Activation mechanisms tested by global fitting to single-channel data. The mechanisms shown here are the most general forms of each of the 44 mechanisms we tested. In all schemes, the letter A denotes an agonist molecule and its subscript indicates the number of agonist molecules bound to the receptor. The letters R and R* denote resting shut states and open states of the receptor, respectively. (A) Sequential scheme with five binding steps and open states (nine “subset” schemes tested in all, with two to five binding steps, allowing opening either from each bound state or only from the one or two highest liganded states). (B) Jones and Westbrook–type scheme (Jones and Westbrook, 1995), with an open state and a distal desensitized state (indicated by D) for each binding step (five schemes tested with one to three binding steps, each leading to a desensitized state, allowing the channel to open only from the highest liganded states). (C) Flip-type scheme, where an intermediate flipped shut state (F) connects resting and open states (Burzomato et al., 2004; five schemes tested with one to five binding steps, a single flipped state, and a single open state; eight schemes tested, where the number of flipped and open states was either the same as the number of binding steps or smaller; three schemes tested, where, after the first binding step, the agonist can bind only to flipped states; three schemes with additional flipped states). (D) A subset of the primed mechanism (generalization of the flipped mechanism [Mukhtasimova et al., 2009]; 11 schemes tested with two to three binding steps and fewer open states than the maximum shown here). Several independent priming steps can occur. Here, the channel can open after one, two, or three priming steps (indicated as F, FF, or FFF).