Polyunsaturated fatty acid‐containing phosphatidic acid interacts with synaptojanin‐1 and enhances its phosphoinositide D‐4‐phosphatase activity

Abstract

Introduction Phosphatidic acid (PA, 1,2-diacyl-sn-glycerol-3-phosphate), the simplest glycerophospholipid interacts with wide variety of proteins to facilitate intracellular signal transduction. PA consists of a variety of molecular species that have different fatty acyl compositions. Consequently, mammalian cells contain up to 50 structurally distinct molecular species. In our previous studies, we identified α-synuclein as a monounsaturated fatty acid (MUFA)-containing PA (18:1/18:1-PA)-binding protein (MUFA-PABP) from the mouse brain. However, polyunsaturated fatty acid (PUFA)-PABPs have not ever been isolated from the mouse brain. Results In the present study, we attempted to identify 18:0/22:6-PABPs from the mouse brain the a screening method using 18:0/22:6-PA-containing liposome precipitation and MS/MS analysis. Intriguingly, we found for the first time that synaptojanin-1 (SYNJ1) bound to 18:0/22:6-PA. SYNJ1 is a brain-enriched lipid phosphatase that dephosphorylates D-5 and D-4 position phosphates from phosphatidylinositol (4,5)-bisphopshate (PI(4,5)P2). PI(4,5)P2 is a major membrane-bound signaling molecule that regulates intracellular organelle transport. Further characterization revealed that SYNJ1 specifically bound to PA and did not associate with other phospholipids, such as phosphatidylcholine, phosphatidylserine, phosphatidylglycerol, phosphatidylinositol and cardiolipin. Interestingly, in addition to 18:0/22:6-PA, 18:0/20:4-PA also interacted with SYNJ1. Moreover, 18:0/20:4- and 18:0/22:6-PA strongly enhanced D-4-phosphatase activity, but not D-5-phosphatase activity, of SYNJ1. Conclusion We demonstrated for the first time that SYNJ1 specifically interacts with PUFA-PAs and is functionally controlled by them. Moreover, we revealed that SYNJ1 is a very unique PABP that possesses substantial preference to PA. We have already reported that, unlike SYNJ1, α-synuclein bound to MUFA-PA (e.g. 18:1/18:1-PA) in the brain. Taken together, these results indicate that there are many functionally distinct PABPs having different selectivities to PA species. Therefore, it is likely that each PA species individually regulate diverse physiological functions through diffeent PABPs such as SYNJ1 and α-synuclein.