During cellular differentiation, may mediate an extracellular cyclic AMP stimulated signal transduction pathway that regulates prespore and prestalk B-cell proportions through inhibition of stalk cell formation and induction of prespore cell differentiation. The cAMP receptor carC appears to activate gskA via the tyrosine kinases zakA and zak2, to stimulate prespore differentiation, while carD appears to negatively regulate gskA, to promote prestalk formation. ATP + L-seryl-[tau protein] = ADP + H(+) + O-phospho-L-seryl-[tau protein] ATP + L-threonyl-[tau protein] = ADP + H(+) + O-phospho-L-threonyl-[tau protein] Inhibited by lithium. Lithium inhibition is competitive with respect to magnesium but non-competitive with respect to the peptide substrate. Expressed in growing cells and throughout development. Levels increase during mound formation (12 hours) and peak at approximately twice the level seen in growing cells before decreasing again at the start of culmination (16 hours). Cells grow normally in both axenic medium and with bacteria. Under starvation conditions, mutants lacking gskA aggregate more quickly and form smaller mounds than wild type, fail to form slugs and take longer than wild type to reach culmination. At culmination, mutants lacking gskA form abnormal fruiting bodies characterized by an unusually large mound-like basal disk. Prestalk B (pstB) cells are formed at the expense of prespore cells, and the proportion of gskA null cells that differentiate into spores is greatly reduced when compared to wild-type cells. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. GSK-3 subfamily.