Dictyostelium extracellular cAMP relay / aggregation oscillator

The defining Dictyostelium discoideum aggregation system — a self-organising extracellular cAMP oscillator that propagates chemotactic waves so that starving amoebae stream together into a mound. Secreted cAMP is detected by the seven-transmembrane receptor cAR1 (carA), which couples through the heterotrimeric G protein Galpha2 (gpaB); receptor activation recruits the PH-domain adaptor CRAC (dagA) that is required to activate the aggregation adenylyl cyclase ACA (acaA), which synthesises cAMP. The newly made cAMP is secreted and activates cAR1 on neighbouring cells (the RELAY, a positive feedback), while the extracellular phosphodiesterase PdsA degrades secreted cAMP and the intracellular response-regulator phosphodiesterase RegA degrades internal cAMP — the negative feedback / adaptation that lets the system oscillate and produce propagating waves rather than saturate. This is the extracellular-cAMP CHEMOTACTIC oscillator, distinct from the generic intracellular GPCR->cAMP->PKA cassette. OUT OF SCOPE: the downstream PKA-driven developmental gene expression (cAMP effector; see the pkaC review and the SDF-2 relay module), the actin/PI3K chemotaxis motor, and the later adenylyl cyclases ACG/ACR. Phrased over functions and family selectors with D. discoideum representatives.

Dictyostelium-specific extracellular-cAMP chemotactic oscillator grounded to cAR1 (P13773, GO:0001646), Galpha2 (P16051, GO:0003925), CRAC (P35401, GO:0008047), ACA (Q03100, GO:0004016), PdsA (P12019, GO:0004115) and RegA (Q23917, GO:0004115). GO molecular-function terms were taken from the completed DICDI reviews; nodes use PANTHER family selectors plus D. discoideum representative members. Explicitly named "extracellular_camp_relay" so it does not collide with the generic gpcr_camp_pka_signaling module: the discriminating feature is that the cAMP ligand is SECRETED and relayed between cells, driving chemotactic waves. PKA (the intracellular cAMP effector) is downstream and out of scope here. Later, at the mound and slug stage, cAR1 also signals to the transcription factor STATa without needing G-beta; that output is modelled in dicty_jak_independent_stat_signaling.