Constitutively active RAS prolongs Cdc42 signalling while attenuating MAPK signalling during fission yeast mating

The small GTPase RAS is a signalling hub activating multiple pathways, which may respond differently to a constitutively active RAS mutation. We explored this issue using fission yeast, where RAS-mediated pheromone signalling (PS) activates the MAPK ^Spk1 and Cdc42 pathways. We observed that in cells with the yeast RAS mutant, ras1.G17V , the MAPK ^Spk1 activation was attenuated similarly to that in wildtype cells, whereas Cdc42 activation was prolonged. We built a mathematical model implementing 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 20 PS mutants. In support of the model, a recombinant Cdc42-GEF ^Scd1 competed with MAPKKK ^Byr2 for Ras1 binding and overexpression of the Ras binding domain of either Cdc42-GEF ^Scd1 or MAPKKK ^Byr2 inhibited both downstream pathways. Our study has established that the constitutively active RAS signalling propagates differently to downstream pathways where the system prevents MAPK overactivation.