Structural organization and regulation of the guanine nucleotide exchange factor C3G

Abstract

C3G is a guanine nucleotide exchange factor (GEF) for some members of the Ras family of GTPases including Rap1 and R-Ras. C3G is involved in the regulation of a wide range of cellular processes such as proliferation, differentiation, cytoskeletal remodelling, transformation, and apoptosis. C3G (120 kDa) is a tripartite protein according to its structural and functional features: (i) The N-terminal part contains an α-helical rich region that binds to the cytoplasmic tail of E-cadherin. (ii) The central segment contains five Pro-rich sequences (P0-P4) that mediate binding to the SH3 domains of at least Crk, p130Cas, Grb2, Hck and c-Abl proteins. Furthermore Tyr 504 in this region is phosphorylated by Src-family kinases during activation of C3G. (iii) The GEF activity of C3G lies in the C-terminal third, which consists of a Ras Exchange Motif (REM) and a catalytic Cdc25H domain. Here we show that C3G adopts a closed conformation stabilized by a head-tail interaction. We have identified some residues involved in this intramolecular interaction. Other GEFs of the Cdc25H family are autoinhibited by analogous head-tail interactions. We are currently analyzing the relationship between the conformational state of C3G and its GEF activity. In this context we are also analyzing the effect of post-transductional modifications on the structural organization of C3G and its GEF catalytic activity.