Chloride ion trapped by three Lys139 within a large water cavity at the bottom center of the pre-pore Cry4Aa trimer. (a) side view, and (b) magnified view of three α3-α4 hairpins showing the water cavity in Cry4Aa trimer. The three hairpins are depicted as ribbons and colored blue, red, and green. Chloride ion is rendered as a cyan sphere. Three Lys139 and water are rendered as blue and yellow sticks, respectively.