Figure 2.

(A–C) Spectral cutouts of 2D ¹³C-¹³C correlation (A and C) and ¹⁵N-¹³C (B) correlation spectra recorded in the presence of 150 mM KCl at pH 4.0 in different anionic phospholipid environments. KcsA was reconstituted into proteoliposomes composed of DOPC/DOPG, DOPC/DOPA, or DOPC/CL) at a 3:7 molar ratio. Data obtained using CL-rich lipids (pH 4.0) are shown in green. ssNMR spectra recorded for DOPC/DOPG (data not shown) and DOPC/DOPA lipids (pH 4.0, red) were identical and are denoted by PA/PG for brevity. As a reference, data obtained using asolectin at pH 7.0 or pH 4 are indicated in black and blue, respectively. Specific residues for each state are color-coded correspondingly. Experimental data were recorded at magnetic fields corresponding to a 500–700 MHz ¹H frequency and at an effective sample temperature of 273 K. (D) Cα chemical-shift changes of the selectivity filter in the collapsed state of PA, PG, and CL (left), and the conductive SF conformation (denoted by 2) appearing for CL-rich membranes at pH 4.0 (middle) and pH 3.5 (right). The color-coding refers to differences in Cα chemical-shift values measured for the indicated KcsA lipid preparations and pH values relative to ssNMR shifts obtained for the conductive state of KcsA at pH 7 in asolectin lipids (26). To see this figure in color, go online.