The MEKK1, MKK1/MKK2 and MPK4 function in a signaling pathway that modulates the expression of genes responding to biotic and abiotic stresses and also plays an important role in pathogen defense by negatively regulating innate immunity. Involved in the innate immune MAP kinase signaling cascade (MEKK1, MKK4/MKK5 and MPK3/MPK6) downstream of bacterial flagellin receptor FLS2. May be involved in the cold and salinity stress-mediated MAP kinase signaling cascade (MEKK1, MKK1/MKK2 and MPK4/MPK6). Activates by phosphorylation the downstream MKK2, MKK4 and MKK5 in a calcium-dependent manner. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Activated by cold via CRLK1-mediated phosphorylation and leading to elevated kinase activity towards MKK2. Interacts with MKK1, MMK2 and MPK4. May form a ternary complex composed of MEKK1 and MKK1/MKK2 and MPK4 (PubMed:18982020, PubMed:9878570). Interacts with RACK1A, RACK1B and RACK1C (PubMed:25731164). Binds to CRLK1 (PubMed:20724845). By touch, cold and salinity stress. Phosphorylated by CRLK1 in response to cold. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase kinase subfamily. Sequencing errors.