Data and refinement statistics.

◊Rsym = ΣhΣi|I(h) - I(h)i|IΣhΣi I(h)i, where I(h) is the mean intensity after rejections.§Numbers in parentheses correspond to the highest resolution shell of data, which were 2.98 to 2.90 for the cGMP, 2.53 to 2.49 Å for the cAMP and 2.85 to 2.75Å for APO.||Rwork = Σh||Fobs(h)| -|Fcalc(h)||IΣh|Fobs(h)|; no I/σ cutoff was used during refinement.¶5.0% of the observed intensities was excluded from refinement for cross validation purposes.