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 through 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 1 currently includes the following proteins:

• Periplasmic maltose/maltodextrin-binding proteins of Enterobacteriaceae (gene malE) and homologous lipoprotein in Streptococcus pneumoniae (gene malX).
• Periplasmic multiple oligo-saccharide binding protein of Streptococcus mutans (gene msmE), which is involved in the uptake of melibiose, raffinose and isomaltotriose.
• Periplasmic glycerol-3-phosphate-binding protein of Escherichia coli, a component of the phosphate-limitation inducible uptake system for sn-glycerol-3-phosphate and glycerophosphoryl diesters.
• Periplasmic iron-binding protein from Serratia marcescens (gene sfuA) and the homologous proteins (gene fbp) from Haemophilus influenzae and Neisseria, which are part of the iron-acquisition system.
• Periplasmic thiamine-binding protein (gene tbpA) from Escherichia coli and Haemophilus influenzae.

The signature pattern for this family is located in the central section of the mature proteins.