May mediate extracellular signals to regulate transcription in differentiating cells. Probably involved first at the cortical polarity site, to restrict MAPK signaling and promote asymmetric cell division (ACD), and second in the nucleus of stomatal lineage ground cells (SLGCs) or meristemoids, to limit cell division and to promote differentiation into pavement or stomatal guard cells, respectively (PubMed:30429609). Phosphorylate YDA and SPCH in vitro (PubMed:30429609). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Binds to KIB1 (PubMed:28575660). Component of a complex made of POLAR, BASL, ASK7/BIN2 and ASK3/SK12 (PubMed:30429609). Binds to POLAR and BASL (PubMed:30429609). Localized throughout the plasma membrane in asymmetric cell division (ACD) precursors (PubMed:30429609). Accumulates transiently in a polarized pattern at the plasma membrane immediately before ACD, relocalizing to well-defined foci, in a POLAR-dependent and in the presence of BASL (PubMed:30429609). Roots, shoots and leaves. Expressed in protodermal cells in young seedlings. Autophosphorylated mainly on threonine and serine residues. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. GSK-3 subfamily.