SYNERGISTIC ACTIVATION BY RAS AND 14-3-3-PROTEIN OF A MITOGEN-ACTIVATED PROTEIN-KINASE KINASE KINASE NAMED RAS-DEPENDENT EXTRACELLULAR SIGNAL-REGULATED KINASE KINASE STIMULATOR

We have identified, in Xenopus oocyte cytosol, a protein kinase named REKS (Ras-dependent extracellular signal-regulated kinase (ERK)/mitoge n-activated protein kinase kinase (MER) stimulator), which phosphoryla tes and activates recombinant ERK2 through recombinant MEK in a recomb inant GTP gamma S (guanosine 5'-(3-O-thio)triphosphate)-Ras-dependent manner. We show here that this REKS activity is synergistically enhanc ed by a combination of mammalian recombinant GTP gamma S-Ki-Ras and 14 -3-3 protein purified from rat brain. 14-3-3 protein is known to activ ate tyrosine and tryptophan hydroxylases, to modulate the protein kina se C activity, to stimulate secretion, and to show phospholipase A(2) activity per se. 14-3-3 protein did not affect the MEK activity. 14-3- 3 protein neither interacted with Ki-Ras nor affected the neurofibromi n activity to stimulate the GTPase activity of Ki-Ras under the condit ions where the recombinant N-terminal fragment of c-Raf-1 inhibited it . These results suggest that 14-3-3 protein has an additional function in the regulation of the Ras-MEK-ERK cascade pathway through the acti vation of REKS.