Constitutively active RAS in S. pombe causes persistent Cdc42 signalling but only transient MAPK activation

The small GTPase RAS is a signalling hub. Oncogenic RAS mutations are assumed to over-activate all of the downstream pathways. We tested this assumption in fission yeast, where, RAS-mediated pheromone signalling (PS) activates the MAPK Spk1 and Cdc42 pathways. Unexpectedly, we found that constitutively active Ras1.G17V induced acute transient MAPK Spk1 activation, whilst Cdc42 activation persisted. Acute transient MAPK Spk1 activation was also seen in the deletion mutant of Cdc42-GEF Scd1 , a Cdc42 activator. We have built a mathematical model using PS negative-feedback circuits and competition between the two Ras1 effectors, MAPKKK Byr2 and Cdc42-GEF Scd1 . The model robustly predicted the MAPK Spk1 activation dynamics of an additional 21 PS mutants. Supporting the model, we show that a recombinant Cdc42-GEF Scd1 fragment competes with MAPKKK Byr2 for Ras1 binding. Our study has established a concept that constitutive RAS yields biased Cdc42/Rac over-activation, providing a strong rationale to interfere with this process to target oncogenic RAS in humans.