Regulation of Dictyostelium adenylylcyclases by morphogen-induced modulation of cytosolic pH or Ca2+ levels.
Inhibition of cAMP production and consequent inactivation of protein kinase A (PKA) by the putative morphogen ammonia has been suggested to block culmination and stalk cell differentiation in Dictyostelium. Since other weak bases mimic and weak acids act oppositely to ammonia, its effects were attributed to cytosolic or vesicular alkalinization; the latter resulting in impaired Ca2+ sequestration. We investigated whether weak bases and acids modulate the activity of the two Dictyostelium adenylylcyclases ACA and ACG in a manner consistent with their effects on development. It appeared that ammonia inhibits both ACG activity and ACA activation only transiently and does not significantly affect cAMP levels in slugs. Surprisingly, weak acids inhibit both ACA and ACG permanently, but do not affect secretion of cAMP as was suggested earlier. The effects of weak acids, which reduce cytosolic pH, are consistent with the pH dependence of ACA and ACG. In lysates, basal and GTP gamma S-stimulated ACA activity as well as ACG activity are optimal at pH 8 and are virtually absent below pH 7. ACG activity in cell lysates is completely insensitive to Ca2+, while GTP gamma S-stimulated ACA activity is maximally 50% reduced by supraphysiological Ca2+ concentrations. The observation that weak acids strongly inhibit ACA and ACG while promoting a PKA-dependent process such as stalk cell differentiation suggests that in Dictyostelium PKA can be activated in the absence of cAMP production.