uptake: biochemical properties and regulation by the Aau two-component system. Microbiology (Reading, Engl.) 154, 797-809
3. Structural, functional, and evolutionary relationships among extracellular solute-binding receptors of bacteria. Microbiol. Rev. 57, 320-46
4. The novel transcription factor SgrR coordinates the response to glucose-phosphate stress. J. Bacteriol. 189, 2238-48

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