Nir1 constitutively localizes at ER–PM junctions and promotes Nir2 recruitment for PIP(2) homeostasis

Homeostatic regulation of plasma membrane (PM) phosphatidylinositol 4,5-bisphosphate (PIP(2)) in receptor-stimulated cells is mediated by the lipid transfer protein Nir2. Nir2 is dynamically recruited to endoplasmic reticulum–plasma membrane (ER–PM) junctions to facilitate replenishment of PM PIP(2) hydrolyzed during receptor-mediated signaling. However, our knowledge regarding the activation and sustainment of Nir2-mediated replenishment of PM PIP(2) is limited. Here, we describe the functions of Nir1 as a positive regulator of Nir2 and PIP(2) homeostasis. In contrast to the family proteins Nir2 and Nir3, Nir1 constitutively localizes at ER–PM junctions. Nir1 potentiates Nir2 targeting to ER–PM junctions during receptor-mediated signaling and is required for efficient PM PIP(2) replenishment. Live-cell imaging and biochemical analysis reveal that Nir1 interacts with Nir2 via a region between the FFAT motif and the DDHD domain. Combined, results from this study identify Nir1 as an ER–PM junction localized protein that promotes Nir2 recruitment for PIP(2) homeostasis.