S1P mimics phosphorylation to mediate PDK1-independent activation of aPKC with an opposing downstream effect

Atypical protein kinase C (aPKC) isozymes play opposing roles in oncogenic and tumor-suppressive signaling pathways. These kinases are activated by phosphoinositide-dependent kinase 1 (PDK1) as well as by direct binding of the lipid second messenger sphingosine 1-phosphate (S1P). Here, we found that S1P and PDK1 provide mutually exclusive activation of aPKC by a common switch mechanism. In silico docking, molecular dynamics, and biochemical approaches revealed that the phosphate on S1P occupied the same position as the phosphate of the PDK1-phosphorylated Thr residue in the activation loop to structure the active site for catalysis. The interaction of either phosphate with the activation loop was coordinated by conserved Arg and Lys residues. When aPKC was phosphorylated, these residues positioned the activation loop in a manner that made the binding of S1P energetically prohibitive. Cellular studies revealed that S1P sustained basal signaling of aPKC, which suppressed apoptosis. In contrast, PDK1-dependent phosphorylation promoted apoptosis. These data identify a regulatory mechanism by which a ligand mimics a posttranslational modification and reveal that aPKC is at the crossroads of lipid second messenger signaling through cytosolic S1P pathways and canonical PDK1 signaling.