Figure 4.

Quantification of the effect on CSI caused by single- and double-point mutations in S4S5_N and S6 of monomeric Kv4.2 constructs. The mutation-induced changes in apparent affinity for a transitory state (left, open bars) and steady-state inactivation (right, solid bars) are expressed as ln (K_trans/K_trans,wt) and ln (K_steady/K_steady,wt), respectively (green, S4S5_N single mutations; blue, S6 single mutations; red, S4S5_N/S6 double-point mutations). Positive (i.e., mutation-induced increases in K) and negative (i.e., mutation-induced decreases in K) values indicate a decreased and increased likelihood, respectively, for entering the respective closed-inactivated state. The data for individual S4S5/S6 coupling pairs are grouped in bar triplets to illustrate additivity or nonadditivity. (A–C) Separate graphs show the data for the S4S5/S6 coupling pairs with the S4S5_N residues G309 (A), R311 (B), and L313 (C). Inclusion of data with ss = 10⁻⁶ (three of four experiments with [313:400]; see Materials and Methods) resulted in a mean ln (K_steady/K_steady,wt) value of −18.4 kT. Note that the mutation-induced effects on K_trans and K_steady are qualitatively identical (except for [313:407], asterisk). X: double-exponential fitting was not possible, and therefore K_trans data are not available for [311:407] and [311:408] (see Materials and Methods).