EPAC1, a cAMP-activated GEF for Rap GTPases, is a major transducer of cAMP signaling and a therapeutic target in cardiac diseases. The recent discovery that cAMP is compartmentalized in membrane-proximal nanodomains challenged the current model of EPAC1 activation in the cytosol. Here, we discover that anionic membranes are a major component of EPAC1 activation. We find that anionic membranes activate EPAC1 independently of cAMP, increase its affinity for cAMP by two orders of magnitude, and synergize with cAMP to yield maximal GEF activity. In the cell cytosol, where cAMP concentration is low, EPAC1 must thus be primed by membranes to bind cAMP. Examination of the cell-active chemical CE3F4 in this framework further reveals that it targets only fully activated EPAC1. Together, our findings reformulate previous concepts of cAMP signaling through EPAC proteins, with important implications for drug discovery.

EPAC1, a cAMP-activated GEF for Rap GTPases, is a major physiological transducer of cAMP signaling and a therapeutic target in cardiac diseases. The recent discovery that cAMP is compartmentalized in membrane-proximal nanodomains challenged the current model of EPAC1 activation in the cytosol. Here, we discover that anionic membranes are a major component of EPAC1 activation. We find that anionic membranes activate EPAC1 in the absence of cAMP, that they increase its affinity for cAMP by 2 orders of magnitude, and that they synergize with cAMP to yield maximal GEF activity. Thus, EPAC1 has four states that differ in their affinity for cAMP and GEF efficiency. In the cell cytosol, where cAMP is low, EPAC1 must thus be primed by membranes to bind cAMP. Examination of the cardiomyocyte-active chemical CE3F4 in this new framework further reveals that inhibition affects only membrane- and cAMP-activated EPAC1. Together, our findings reformulate previous concepts of cAMP signaling to include a hitherto overlooked role of membranes through EPAC proteins, with important implications for drug discovery.