Phosphorelay intermediate protein that is part of the bifurcated SLN1-YPD1-SKN7/SSK1 two-component regulatory system, which controls activity of the HOG1 pathway and gene expression in response to oxidative stress and probably to changes in the osmolarity of the extracellular environment. Catalyzes the phosphoryl group transfer from the membrane-bound histidine kinase SLN1 to two distinct response regulators SSK1 and SKN7. Belongs to the YPD1 family.