Control of Phosphoinositide Synthesis and Degradation by the Acyltransferase Lycat

Phosphoinositides (PIPs) are important regulators of various cellular phenomena including intracellular signaling, membrane traffic and cell migration. PIPs are formed as a result of the regulated phosphorylation of the inositol headgroup of phosphatidylinositol (PI) on specific positions by certain lipid kinases and phosphatases. It is well appreciated that the enrichment of specific PIPs, defined by inositol headgroup phosphorylation, within specific membrane compartments plays a critical role in organelle identity and membrane traffic. However, while much attention has been given to understanding of the role of inositol headgroup phosphorylation in PIP function, much less is known about the role of dynamic incorporation of specific acyl groups into these phospholipids. Importantly, PI and PIPs exhibit remarkable and unique selectivity for certain acyl groups. For example, about 45% of PIs (but not other phospholipids) are rich in 1-steroyl 2-arachidonyl. We recently identified that the possible control of the selective incorporation of steric acid at the sn-1 position is by the acyltransferase LYCAT, which controls the levels, acyl profile and function of phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2) (Bone et al. Mol Biol Cell 2017. 28:161-172). Here we examine how perturbation of LYCAT leads to a reduction in the levels of PI(4,5)P2 and phosphatidylinositol-3,4,5-trisphosphate (PI(3,4,5)P3). To measure the rate of PI(4,5)P2 synthesis, we treated cells with ionomycin to first ablate this PIP, followed by washout of the drug and monitoring of rate of reappearance via localization of a fluorescent PI(4,5)P2 probe. To measure the rate of PI(4,5)P2 degradation, we arrested PI(4,5)P2 synthesis by a pharmacological inhibitor, phenylarsine oxide (PAO) and monitored the loss of cellular PI(4,5)P2. Lastly, to examine the production of PI(3,4,5)P3, we treated cells with epidermal growth factor (EGF) and monitored the production of this PIP. Together, this work provides new information about how the dynamic and selective remodeling of specific phospholipids controls their levels, localization and function.