Proc. Natl. Acad. Sci. USA 109, 18613–18618 (2012)

Exchange protein directly activated by cAMP (EPAC), a guanine nucleotide exchange factor (GEF), is one of the targets directly downstream of the second messenger cAMP. Distinguishing signals mediated by EPAC from another target protein kinase A (PKA) is challenging. Tsalkova et al. now report a small-molecule inhibitor of EPAC2. The authors performed a targeted high-throughput screen, looking for compounds that inhibit EPAC GEF activity. Two (ESI-05 and ESI-07) of the seven hit compounds were selective for EPAC2 over EPAC1 in vitro and in cells, showing dose-dependent activity with apparent IC50 values in the low micromolar range. Neither compound inhibited PKA activity in vitro or in cells. To determine the molecular basis for the compounds' selectivity for EPAC2, the authors performed deuterium exchange MS, which allowed them to map regions of lower solvent exposure in response to ESI-07. These data in conjunction with published crystal structure data allowed them to propose a model where ESI-07 binds at the interface of the two cAMP-binding domains, locking EPAC2 in an autoinhibited conformation. This mechanism would not be effective for EPAC1 because it has only one cAMP-binding domain. Although the mechanistic details remain to be confirmed, ESI-05 and ESI-07 are the first chemical probes shown to be selective for EPAC2.