Extrapolated expansion of the transmembrane barrel. Wire representations of the TM3 barrel in the initial (A) and expanded (B) conformations, with the side and top views. The gate position is represented by α carbons of residues L105 and L109 (balls). In B, 10 similar conformations obtained in independent extrapolated trajectories are shown as a bundle (thin blue lines) around the consensus conformation (thicker tube) closely retracing the average backbone position. Expansion trajectories presented in the coordinates of TM3a helical tilt versus pore diameter (C). Helical tilt was defined as the angle between the pore axis and TM3a helices, measured for the backbone of all residues between V96 to N112. The average surface-to-surface distance at the level of the gate was taken as the constriction diameter. The initial position is denoted with square (around the 7-Å 25° point); the groups of trajectories obtained with the amplification coefficients of 1 and 1.05 are show in blue and red, respectively. The rectangle around the 15-Å, 30° point designates the group of open-like states that satisfy the experimental restraints on the channel geometry. An in-plane expansion area versus conductance plot for nine consecutive conformations from a typical expansion trajectory is shown in D. The green rectangle shows the range for experimentally observed parameters within 10% deviation.