SGK1 Inhibits ULK2 Autophagy Activity

Serum- and glucocorticoid-induced kinase 1 (SGK1) is known to have consensus sequence of phosphorylation site R-x-R-x-x-(S/T)-Φ, where Φ is any hydrophobic amino acid and arginine residues are conserved at positions −5 and −3 relative to positions of Ser/Thr residues that are phosphorylated in the presence of SGK1. UNC-21-like kinase 2 (ULK2) also harbors putative SGK1 phosphorylation sites at both Ser507 (⁵⁰²RsRnsSG⁵⁰⁸) and Ser750 (⁷⁴⁵RtRttSV⁷⁵¹) residues. Thus, the objective of this study was to determine whether Ser507 and Ser750 residues of ULK2 could be phosphorylation sites of SGK1 as one of its authentic substrate proteins. Using ULK2 507 and 750 serine residue un- or phosphorylation analog (S507AS750A or 507DS750D), we observed that modification of Ser507 or Ser750 residue was required to activate the kinase activity of ULK2 and sensitize ULK2 to stress or starvation while simultaneously enhancing its active state and autophagy characteristics, suggesting that phosphorylation at Ser750 or Ser507 residue could modulate its subcellular localization and protein interaction with AMPK1α to activate ULK2. We also observed that ULK2 autophagy activity was enhanced by GSK650394 (an SGK1 inhibitor) to compensate survival capacity through increasing its association with LC3 and phosphorylation. When SGK1 known to be associated with cell survival was inhibited by GSK650394, ULK2 autophagy pathway was activated to avoid cell death alternatively. Thus, our observations indicate that phosphorylation of ULK2 by SGK1 can regulate cell survival as an alternative modulation of ULK2 functions.