Fig. 2. (A) Structure of the MscL channel in the open state viewed from the extracellular side. (B) Side views of the transmembrane domains TM1 and TM2 of the open channel. The inset shows the structure of the channel monomer. Modified with permission from Nature (Perozo et al., 2002a). (C) Assymetry of the tension gradient across the lipid bilayer is required for MscL opening. Changes in the spectral line shape from position Ile24 obtained by the EPR spectroscopy were used to monitor the influence of different bilayer tension gradients produced by different lipid environments on the conformation of MscL: symmetric phosphatidylcholine (PC18; closed), asymmetric lysophosphatidylcholine (LPC; open), detergent solution (closed), and symmetric LPC (closed). Note, narrowing of the spectral line after addition of LPC to one monolayer indicates a large increase in spin probe mobility together with complete elimination of any intersubunit spin-spin interaction, which is characteristic of the open channel. Reproduced with permission from Nature (Perozo et al., 2002b).