Involved in regulation of actin cytoskeleton organization during cell motility; F-actin fragmenting and capping protein allowing dynamic rearrangements of the actin cytoskeleton. Also part of a regulatory pathway from the centrosome to the midzone, thus regulating the completion of cell division. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Phosphorylation of severin is reduced with high concentrations of calcium ions. Optimum pH is 7.5. Shows overall localization in the cytosol and, to a lesser extent, in the nucleus. Enriched around the centrosome. In dividing cells, forms a cloud around the spindle and nuclei. Enriched in the cleavage furrow during late cytokinesis. Autophosphorylated. Defects in cytokinesis, leading to multinucleated cells caused by failure to sever the final connecting bridge between the 2 daughter cells. Cells also display defects in development and directed slug movement. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.