[Analysis of the cellular functions of the small GTP-binding protein rho p21 with Clostridium botulinum C3 exoenzyme].

rho p21 is a member of the ras superfamily of small GTPases. Clostridium botulinum C3 exoenzyme ADP-ribosylates rho p21 at the Asp41 residue located at an effector domain and inhibits its biological activity by interfering with the interaction with its downstream effectors. The amount of rho p21 in cells or tissues is determined by the in vitro ADP-ribosylation reaction with C3 exoenzyme and 32P NAD. The studies using C3 exoenzyme have revealed that rho p21 is involved in the regulation of stress fiber formation, cell adhesion, contractile ring formation during cytokinesis and serum response factor-mediated activation of immediate early genes. C3 exoenzyme is a valuable tool for elucidating the unidentified function of rho p21 because the exoenzyme specifically inhibits rho p21-mediated signal transduction pathways. A Glu173 substitution mutant of the C3 exoenzyme lacking ADP-ribosyltransferase activity is useful for a control experiment.