1-Stearoyl-2-docosahexaenoyl-phosphatidic acid interacts with and activates Praja-1, the E3 ubiquitin ligase acting on the serotonin transporter in the brain

Serotonin transporter (SERT) is involved in serotonergic system regulation and in the pathophysiology/therapeutics of serotonin-/SERT-related diseases such as obsessive-compulsive disorder, depression, autism, and schizophrenia. We recently revealed that diacylglycerol (DG) kinase (DGK) δ induces ubiquitination/degradation of SERT in a DGK activity-dependent manner through Praja-1 E3 ubiquitin-protein ligase. However, it is still unclear how Praja-1 activity is regulated by DGKδ. Here, we reveal that 1-stearoyl-2-docosahexaenoyl (18:0/22:6)-phosphatidic acid (PA) and 18:0/22:6-DG are simultaneously decreased and accumulated, respectively, in the DGKδ-knockout mouse brain, indicating that DGKδ selectively phosphorylates 18:0/22:6-DG to generate 18:0/22:6-PA. Moreover, we find that 18:0/22:6-PA selectively binds to Praja-1 and enhances its activity. These results strongly suggest that 18:0/22:6-PA generated by DGKδ activates Praja-1 to degrade SERT in the brain.