Dictyostelium SDF-2 spore-encapsulation relay (AcbA-TagC-DhkA-RegA-PKA)

The non-cell-autonomous signalling relay that triggers rapid, synchronous encapsulation of prespore cells into spores during culmination in Dictyostelium discoideum. The acyl-CoA-binding protein AcbA is unconventionally secreted by prespore cells and cleaved at the prestalk cell surface by the ABC-transporter/ serine-protease TagC to release the peptide signal SDF-2. SDF-2 binds the membrane sensor histidine kinase DhkA and INHIBITS its phosphorelay; because DhkA (through the histidine-phosphotransfer protein RdeA) normally keeps the response-regulator cAMP phosphodiesterase RegA active, inhibiting DhkA lowers RegA activity, so intracellular cAMP rises and activates cAMP-dependent protein kinase (PKA-C released from PKA-R), which drives spore encapsulation. The net logic is a double negative — SDF-2 inhibits DhkA, DhkA activates RegA, RegA suppresses cAMP/PKA — so SDF-2 ultimately ACTIVATES encapsulation. The relay is self-amplifying (encapsulating cells release more AcbA). Phrased over functions and family selectors with D. discoideum representatives rather than a fixed gene list. OUT OF SCOPE: the parallel GABA/GrlE input that also triggers AcbA release (a separate module), the ACR/PKA culmination cAMP source, and spore-coat assembly. The histidine-phosphotransfer intermediate RdeA is described in the DhkA->RegA connection but is NOT separately grounded here because it has not yet been reviewed.

Dictyostelium-specific culmination signalling relay grounded to six reviewed proteins: AcbA (Q5FXM5), TagC (Q23868, GO:0004252), DhkA (Q54U87, GO:0000155), RegA (Q23917, GO:0004115), PKA-R (P05987, GO:0004862) and PKA-C (P34099, GO:0004691). GO molecular-function terms were taken from the completed DICDI reviews; each node uses a PANTHER family selector plus the D. discoideum representative member. The DhkA->RegA direction follows the Wang/Anjard-Loomis inhibition model adopted in the dhkA review (SDF-2 inhibits DhkA, lowering RegA and raising cAMP/PKA); the alternative Tekinay model is recorded as contested there. The histidine-phosphotransfer intermediate RdeA (Q54RR8, GO:0009927) is now grounded as its own node (rdea_relay) between DhkA and RegA, following the completed rdeA review. Named to signal Dictyostelium scope so it does not collide with a generic two-component / cAMP-PKA signalling module.