Ral Signals through a MAP4 Kinase-p38 MAP Kinase Cascade in C. elegans Cell Fate Patterning

Summary: C. elegans vulval precursor cell (VPC) fates are patterned by an epidermal growth factor (EGF) gradient. High-dose EGF induces 1° VPC fate, and lower dose EGF contributes to 2° fate in support of LIN-12/Notch. We previously showed that the EGF 2°-promoting signal is mediated by LET-60/Ras switching effectors, from the canonical Raf-MEK-ERK mitogen-activated protein (MAP) kinase cascade that promotes 1° fate to the non-canonical RalGEF-Ral that promotes 2° fate. Of oncogenic Ras effectors, RalGEF-Ral is by far the least well understood. We use genetic analysis to identify an effector cascade downstream of C. elegans RAL-1/Ral, starting with an established Ral binding partner, Exo84 of the exocyst complex. Additionally, RAL-1 signals through GCK-2, a citron-N-terminal-homology-domain-containing MAP4 kinase, and PMK-1/p38 MAP kinase cascade to promote 2° fate. Our study delineates a Ral-dependent developmental signaling cascade in vivo, thus providing the mechanism by which lower EGF dose is transduced. : Ral is an oncogenic effector of the oncoprotein Ras. Surprisingly, in vivo signals downstream of Ral have mostly defied identification. Shin et al. find that, during developmental patterning of the C. elegans VPCs, RAL-1/Ral signals through EXOC-8/Exo84, GCK-2/MAP4K, and PMK1/p38 MAP kinase to promote 2° fate in support of LIN-12/Notch. Keywords: MAP4K, Sec5, RalBP1, RLBP-1, EXOC-8, SEC-5, MLK-1, MIG-15, Misshapen, Happyhou