CONSTITUTIVE EFFECTOR REGION ON THE C-TERMINAL SIDE OF SWITCH-I OF THE RAS PROTEIN

The ''switch I'' region (Asp(30)-Asp(38)) of the Ras protein takes rem arkably different conformations between the GDP- and GTP-bound forms a nd coincides with the so-called ''effector region.'' As for a region o n the C-terminal side of switch I, the V45E and G48C mutants of Pas fa iled to promote neurite outgrowth of PC12 cells (Fujita-Yoshigaki, J., Shirouzu, Pd, Koide, H., Nishimura, S., and Yokoyama, S. (1991) FEBS Lett 294, 187-190). In the present study, we performed alanine-scannin g mutagenesis within the region Lys(42)-Ile(55) Of Pas and found that the K42A, I46A, G48A, E49A, and L53A mutations significantly reduced t he neurite-inducing activity. This is an effector region by definition , but its conformation is known to be unaffected by GDP --> GTP exchan ge. So, this region is referred to as a ''constitutive'' effector (E(c )) region, distinguished from switch I, a ''switch'' effector (E(s)) r egion. The E(c) region mutants exhibiting no neurite-inducing activity were found to be correlatably unable to activate mitogen-activated pr otein (MAP) kinase in PC12 cells. Therefore, the E(c) region is essent ial for the MAP kinase activation in PC12 cells, whereas mutations in this region only negligibly affect the binding of Ras to Raf-1 (Shirou zu, M., Koide, H., Fujita-Yoshigaki, J., Oshio, Il., Toyama, Y., Yamas aki, H., Fuhrman, S. A., Villafranca, E., Kaziro, Y., and Yokoyama, S. (1994) Oncogene 9, 2153-2157).