Regulates the transition between growth and differentiation. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Expressed at the same level in vegetative cells and throughout development, with a slight peak at the time when the cells are aggregating. Cells initiate multicellular development prematurely. In the N-terminal section; belongs to the GDT family. In the C-terminal section; belongs to the protein kinase superfamily. TKL Ser/Thr protein kinase family.