Histidine kinase that acts as an osmosensor at the plasma membrane. 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 changes in the osmolarity of the extracellular environment. Under normal osmotic conditions, the histidine kinase autophosphorylates His-576. This phosphate is subsequently transferred to Asp-1144, from where it is relayed to 'His-64' of the phosphorelay intermediate protein YPD1. Under high osmolarity conditions, the histidine kinase is no longer active. ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. Interacts with DJP1, MOG1 and YPD1. The phosphorelay mechanism involves the sequential transfer of a phosphate group from His-576 (H1) in the histidine kinase domain (transmitter domain) to Asp-1144 (D1) of the response regulatory domain (receiver domain). This transfer probably occurs between two SLN1 molecules, rather than intramolecularly. The phosphate group is further transferred to 'His-64' (H2) of YPD1 and finally to 'Asp-554' (D2) of SSK1 or 'Asp-427' (D2) of SKN7. Present with 656 molecules/cell in log phase SD medium.