In Saccharomyces cerevisiae, a multi-component phosphorelay signal transduction pathway mediates cellular responses to environmental stress. A histidine-containing phosphotransfer protein, YPD1, represents a bifurcation point between the SLN1-YPD1-SSK1 pathway responsible for osmotic stress responses and the SLN1-YPD1-SKN7 pathway involved in cell wall biosynthesis and cell cycle control. The phosphorelay protein YPD1 must physically interact with and transfer phosphoryl groups between three homologous response regulator domains, designated SLN1-R1, SSK1-R2, and SKN7-R3. In this comparative study, the molecular basis of interaction was examined between YPD1 and each of the three response regulator domains utilizing alanine scanning mutagenesis combined with a yeast two-hybrid assay. Results from the yeast two-hybrid assay indicate that all three response regulator domains bind to a common area, largely hydrophobic in nature, on the surface of YPD1. We postulate that other YPD1 surface residues surrounding this common docking site are involved in making specific interactions with one or more of the response regulator domains.