Phosphorelay intermediate protein that is part of the branched SLN1-YPD1-SKN7/SSK1 two-component regulatory system, which controls activity of the HOG1 pathway and gene expression in response to changes in the osmolarity of the extracellular environment. Catalyzes the phosphoryl group transfer from the membrane-bound osmosensing histidine kinase SLN1 to two distinct response regulator proteins, SSK1 in the cytoplasm, and transcription factor SKN7 in the nucleus. Interacts with the response regulatory domains of SLN1 and SSK1. Localizes constitutively to the cytoplasm and the nucleus, independent on osmotic conditions and phosphorylation status of the protein. The phosphorelay mechanism involves the sequential transfer of a phosphate group from 'His-576' (H1) to 'Asp-1144' (D1) of SLN1, then to His-64 (H2) of YPD1 and finally to 'Asp-554' (D2) of SSK1 or 'Asp-427' (D2) of SKN7. Present with 6330 molecules/cell in log phase SD medium. Belongs to the YPD1 family.