PDE5 is converted to an activated state upon cGMP binding to the GAF A domain

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Abstract
CGMP‐specific, cGMP‐binding phosphodiesterase (PDE5) regulates such physiological processes as smooth muscle relaxation and neuronal survival. PDE5 contains two N‐terminal domains (GAF A and GAF B), but the functional roles of these domains have not been determined. Here we show that recombinant PDE5 is activated directly upon cGMP binding to the GAF A domain, and this effect does not require PDE5 phosphorylation. PDE5 exhibited time‐ and concentration‐dependent reversible activation in response to cGMP, with the highest activation (9‐ to 11‐fold) observed at low substrate concentrations (0.1 μM cGMP). A monoclonal antibody directed against GAF A blocked cGMP binding, prevented PDE5 activation and decreased basal activity, revealing that PDE5 in its non‐activated state has low intrinsic catalytic activity. Activated PDE5 showed higher sensitivity towards sildenafil than non‐activated PDE5. The stimulatory effect of cGMP binding on the catalytic activity of PDE5 suggests that this mechanism of enzyme activation may be common among other GAF domain‐containing proteins. The data also suggest that development of agonists and antagonists of PDE5 activity based on binding to this site might be possible.