Dissociation mechanism of GDP from Cdc42 via DOCK9 revealed by molecular dynamics simulations.

Cell division control protein 42 homolog (Cdc42) influences a variety of cellular responses such as cell migration and polarity. Deregulation of Cdc42 has been associated with several human diseases and developmental disorders. Over-activation of Cdc42 through guanine nucleotide exchange factor (GEF) is a critical event for Cdc42 involved cancer metastasis. Members of DOCK family of GEF are important activators of Cdc42. However, this activation mechanism is still unknown. Molecular dynamics (MD) simulations and molecular mechanics-Poisson-Boltzmann surface area (MM-PBSA) calculations were employed to investigate the central step of the activation of Cdc42: the dissociation mechanism of GDP from Cdc42 via DOCK9. Simulation results show that Mg2+ ion has a remarkable influence on the conformational change of switch I of Cdc42 through residue Pro34 which functions as a "clasp" to control the flexibility of switch I. In the GDP dissociation process, the Mg2+ ion leave first to result in a suitable conformation of Cdc42 for following DOCK9 binding to. When DOCK9 binds to Cdc42, it changes the orientations of residues Lys16, Thr17, Cys18 and Phe28 of Cdc42 to weaken the interactions between Cdc42 and GDP to release GDP. This study first elucidates the dissociation mechanism of GDP from Cdc42 via DOCK9 and identifies the essential residues of Cdc42 in this process. These simulation results are consistent with the recent findings of biochemical and amino acid mutational studies, and the observations are beneficial to understand the activation mechanism of Cdc42 and to provide insights for designing compounds targeting on Cdc42 related cancer metastasis.