Suppressor Scanning at Positions 177 and 236 in the Escherichia coli Lactose/H+ Cotransporter and Stereotypical Effects of Acidic Substituents That Suggest a Favored Orientation of Transmembrane Segments Relative to the Lipid Bilayer

摘要

Acidic substituents for Ala-177 (helix 6) or Tyr-236 (helix 7) in LacY cause effects on sugar recognition and cosubstrate coupling that are stereotypical of neutral substituents. Thus, helices 6 and 7 are probably oriented to produce little side-chain contact with the low dielectric lipid bilayer at positions 177 and 236.