Table 2.

Sectional resistances along the pore of Shaker variants

Variant	rC	RT	$R_T^S$	RAccIn	$R_{\mathit{AccIn}}^S$	RAccEx	RSucOc	RSucFr	Rf	RSucFr + Rf	%Occ	Rf/RT
	Å
WT	0.82	39	90	2.33	16.6	2.1	5.98	25.60	3.0	28.60	0.19	0.08
475Q	0.82	20	77	2.33	16.6	1.5	7.00	6.30	3.0	9.30	0.53	0.15
475D	1.64	6.6	16	1.16	8.30	2.0	0.37	0.07	3.0	3.07	0.84	0.45
BK	4.4a	4.0	8.0	0.29	2.07	0.9b	0.36	0.45	2.0c	2.44	0.45	0.50

Parameters of the in-series connected resistance model. The conduction pathway was divided in five resistors connected in-series (Fig. 7). Resistors are: the external access (R_AccEx), measured with Eq. 2; the internal access (R_AccIn), calculated from Eq. 3; the sucrose-accessible section (R_SucOc); the sucrose-free section (R_SucFr); and the selectivity filter (R_f). The capture radius, r_C, is expressed in Å. R_T is the total channel resistance in control, and $R_T^S$ is the total channel resistance after the addition of 2 M sucrose to the internal side. %Occ is the fraction occupied by the sucrose-accessible section, and R_f /R_T is the fraction of the total resistance contributed by the selectivity filter. Values for BK channels were calculated from data in Brelidze and Magleby (2005), who used 150-mM KCl solutions. All resistance values are given in GΩ.

^aThe radius of capture assumes intact charge rings and conductivity of a 150-mM KCl solution.

^bR_AccEx was calculated assuming r_C = 1.4 Å (Yellen, 1984).

^cR_f was set to two thirds of Shaker’s because of the use of 150 mM K⁺ instead of 100 mM K⁺, and conductance increases nearly linear with K⁺ concentration in that range.