Serine/threonine protein kinase required for cell-type-specific transcription and signal transduction in yeast. It is thought that it phosphorylates the STE7 protein kinase which itself, phosphorylates the FUS3 and or KSS1 kinases (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Homodimer. Interacts (via SAM domain) with STE50 (via SAM domain). Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase kinase subfamily.