G alpha 3 regulates the cAMP signaling system in Dictyostelium

The Dictyostelium discoideum developmental program is initiated by starvation and its progress depends on G-protein-regulated transmembrane signaling. Disruption of the Dictyostelium G-protein alpha-subunit G alpha 3 (g alpha 3-) blocks development unless the mutant is starved in the presence of artificial cAMP pulses. The function of G alpha 3 was investigated by examining the expression of several components of the cAMP transmembrane signaling system in the g alpha 3- mutant. cAMP receptor 1 protein, cyclic nucleotide phosphodiesterase, phosphodiesterase inhibitor, and aggregation-stage adenylyl cyclase mRNA expression were absent or greatly reduced when cells were starved without exogenously applied pulses of cAMP. However, cAMP receptor 1 protein and aggregation-stage adenylyl cyclase mRNA expression were restored by starving the g alpha 3- cells in the presence of exogenous cAMP pulses. Adenylyl cyclase activity was also reduced in g alpha 3- cells starved without exogenous cAMP pulses compared with similarly treated wild-type cells but was elevated to a level twofold greater than wild-type cells in g alpha 3- cells starved in the presence of exogenous cAMP pulses. These results suggest that G alpha 3 is essential in early development because it controls the expression of components of the transmembrane signaling system.