G-ALPHA-3 REGULATES THE CAMP SIGNALING SYSTEM IN DICTYOSTELIUM

The Dictyostelium discoideum developmental program is initiated by sta rvation and its progress depends on G-protein-regulated transmembrane signaling. Disruption of the Dictyostelium G-protein alpha-subunit G a lpha 3 (g alpha 3(-)) blocks development unless the mutant is starved in the presence of artificial cAMP pulses. The function of G alpha 3 w as investigated by examining the expression of several components of t he cAMP transmembrane signaling system in the g alpha 3(-) mutant. cAM P receptor 1 protein, cyclic nucleotide phosphodiesterase, phosphodies terase inhibitor, and aggregation-stage adenylyl cyclase mRNA expressi on were absent or greatly reduced when cells were starved without exog enously applied pulses of cAMP. However, cAMP receptor 1 protein and a ggregation-stage adenylyl cyclase mRNA expression were restored by sta rving the g alpha 3(-) cells in the presence of exogenous cAMP pulses. Adenylyl cyclase activity was also reduced in g alpha 3(-) cells star ved 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 de velopment because it controls the expression of components of the tran smembrane signaling system.