Serine/threonine-protein kinase involved in regulation of lymphocyte migration. Phosphorylates MSN, and possibly PLK1. Involved in regulation of lymphocyte migration by mediating phosphorylation of ERM proteins such as MSN. Acts as a negative regulator of MAP3K1/MEKK1. May also act as a cell cycle regulator by acting as a polo kinase kinase: mediates phosphorylation of PLK1 in vitro; however such data require additional evidences in vivo (By similarity). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Inhibited by the pyrrole-indolinone inhibitor SU11274 (K00593): intercalates between the ATP-binding Lys-65 and alpha-C glutamate (Glu-81), resulting in a partial disordering of the lysine side chain. Also specifically inhibited by erlotinib. Slightly inhibited by gefitinib (By similarity). Homodimer; homodimerization is required for activation segment autophosphorylation. Autophosphorylates following homodimerization, leading to activation of the protein. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.