Functional Characterization of Amphipathic alpha-Helix in the Osmoregulatory ABC Transporter OpuA

The ATP-binding-cassette transporter OpuA from Lactococcus lactis is composed of two ATPase subunits (OpuAA) and two subunits (OpuABC) with the transmembrane domain fused to an extracellular substrate-binding protein. Of the almost 1900 homologues of OpuA known to date, a subset has an amino-terminal amphipathic helix (plus extra transmembrane segment) fused to the core of the transmembrane domain of the OpuABC subunit. FRET measurements indicate that the amphipathic alpha-helix is located close to the membrane surface, where its hydrophobic face interacts with the transport protein rather than the membrane lipids. Next, we determined the functional role of this accessory region by engineering the amphipathic alpha-helix. We analyzed the consequence of the mutations in intact cells by monitoring growth and transport of glycine betaine under normal and osmotic stress conditions. More detailed studies were performed in hybrid membrane vesicles, proteoliposomes, and bilayer nanodisks. We show that the amphipathic alpha-helix of OpuA is necessary for high activity of OpuA but is not critical for the biogenesis of the protein or the ionic regulation of transport.