Final receptor of the SLN1-YPD1-SSK1 two-component regulatory system, which controls activity of the HOG1 pathway in response to changes in the osmolarity of the extracellular environment. Under normal osmotic conditions, maintained in a phosphorylated and inactive state by the phosphorelay intermediate protein YPD1. Under conditions of high osmolarity, the histidine kinase SLN1 is no longer active and the unphosphorylated form of SSK1 interacts with and activates SSK2 and SSK22, two MAPKKKs that further stimulate the PBS2-HOG1 MAPKK-MAPK cascade. Unphosphorylated SSK1 is subsequently degraded by the UBC7-dependent ubiquitin-proteasome system to down-regulate the HOG1 pathway after completion of the osmotic adaptation. Interacts with SSK2, SSK22 and YPD1. 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. Present with 1200 molecules/cell in log phase SD medium.