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. 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. Expressed predominantly in lymphoid organs such as spleen, thymus, and bone marrow. Autophosphorylates following homodimerization, leading to activation of the protein. Mice do not show any obvious abnormalities. Lymphocytes develop normally but activated lymphocytes show enhanced cell adhesion. Decreased phosphorylation of ERM proteins. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.