<p>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 <db_xref db="INTERPRO" dbkey="IPR003439"/>) and a high affinity periplasmic solute-binding protein. The latter 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.</p><p>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 homologue 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.</p><p>In addition, at least some solute-binding proteins function in the initiation of sensory transduction pathways.</p><p>On the basis of sequence similarities, the vast majority of these solute-binding proteins can be grouped [<cite idref="PUB00003610"/>] into eight families or clusters, which generally correlate with the nature of the solute bound.</p><p>Family 3 groups together specific amino acids and opine-binding periplasmic proteins and a periplasmic homologue with catalytic activity.</p>