Epac, Guanine-nucleotide-exchange factor for GTP-binding protein Rap
Mus musculus (house mouse)
Rap1 is a small GTPase that could suppress the oncogenic transformation of cells by Ras, and involved in cellular processes such as proliferation and cell differentiation. Several extracellular stimuli including cyclic AMP, are able to activate Rap1 by promoting its release of the guanine nucleotide GDP and its binding to GTP. Epac proteins (exchange proteins directly activated by cAMP) are guanine-nucleotide-exchange factors (GEFs) for the small GTP-binding proteins Rap1 and Rap2. Epac composed of N-terminal regulatory region and catalytic region. The regulatory region contains two cAMP-binding site (cNBD-A and cNBD-B) and a Dishevelled, Egl-10, Pleckstrin (DEP) domain, which is involved in membrane localization. This region is an auto-inhibitory domain that blocks GEF activity for Rap proteins. The catalytic domain contains a Ras-exchange motif (REM) domain, a Ras-association domain and a CDC25-homology domain (CDC25-HD).
The structure shown here is that of the Epac2 in the auto-inhibited state. The regulatory region sterically blocked binding of the Rap to the catalytic region. The regulatory domain and the catalytic domain made two loose contacts. The first one is the five strand, beta-sheet like tripartite structure formed by strands provided by cNBD-B and the REM domain and the loop of the helical hairpin of CDC25-HD, called "switchboard". The second one is an ionic latch formed between charged residues of the CDC25-HD and cNBD-B. The large number of main-chain interaction suggests the rigidity of the regulatory region. Access of the Rap1 to catalytic region become possible by rigid body movement of the regulatory region facilitated by two hinge-like loose connection, not by the structural change of the regulatory region upon cAMP-binding.
The authors of this paper suggest that the beta-strand at the C terminus of the cNBD-B domain serve as the lid to cover bound cAMP, and Movement of the second cNBD towards this lid would uncover the Rap binding site in CDC25-HD.
Protein Data Bank (PDB)
author: Miho Higurashi