title: Identification of tyrosine residues critical for the function of an ion-coupled multidrug transporter.
abstract: Aromatic residues may play several roles in integral membrane proteins, including direct interaction with substrates. In this work, we studied the contribution of tyrosine residues to the activity of EmrE, a small multidrug transporter from Escherichia coli that extrudes various drugs across the plasma membrane in exchange with protons. Each of five tyrosine residues was replaced by site-directed mutagenesis. Two of these residues, Tyr-40 and Tyr-60, can be partially replaced with hydroxyamino acids, but in the case of Tyr-40, replacement with either Ser or Thr generates a protein with modified substrate specificity. Replacement of Tyr-4 with either Trp or Phe generates a functional transporter. A Cys replacement at this position generates an uncoupled protein; it binds substrate and protons and transports the substrate downhill but is impaired in uphill substrate transport in the presence of a proton gradient. The role of these residues is discussed in the context of the published structures of EmrE.
authors: Alexander Silberman Institute of Life Sciences, Hebrew University of Jerusalem, 91904 Jerusalem, Israel.
authors: Dvir
authors: D
authors: Rotem
authors: Sonia
authors: S
authors: Steiner-Mordoch
authors: Shimon
authors: S
authors: Schuldiner