Adenylyl cyclase G, an osmosensor controlling germination of Dictyostelium spores

Dictyostelium cells express a G-protein-coupled adenylyl cyclase, ACA, during aggregation and an atypical adenylyl cyclase, ACG, in mature spores, The ACG gene was disrupted by homologous recombination. acg(-) cells developed into normal fruiting bodies with viable spores, but spore germination was no longer inhibited by high osmolarity, a fairly universal constraint for spore and seed germination, ACG activity, measured in aca(-)/ACG cells, was strongly stimulated by high osmolarity with optimal stimulation occurring at 200 milliosmolar. RdeC mutants, which display unrestrained protein kinase A (PKA) activity and a cell line, which overexpresses PKA under a prespore specific promoter, germinate very poorly, both at high and low osmolarity, These data indicate that ACG is an osmosensor controlling spore germination through activation of protein kinase A.