Display of 3-D variability in the SP4 dataset. (A) Side view of 11 (from a total of 20) structures generated from one of the principal components from the 3-D variability analysis. This series captures the conformational transition from the C2 symmetric conformation to the asymmetric conformation with a bent TMD. CTDs from each structure have been aligned. Helices in the TMD are rainbow colored from blue to red, the CTD is gray, and the Fab chains are orange and red. (B) Front view of the same 11 structures showing the bending of the M2 helix (cyan indicated by arrow) during the conformational transition. (C) View from the cytoplasm toward Zn site A for a subset of five structures from the 3-D variability analysis illustrating the bending of M2 (cyan) and M5 (orange) during the conformational transition. (D) Plots of distances between C_α atoms from Leu154 and Leu199 on the cytoplasmic side of the TMD and between Ala43 and Ala185 on the periplasmic side for each of the components derived from 3-D variability analysis of the SP4 dataset. These plots are analogous to those in Fig. 8. Evidence of TM gate closure can be seen in components 1 and 2.