Fig. 1.

Postulated mechanisms of EPAC activation and associated biological functions. Under the G-protein-coupled receptor (GPCR) stimulation, adenylate cyclases (ACs) convert adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP). The excessive cAMP can be degraded into 5′-AMP by phosphodiesterases (PDEs). The binding of cAMP to inactive EPAC leads to the activation of EPAC, which facilitates the exchange of guanosine diphosphate (GDP) to guanosine triphosphate (GTP) and controls Rap-mediated biological functions. Meanwhile, Rap-GTPase-activating proteins (Rap-Gap) facilitate the intrinsic GTPase activity of Rap to breakdown GTP into GDP and phosphorus inorganic (Pi).