Functions in the NACK-PQR (NPK1-NQK1/MEK1-NRK1) MAP kinase signaling pathway, which is essential for somatic cell cytokinesis, especially for the cell-plate formation and its expansion, and depends on NACK1 and NACK2 kinesin-related proteins. Functions in the regulation of resistance gene-mediated resistance responses such as the N-mediated resistance to tobamovirus (TMV) and the Rx-mediated hypersensitive response (HR) to potato virus X (PVX). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts (via C-terminus) with NACK1 (via C-terminus). In interphase and prophase, detected in the nucleus. From prometaphase to metaphase, found in the cytoplasm in patches. At anaphase, distributed around the spindle midzone and on the phragmoplast equator at telophase and then redistributed to the newly forming daughter nuclei. Expressed in root apex, lateral root primordia, cotyledons, shoot apical meristem, stems, young leaves, vasculature floral meristem, floral organ primordia, stigma, style and ovules in the immature gynoecium. During the M phase of the cell cycle (at protein level). Phosphorylated at Ser-575, Ser-669 and Ser-687 by CDKAs and CDKBs. The phosphorylation occurs before metaphase and inhibits the interaction with NACK1 preventing the transition to cytokinesis. Plants overexpressing the NPK1 kinase domain produced seeds defective in embryo and endosperm development (PubMed:9790195). Expression of kinase-negative NPK1 causes inhibition of phragmoplast expansion and multinucleate cells (PubMed:11159915). Plants silencing NPK1 exhibit reduced cell size, defective cytokinesis and an overall dwarf phenotype (PubMed:12194859). Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. MAP kinase kinase kinase subfamily.