Regulator of the myosin I component of the cytoskeleton: required for regulation of cytokinesis, phagocytosis and pinocytosis. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with rac1A, rac1B, rac1C, racA, racB, racC and racF1. Mainly cytosolic but enriched at the leading edge of migrating cells and at macropinocytic and phagocytic cups, possibly due to binding to acidic phospholipids. Autophosphorylated at Ser-8. This may stimulate interaction with GTP-bound Rac family members which then further stimulates autophosphorylation and kinase activity. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.