Figure 1.

Electrical charge in the TM2 extracellular ring (position 20′) is the main determinant of Ca²⁺ permeability. The sum of the electrical charges associated with charged amino acids in eight different regions of each nAChR reported in Table 1 has been calculated and plotted against the Pf-value of the same nAChR. (A) extracellular domain; (B) intracellular domain between TM3 and TM4; (C) position {−5′ −4′} (where 0′ is the conserved lysine residue immediately preceding TM2, according to Miller, 1989); (D) position {−1′}; (E) position {19′}; (F) position {20′}; (G) position {21′–24′}, (H) position {25′–29′}. In each panel the angular coefficient m of the linear regression is reported, expressed in nuec⁻¹, as well as the R²-value, to evaluate the fit. Please note the significant linear correlations reported in (F) for position {20′}, and in (H) for position {25′–29′}, with the indicated p-values.