Cell movement and cell-type-specific gene expression during Dictyostelium development are regulated by cAMP, which functions both as an extracellular hormone-like signal and an intracellular second messenger. Previous data indicated that aca-mutants, which lack adenylyl cyclase activity, fail to aggregate and do not express cell-type-specific genes. We show here that overexpression of ACG, a constitutively active adenylyl cyclase, which in wild-type cells is only expressed during spore germination, partially restores the coordination of cell movement and completely restores developmental gene expression. The aca-cells can also be induced to develop into viable spores by synergy with wild-type cells and, furthermore, form small but normal fruiting bodies, after a developmentally relevant regimen of stimulation with nanomolar cAMP pulses followed by micromolar cAMP concentrations. 2'-Deoxy cAMP, a cAMP analog that activates the cell-surface cAMP receptors but not cAMP-dependent protein kinase (PKA), also induces fruiting body formation as well as expression of prespore-specific and prestalk-enriched genes in aca-cells. Intracellular cAMP levels were not altered in aca-cells after stimulation with 2'-deoxy cAMP. Our data indicate that ACA is not required to provide intracellular cAMP for PKA activation but is essential to produce extracellular cAMP for coordination of cell movement during all stages of development and for induction of developmental gene expression.

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Two open reading frames, YIL042c (PKP1 ) and YGL059w, with 25% sequence similarity to human pyruvate dehydrogenase kinases, were shown to have protein kinase activity. Using GFP fusions, it was demonstrated that the proteins localize in discrete submitochondrial regions. Strains with a null mutation in these loci grew poorly on acetate and ethanol as carbon sources. Doubling times increased from ca. 4 h in the wild-type to >6 h for the mutants. Growth rates of the mutants could be restored to wild-type levels by simultaneous disruption of the PDA1 gene, encoding the E1α subunit of the pyruvate dehydrogenase complex. This observation and the pyruvate dehydrogenase activities measured in the mutant strains and the wild-type grown on glucose or acetate suggest that the slow growth phenotype on C2 carbon sources is caused by a futile cycle in which phosphoenolpyruvate is converted back to acetyl coenzyme A.

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.

During development of the cellular slime mold Dictyostelium discoideum, cAMP induces chemotaxis and expression of different classes of genes by means of interaction with surface cAMP receptors. We describe a cAMP derivative, 8-p-chlqrophenylthioadenosine 3',5'-cyclic monophosphate (8-CPT-cAMP), which inhibits cAMP-induced chemotaxis at low concentrations but induces chemotaxis at supersaturating concentrations. This compound, moreover, selectively activates expression of aggregative genes but not of p reptive genes. 8-CPT-cAMP induces normal cGMP and cAMP accumulation but in contrast to cAMP, which increases inositol 1,4,5-trisphosphate levels, 8-CPT-cAMP decreases inositol 1,4,5-trisphosphate levels. The derivative induces reduced activation of guanine nucleotide regulatory proteins, which may cause its defective activation of inositol 1,4,5-trisphosphate production. Our data suggest that disruption of inositolphospholipid signaling impairs chemotaxis and expression of a subclass of cAMP-regulated genes.