FIGURE 3.

Quenching of KcsA fluorescence in mixtures with phosphatidylserine. KcsA was reconstituted into bilayers containing mixtures of BrPC and DOPS (□), BrPS and DOPC (○), and DOPC and DOPS (▵). Fluorescence intensities are expressed as F/F_o where F_o is the fluorescence intensity in the nonbrominated lipid. In A, the solid lines for the BrPC/DOPS and BrPS/DOPC experiments show fits to Eq. 2 giving the values for relative binding constants listed in Table 2. The broken line shows an attempt to fit the data for the BrPS/DOPC experiment to the binding constant determined from the BrPC/DOPS experiment, as described in the text. In B, the solid line for the BrPC/DOPS experiment shows a fit to Eq. 2 giving the annular binding constant and the lines for the BrPS/DOPC experiments show fits to the annular/non-annular binding site model, assuming (solid line) that all the fluorescence can be quenched from the non-annular sites (Eq. 5), and (broken line) that only 60% of the fluorescence can be quenched from the non-annular sites (Eq. 6). Values for the binding constants are listed in Table 3. The buffer was 20 mM HEPES and 1 mM EGTA at pH 7.2.