Human PHOSPHO1 Exhibits Phosphatidylcholine- and Phosphatidylethanolamine-phospholipase C Activities and Interacts with Diacylglycerol Kinase δ

Phosphatidylcholine- and phosphatidylethanolamine-specific phospholipase C (PC-PLC and PE-PLC) activities, which generate diacylglycerol (DG) and are tricyclodecan-9-yl-xanthogenate (D609)-sensitive, have been detected in both the membrane and cytosolic fractions. We have previously demonstrated that sphingomyelin synthase isozymes, which are transmembrane proteins, exhibit PC-/PE-PLC activities. However, mammalian cytosolic PC-PLC and PE-PLC remain unidentified. Here, we demonstrated that phosphatase orphan 1 (PHOSPHO1), a cytosolic protein, exhibits D609-sensitive PC-PLC and PE-PLC activities. Moreover, the overexpression of PHOSPHO1 in HEK293 cells significantly increased the levels of cellular saturated and/or monounsaturated fatty acid-containing DG. Furthermore, DGKδ cosedimented and colocalized with PHOSPHO1. Collectively, these in vitro findings provide, for the first time, a promising candidate for the long-sought cytosolic PC-/PE-PLC, which may act as DG supply enzyme upstream of DGKδ.