Phosphatidylinositol 4,5-Bisphosphate Homeostasis Regulated by Nir2 and Nir3 Proteins at Endoplasmic Reticulum-Plasma Membrane Junctions

Phosphatidylinositol (PI) 4,5-bisphosphate (PIP 2 ) at the plasma membrane (PM) constitutively controls many cellular functions, and its hydrolysis via receptor stimulation governs cell signaling. The PI transfer protein Nir2 is essential for replenishing PM PIP 2 following receptor-induced hydrolysis, but key mechanistic aspects of this process remain elusive. Here, we demonstrate that PI at the membrane of the endoplasmic reticulum (ER) is required for the rapid replenishment of PM PIP 2 mediated by Nir2. Nir2 detects PIP 2 hydrolysis and translocates to ER-PM junctions via binding to phosphatidic acid. With distinct phosphatidic acid binding abilities and PI transfer protein activities, Nir2 and its homolog Nir3 differentially regulate PIP 2 homeostasis in cells during intense receptor stimulation and in the resting state, respectively. Our study reveals that Nir2 and Nir3 work in tandem to achieve different levels of feedback based on the consumption of PM PIP 2 and function at ER-PM junctions to mediate nonvesicular lipid transport between the ER and the PM. Background: Mechanisms coupling phosphatidylinositol (PI) 4,5-bisphosphate (PIP 2 ) hydrolysis to its rapid replenishment remain elusive. Results: Phosphatidic acid production triggers PIP 2 replenishment mediated by Nir2 and Nir3 at endoplasmic reticulum (ER)-plasma membrane (PM) junctions with PI at the ER membrane. Conclusion: Nir2 and Nir3 are feedback regulators for PIP 2 homeostasis. Significance: Nir2 and Nir3 maintain PIP 2 homeostasis via a nonvesicular mechanism at ER-PM junctions.