REGULATION OF DICTYOSTELIUM ADENYLYLCYCLASES BY MORPHOGEN-INDUCED MODULATION OF CYTOSOLIC PH OR CA2+ LEVELS

Inhibition of cAMP production and consequent inactivation of protein k inase A (PKA) by the putative morphogen ammonia has been suggested to block culmination and stalk cell differentiation in Dictyostelium. Sin ce other weak bases mimic and weak acids act oppositely to ammonia, it s effects were attributed to cytosolic or vesicular alkalinization; th e latter resulting in impaired Ca2+ sequestration. We investigated whe ther weak bases and acids modulate the activity of the two Dictyosteli um adenylylcyclases ACA and ACG in a manner consistent with their effe cts on development. It appeared that ammonia inhibits both ACG activit y and ACA activation only transiently and does not significantly affec t 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 con sistent with the pH dependence of ACA and ACG. In lysates, basal and G TP 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 lys ates is completely insensitive to Ca2+, while GTP gamma S-stimulated A CA activity is maximally 50% reduced by supraphysiological Ca2+ concen trations. The observation that weak acids strongly inhibit ACA and ACG while promoting a PKA-dependent process such as stalk cell differenti ation suggests that in Dictyostelium PKA can be activated in the absen ce of cAMP production. (C) 1995 Academic Press,Inc.