Figure 4.

Comparative S672R versus WT analysis of HCN4 CBD dynamics. Order parameter (S²) distribution for WT (black) and S672R (red) in the absence (A) or presence of saturating amounts of cAMP (B). Data were fit with either one (solid line) or two (dashed line) Gaussian distributions and as summarized in supplemental Table S1. C–J, two-dimensional reduced spectral density plots for the apo-WT (black) and S672R (red) HCN4 CBD. The following four regions were probed: the β2-β3 loop (aa 621–636), BBR (aa 639–652), extended PBC (aa 659–676), and Lid region (aa 699–724), for both apo (top row) and cAMP-bound (bottom row) samples. The Lid is further dissected into the Lid and CTL in supplemental Fig. S6. The solid black line represents the expected spectral densities for an isotropic rigid rotor, and the black and red ellipsoids are centered on the average spectral density values with axes defined by ±2 S.D. for the WT and mutant, respectively. K, map of apo residues with S672R versus WT enhanced picosecond to nanosecond (red spheres and surface) and microsecond to millisecond (orange spheres and surface) dynamics. L, map of apo residues with S672R versus WT quenched picosecond to nanosecond (blue sphere and surface) and microsecond to millisecond (cyan sphere and surface) dynamics. M and N, as in K and L except for the cAMP-bound sample.