Fig. 2: tmFRET between the C-terminal end of the S4 helix and A′ or B′ helix of the C-linker.

a Cartoon showing tmFRET between W355Anap in the S4 helix and Cu²⁺-TETAC attached to L481C in the A′ helix. b Cartoon showing tmFRET between W355Anap and Cu²⁺-TETAC attached to E506C in the B′ helix. c Summary of the fraction of Anap fluorescence quenched by Cu²⁺-TETAC for the spHCN-W355Anap channels in panels a and b, without (left) and with (middle, right) the introduced cysteines L481C or E506C in four conditions: the presence and absence of cAMP, and at 0 and −100 mV; n = 3–5 independent patches. Data shown are mean ± s.e.m.; Source data in the forms of F_Cys, and F_{no Cys} are provided in the Source Data file. d FRET efficiencies of spHCN-W355Anap, L481C channels measured in four conditions: the presence and absence of cAMP, and at 0 and −100 mV; n = 3 independent patches for cAMP conditions and n = 5 independent patches for no cNMP conditions. **p < 0.001 for the statistical comparisons. e FRET efficiencies of spHCN-W355Anap, E506C channels; n = 3 patches for cAMP and n = 4 patches for no cNMP conditions. **p < 0.001 for the statistical comparisons. Data shown are mean ± s.e.m.; source data are provided in the Source Data file.