Both on- and off-pathway mutations increase unliganded gating. (A) α-subunit interface and TMD of the Torpedo AChR (Protein Data Bank accession no. 2bg9; Unwin, 2005). The putative priming pathway is dark gray (see Fig 1 A). The mutated residues were in αM1, αM2, αM3, and αM4. (B) Example unliganded currents and open-interval histograms (the background was βδ9′S; arrow marks the time constant of brief component; see Fig. 2 A). All of the mutations made the brief openings longer-lived. The gating equilibrium constants are given in Table 3.