TABLE 2.

Lipid binding constants for W26,113L determined from fluorescence quenching plots

Lipid	Annular sites on extracellular side Relative binding constant	Nonannular sites Binding constant (mol fraction−1)	Annular sites on intracellular side Relative binding constant
Phosphatidic acid	0.80 ± 0.17	4.6 ± 2.1	2.93 ± 0.77
Phosphatidylglycerol	0.68 ± 0.07	3.0 ± 0.7	1.88 ± 0.10
Phosphatidylserine	0.94 ± 0.11	7.1 ± 2.7	0.70 ± 0.30
Cardiolipin	0.77 ± 0.10	7.3 ± 1.2	0.70 ± 0.16

Annular lipid binding constants relative to di(C18:1)PC on the extracellular side were determined by fitting quenching data for W26,113L to Eq. 4 using a value for n of 1.69. Nonannular lipid binding constants were determined by fitting quenching data for W26,113L to Eq. 9 using the values for the relative binding constant at the annular sites given in the table with a value for n of 1.69 for all lipids except phosphatidic acid for which n = 2.49 (15). Annular lipid binding constants relative to di(C18:1)PC on the intracellular side were determined by fitting quenching data for wild-type KcsA to Eq. 10. The mol fraction of cardiolipin was calculated on a chain basis, to account for the fact that cardiolipin contains four fatty acyl chains whereas phosphatidylcholine contains only two.