Reconstitution of a yeast protein kinase cascade in vitro: Activation of the yeast MEK homologue STE7 by STE11

The mating-factor response pathway of Saccharomyces cerevisiae employs a set of protein kinases similar to kinases that function in signal transduction pathways of metazoans. We have purified the yeast protein kinases encoded by STE11, STE7, and FUS3 as fusions to glutathione S- transferase (GST) and reconstituted a kinase cascade in which STE11 phosphorylates and activates STE7, which in turn phosphorylates the mitogen- activated protein kinase FUS3. GST-STE11 is active even when purified from cells that have not been treated with α-factor. This observation raises the possibility that STE11 activity is governed by an inhibitor which is regulated by pheromone. We also identify a STE11-dependent phosphorylation site in STE7 which is required for activity of STE7. Conservation of this site in the mammalian STE7 homologue MEK and other STE7 relatives suggests that this may be a regulatory phosphorylation site in all MAP kinase kinases.