Nuclear shuttling of yeast scaffold Ste5 is required for its recruitment to the plasma membrane and activation of the mating MAPK cascade

Localization of Ste5 to G beta at the plasma membrane is essential for tran smission of the pheromone signal to associated MAP kinase cascade enzymes. Here, we show that this crucial localization requires prior shuttling of St e5 through the nucleus. Ste5 shuttles through the nucleus constitutively du ring vegetative growth. Pheromone enhances nuclear export of Ste5, and this pool translocates vectorial ly to the cell periphery. Remarkably, Ste5 tha t cannot transit the nucleus is unable to localize at the periphery and act ivate the pathway, while Ste5 with enhanced transit through the nucleus has enhanced ability to localize to the periphery and activate the pathway. Th is novel regulatory scheme may ensure that cytoplasmic Ste5 does not activa te downstream kinases in the absence of pheromone and could be applicable t o other membrane-recruited signaling proteins.