RapC Is Involved in the Development of Dictyostelium Through the Regulation of Autophagy and Calcium Signaling

RapC is a Rap-subfamily small GTPase in Dictyostelium whose developmental function remains largely uncharacterized. Upon starvation, individual amoebae aggregate and undergo morphogenetic transitions to ultimately produce a fruiting body, a process requiring precise spatiotemporal regulation of a single apical tip as the primary organizing center. Here, we report that RapC is required for tip organizer formation during multicellular development by modulating autophagy and intracellular calcium signaling. rapC null cells develop normally through aggregation but consistently form three to four apical tips per aggregate rather than the single organizer tip characteristic of wild-type cells. Pharmacological phenocopy experiments revealed that caffeine and the selective TORC1 inhibitor rapamycin recapitulate the multi-tip phenotype, whereas the PI3K inhibitor LY294002 and the pan-TOR inhibitor Torin2 do not, implicating TORC1 signaling as the relevant downstream effector. In line with this, rapC null cells exhibit normal GFP-PH:CRAC translocation kinetics and slightly elevated PKB/PKBR1 phosphorylation, ruling out defects in the PI3K–TORC2–PKB signaling axis. Instead, rapC null cells display markedly elevated autophagic flux as assessed by the tandem RFP-GFP-Atg8 reporter assay, consistent with attenuated TORC1-mediated autophagy suppression. Furthermore, aggregation-competent rapC null cells show increased intracellular calcium ([Ca2+]i) upon external calcium stimulation, a response absent in wild-type cells and not reproduced by rapamycin, indicating a TORC1-independent role for RapC in calcium homeostasis. Together, our findings establish that RapC promotes normal Dictyostelium development by supporting TORC1 activity to suppress excessive autophagic flux and by maintaining [Ca2+]i homeostasis through a TORC1-independent mechanism.