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 MAPKSpk1 and Cdc42 pathways. We observed that in cells with the yeast RAS mutant, ras1.G17V, the MAPKSpk1 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, MAPKKKByr2 and Cdc42-GEFScd1. The model robustly predicted the MAPKSpk1 activation dynamics of an additional 20 PS mutants. In support of the model, a recombinant Cdc42-GEFScd1 competed with MAPKKKByr2 for Ras1 binding and overexpression of the Ras binding domain of either Cdc42-GEFScd1 or MAPKKKByr2 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.