Mitogen-activated protein kinase kinase kinase; part of the MCK1-MKK2-MPS1 MAP kinase (MAPK) signal transduction cascade that is essential for appressorium formation, penetration and invasive growth (PubMed:18393612, PubMed:28244240). Beside its role in pathogenesis, the MPS1 cascade is active in conidiation and cellular stress responses (By similarity). Targets downstream of the the MPS1-MAPK pathway include transcription factors MIG1 and SWI6, as well as GSK1 and MPG1 (PubMed:28244240). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with the adapter protein MST50 and MIP11. Leads to hypersensitivity to cell-wall-degrading enzymes (PubMed:18393612). Reduces significantly numbers of conidia and develops appressoria in a slightly retarded manner (PubMed:18393612). Impairs the ability of appressoria to penetrate into plant tissues, thereby abolishing pathogenicity (PubMed:18393612). Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase kinase subfamily.