Binding of cyclic adenosine monophosphate (cAMP) to the guanine nucleotide exchange factor Epac1 triggers its translocation to the plasma membrane, where it activates Rap, a guanosine triphosphatase (GTPase) that promotes cell adhesion. A yeast two-hybrid screen and coimmunoprecipitation assays performed by Gloerich et al. identified RanBP2, a cytosolic component of the nuclear pore complex, as a binding partner for Epac1, an interaction that required the zinc finger domains of RanBP2. In Ovcar3 cells, endogenous Epac1 colocalized with endogenous RanBP2 at the nuclear envelope, a localization pattern that was abolished by depletion of RanBP2 by siRNA. The interaction between exogenous Epac1 and endogenous RanBP2 remained intact in cells stimulated with a cAMP analog. In cells actively undergoing mitosis, RanBP2 is located at kinetochores, whereas Epac1 is found at the mitotic spindle and centrosomes. The authors found that the association of Epac1 with RanBP2 was decreased in mitotically arrested U2OS cells because of phosphorylation of the zinc finger domains of RanBP2. Treatment of cells with okadaic acid to inhibit phosphatases decreased the amount of Epac1 at the nuclear envelope. RanBP2 interacted with the CDC25 homology domain of Epac1, the region of the protein that catalyzes the nucleotide exchange of Rap, and in vitro assays indicated that a construct comprising the zinc finger domains of RanBP2 inhibited the ability of Epac1 to activate Rap1B. In Ovcar3 cells treated with isoproterenol to stimulate the generation of cAMP through the β-adrenergic receptor, depletion of RanBP2 increased the amount of Rap1-GTP and adhesion to fibronectin. Thus, the authors propose that RanBP2 sequesters and inactivates Epac1 at the nuclear envelope.
M. Gloerich, M. J. Vliem, E. Prummel, L. A. T. Meijer, M. G. A. Rensen, H. Rehmann, J. L. Bos, The nucleoporin RanBP2 tethers the cAMP effector Epac1 and inhibits its catalytic activity. J. Cell Biol. 193, 1009–1020 (2011). [Abstract] [Full Text]