Ras proteins have the capacity to bind to and activate at least three famil
ies of downstream target proteins: Raf kinases, phosphatidylinositol 3 (PI
3)-kinase, and Ral-specific guanine nucleotide exchange factors (Ral-GEFs),
We have previously shown that the Ras/Ral-GEF and Ras/Raf pathways oppose
each other upon nerve growth factor stimulation, with the former promoting
proliferation and the latter promoting cell cycle arrest. Moreover, the pat
hways are not activated equally. While the Ras/Raf/Erk signaling pathway is
induced for hours, the Ras/Ral-GEF/Ral signaling pathway is induced for on
ly minutes. Here we show that this preferential down-regulation of Ral sign
aling is mediated, at least in part, by protein kinase C (PKC), In particul
ar, we show that PKC activation by phorbol ester treatment of cells blocks
growth factor-induced Ral activation while it enhances Erk activation. More
over, suppression of growth factor-induced PKC activation enhances and prol
ongs Ral activation. PKC does not influence the basal activity of the Ral-C
EF designated Ral-GDS but suppresses its activation by Ras, Interestingly,
Ras binding to the C-terminal Ras binding domain of Ral-GDS is not affected
by PKC activity, Instead, suppression of Ral-GDS activation occurs through
the region N terminal to the catalytic domain, which becomes phosphorylate
d in response to phorbol eater treatment of cells. These findings identify
a role for PKC in determining the specificity of Ras signaling by its abili
ty to differentially modulate Ras effector protein activation.