FIGURE 6.

Proposed mechanism of IgpB2 GEF function on RhoA. Crystal structures are shown on the left, and in a corresponding schematic drawing on the right hand side of each state. A, State 1: free RhoA·GDP·Mg²⁺ (46) and free IpgB2. B, State 2: IpgB2·RhoA complex A, with Mg²⁺ coordinated to the β-phosphate of the GDP nucleotide. C, State 3: complex B, with Mg²⁺ coordinated to the α-phosphate. D, State 4: proposed architecture of the nucleotide-free IpgB2·RhoA complex. The conformation of GDP-free RhoA was obtained from the complex of RhoA with the DH/PH fragment of PDZ-RhoGEF (52). Switch I and switch II of RhoA and the catalytic loop of IpgB2 are shown as yellow, green, and red schematics, respectively. GDP is shown as orange balls and sticks, and the Mg²⁺ ion and water molecules are yellow and red spheres, respectively. In C and D, the complex of (nucleotide-free) Map·Cdc42 (24) is shown in white for comparison. All GEF and GTPase structures were individually superposed with the corresponding structures of complex A as shown in B.