Seven helix cAMP receptors stimulate Ca2+ entry in the absence of functional G proteins in Dictyostelium.
Surface cAMP receptors (cARs) in Dictyostelium transmit a variety of signals across the plasma membrane. The best characterized cAR, cAR1, couples to the heterotrimeric guanine nucleotide-binding protein (G protein) alpha-subunit G alpha 2 to mediate activation of adenylyl and guanylyl cyclases and cell aggregation. cAR1 also elicits other cAMP-dependent responses including receptor phosphorylation, loss of ligand binding (LLB), and Ca2+ influx through a G alpha 2-independent pathway that may not involve G proteins. Here, we have expressed cAR1 and a related receptor, cAR3, in a g beta- strain (Lilly, P., Wu. L., Welker, D. L., and Devreotes, P. N. (1993) Genes & Dev. 7,986-995), which lacks G protein activity. Both cell lines failed to aggregate, a process requiring the G alpha 2 and G beta- subunits. In contrast, cAR1 phosphorylation in cAR1/g beta- cells showed a time course and cAMP dose dependence indistinguishable from those of cAR1/G beta+ controls. cAMP-induced LLB was also normal in the cAR1/g beta- cells. Finally, cAR1/g beta- cells and cAR3/g beta- cells showed a Ca2+ response with kinetics, agonist dependence, ion specificity, and sensitivity to depolarization agents that were like those of G beta+ controls, although they accumulated fewer Ca2+ ions per cAMP receptor than the control strains. Together, these results suggest that the G beta-subunit is not required for the activation or attenuation of cAR1 phosphorylation, LLB, or Ca2+ influx. It may, however, serve to amplify the Ca2+ response, possibly by modulating other intracellular Ca2+ signal transduction pathways.
Milne
J L
JL
Department of Biological Chemistry, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205-2185.
Wu
L
L
Caterina
M J
MJ
Devreotes
P N
PN
D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone / Q000494:pharmacology; D002412:Cations / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism / Q000494:pharmacology; D004023:Dictyostelium / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D019204:GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D015870:Gene Expression; D007700:Kinetics; D010766:Phosphorylation; D011953:Receptors, Cyclic AMP / Q000096:biosynthesis / Q000302:isolation & purification / Q000378:metabolism*; D012430:Ruthenium Red / Q000494:pharmacology
D002412:Cations; D011953:Receptors, Cyclic AMP; D012430:Ruthenium Red; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone; D000242:Cyclic AMP; D019204:GTP-Binding Proteins; D002118:Calcium