Johnson, R.L., Department of Biological Chemistry, Johns Hopkins University, School of Medicine, Baltimore, MD 21205, United States
Saxe III, C.L., Lab. of Cell./Developmental Biology, NIDDKD (6/B1-12), National Institutes of Health, Bethesda, MD 20892, United States, Department of Anatomy, Emory University, School of Medicine, Atlanta, GA 30322, United States
Gollop, R., Lab. of Cell./Developmental Biology, NIDDKD (6/B1-12), National Institutes of Health, Bethesda, MD 20892, United States
Kimmel, A.R., Lab. of Cell./Developmental Biology, NIDDKD (6/B1-12), National Institutes of Health, Bethesda, MD 20892, United States
Devreotes, P.N., Department of Biological Chemistry, Johns Hopkins University, School of Medicine, Baltimore, MD 21205, United States

Extracellular cAMP acts through cell-surface receptors to coordinate the developmental program of Dictyostelium. A cAMP receptor (cAR1), which is expressed during early aggregation, has been cloned and sequenced previously. We have identified a new receptor subtype, cAR3, that has ∼56% and 69% amino acid identity with cAR1 and cAR2, respectively. cAR1, cAR2, or cAR3 expressed from plasmid in growing Dictyostelium cells can be photoaffinity labeled with 8-N3[32P]cAMP and phosphorylated when stimulated with cAMP. cAR3 RNA was not present during growth but appeared during late aggregation. Its expression peaked at 9 hr and then fell to a reduced level that was maintained until culmination. The expression of cAR3 protein followed a similar pattern, but with a 3-hr lag, and reached a maximum at the mound stage. In contrast, cAR1 protein was expressed predominantly during early aggregation and at low levels during later stages. At their respective peaks of expression, there were ∼5 × 103 cAR3 sites per cell compared with ∼7 × 104 cAR1 sites per cell. The cAR3 gene was disrupted by homologous recombination in several different parental cell lines. Surprisingly, the car3- cell lines display no obvious phenotype.