Mutation of a core aromatic eliminates the long component of unliganded openings. (A; top) Single-channel openings from unliganded, constitutively active AChRs occur in clusters (open is down). The shut periods between clusters are times when all AChRs in the patch are desensitized. The background mutations were at the gate region (βδM2-9′S); the binding sites were WT. (Bottom) Patch open-interval duration histogram and example cluster show both brief and long openings. (B) Example clusters and open-interval duration histograms from AChRs having WT-binding sites plus different background mutations that increase the level of constitutive activity. Both brief and long openings are apparent. (C) Phe mutations of the core aromatic amino acids (Fig. 1 B) eliminate long openings but leave the brief ones nearly unchanged (see A for the activity with only background mutations present). Almost all mutations of these four residues, and only these, abolish long openings.