Conserved Core but Divergent Biochemical Features of RAS-MAPK Signalling in C. elegans

Abstract

Genetic studies in Caenorhabditis elegans have established core principles of RAS-MAPK signalling. These nematodes encode three RAS proteins: LET-60/KRAS is essential for vulval induction, while RAS-1 and RAS-2 are orthologs of RRAS/RRAS2 and MRAS, respectively, but remain biologically and biochemically uncharacterized. We here present a comprehensive biochemical analysis of all three C. elegans RAS proteins, defining nucleotide cycling and effector interactions. We show that conserved RAS scaffolds are tuned by sequence variation to balance signalling with developmental fidelity. LET-60 has significantly impaired nucleotide exchange and changing its sequence to match human KRAS induces a protruding vulva phenotype in vivo. RAS-1 rapidly exchanges GDP-to-GTP, whereas RAS-2 remains consistently GDP-bound. LET-60 engages the RAF ortholog LIN-45 through a divergent interface, and RAS-1 unexpectedly drives SOC-2:RAS:PP1C holophosphatase assembly in a nucleotide-independent manner. Our findings reveal both conserved and distinctive biochemical features underlying RAS signalling and suggest species-specific tuning of this key proliferation pathway.