Coevolving Residues Distant from the Ligand Binding Site are Involved in GAF Domain Function

Ligand binding to GAF domains regulates the activity of associated catalytic domains in a wide variety of proteins. For instance, cGMP binding to the GAFa domain of phosphodiesterase 5 (PDE5) activates the cGMP-hydrolyzing catalytic domain in the protein. However, the residues involved and the mechanism of GAF domain function are not entirely clear. Here, combining computational and experimental analysis, we show that two highly coevolving residues distant from the ligand binding site play a critical role in GAF domain allostery. Specifically, Statistical Coupling Analysis (SCA) of GAF domain sequences revealed the highest coevolution score for residues L267 and F295. Molecular dynamics (MD) simulations of both apo and holo forms of the wild type and mutant (L267A and F295A) PDE5 GAFa domains revealed significant alterations in structural dynamics and interaction with cGMP. Incorporation of the mutations in a Bioluminescence Resonance Energy Transfer (BRET)-based biosensor, which reports a ligand-induced conformational change, revealed a change in the conformation of the GAF domain and an increase in the EC ₅₀ of cGMP-induced conformational change. Similar results were obtained regarding cGMP-induced conformational change in the full-length PDE5 and in the fluorescence of the GAF domain fluorescent protein, miRFP670nano3. Finally, structural analysis of conformers observed in MD simulations revealed a possible mechanism underlying the impact of mutations of these two coevolving residues in the PDE5 GAFa domain. Our results provide insight into the role of distant, coevolving residues in GAF domain allostery, and may aid in understanding evolution of allostery in proteins.