Figure 6

a. Expression of ArhGAP9 R246, 247A mutant disrupts stress fibres in Swiss 3T3 fibroblasts. Swiss 3T3 cells were microinjected with full-length ArhGAP9 (wild type or the R246, 247A (RR) mutant) together with GFP-actin. The cells were imaged for GFP fluorescence. b. Proposed mechanism of negative regulation of MAP kinase by ArhGAP9. ArhGAP9 contains a WW domain which possesses a basic di-Arginine motif while MAP kinase (MAPK) contains a Common Docking (CD) domain that contains conserved acidic residues. In quiescent state, ArhGAP9 interacts with MAPK through electrostatic interaction between the complementary basic and acidic residues in the WW and CD domains, respectively thus blocking the access of MAPK by other docking proteins and negatively regulating MAPK activation. In the induced state, the presumable increase in local concentration of active upstream MAPK kinase (MAPKK) displaces ArhGAP9 by docking onto CD domain of MAPK, causing the diminished binding of ArhGAP9 to MAPK. The interaction between MAPK and MAPKK results in the phosphorylation of MAPK in the kinase activation loop to activate the latter.