A GDI (AGS3) and a GEF (GIV) regulate autophagy by balancing G protein activity and growth factor signals

Autophagy is the major catabolic process responsible for the removal of aggregated proteins and damaged organelles. Autophagy is regulated by both G proteins and growth factors, but the underlying mechanism of how they are coordinated during initiation and reversal of autophagy is unknown. Using protein-protein interaction assays, G protein enzymology, and morphological analysis, we demonstrate here that G alpha-interacting, vesicle-associated protein (GIV, a.k.a. Girdin), a nonreceptor guanine nucleotide exchange factor for G alpha(i3), plays a key role in regulating autophagy and that dynamic interplay between G alpha(i3), activator of G-protein signaling 3 (AGS3, its guanine nucleotide dissociation inhibitor), and GIV determines whether autophagy is promoted or inhibited. We found that AGS3 directly binds light chain 3 (LC3), recruits G alpha(i3) to LC3-positive membranes upon starvation, and promotes autophagy by inhibiting the G protein. Upon growth factor stimulation, GIV disrupts the G alpha(i3)-AGS3 complex, releases G alpha(i3) from LC3-positive membranes, enhances anti-autophagic signaling pathways, and inhibits autophagy by activating the G protein. These results provide mechanistic insights into how reversible modulation of G alpha(i3) activity by AGS3 and GIV maintains the delicate equilibrium between promotion and inhibition of autophagy.