Vulval precursor cells (VPCs) are patterned through graded action of EGF from the anchor cell (AC) in conjunction with LIN-12/Notch-mediated lateral signaling. Together these signals control the highly reproducible 3˚-3˚-2˚-1˚-2˚-3˚ pattern that develops to form the vulva. A key protein in this process is LET-60/Ras, which in mammals is the most mutated oncoprotein. EGFR promotes LET-60/Ras activation to induce 1˚ fate via the canonical Raf-MEK-ERK MAP kinase cascade. We previously demonstrated that LET-60/Ras also switches effectors to activate its effector RalGEF-Ral to induce 2˚ fate in support of Notch. In general, 1˚- and 2˚-promoting signals are strongly antagonistic. Thus, in equipotent cells Ras switches effectors between Raf and RalGEF-Ral to promote mutually antagonistic 1˚ and 2˚ cell fates, respectively. However, the Ral signaling cascade is poorly understood in all systems. We therefore screened candidate Ral binding partners from the literature. Two subunits of the Exocyst complex have been previously shown to bind Ral in other systems. We found that loss of Exo84 but not Sec5 caused the same phenotype as loss of Ral, suggesting that Ral binds to Exo84 to promote 2˚ fate. To move further downstream we analyzed a little known family of MAP4 kinases. In Drosophila Sec5 and Msn/MAP4K, the ortholog of C. elegans MIG-15, physically interact and promote JNK signaling, but function in opposition to Ral signaling. Consistent with the Drosophila genetics, our preliminary data in vulval development do not support the model that Sec5 and Msn function as a Ral effector. We hypothesize that the paralogous MAP4K subfamily, C. elegans GCK-2 and Drosophila Hppy, constitutes the real Ral effector. We found loss of GCK-2 phenocopies loss of Exo84, and GCK-2 functions cell autonomously downstream of Ral in vulva. Activated small GTPases often recruit effectors to membranes. Using CRISPR/Cas9, we tagged the N-terminus of endogenous GCK-2 with mNeonGreen. mNG::GCK-2 is localized in the cytoplasm of 1˚ and 2˚ cells, with perinuclear concentration. Thus, the mechanism by which Ral and Exo84 activate GCK-2 remains unclear. We will tag endogenous Ral with TagRFP to assess co-localization of mNG::GCK-2 and TagRFP::Ral in 1˚ vs. 2˚ before and after EGF induction. We also found that Ral-Exo84-GCK-2 signals through a conserved MAP3K/MLK-1-MAP2K/MEK-1-p38/PMK-1 cascade. This cascade may further activate MAPKAP kinase, MAK-2. We will generate p38::mNeonGreen to measure the activity of Ral signaling cascade through activation-dependent cytoplasmic-to-nuclear translocation of p38.