Collybistin (CB)

ARHGEF9 belongs to a family of Rho-like GTPases that act as molecular switches by cycling from the active GTP-bound state to the inactive GDP-bound state.ARHGEF9 contains an N-terminal SRC homology 3 (SH3) domain, followed by tandem DBL homology (DH) and pleckstrin homology (PH) domains and a C-terminal proline-rich sequence. Northern blot analysis detected a transcript of about 6 kb expressed almost exclusively in brain. ARHGEF9, expressed in COS-7 cells, showed an apparent molecular mass of about 70 kD.ARHGEF9 specifically activates CDC42 and not Rac or RhoA. Ectopic expression in mouse fibroblasts revealed a Cdc42 phenotype featuring filopodia formation, followed by cortical actin polymerization and cell rounding.