Nir1-Nir2 Heterodimerization Confers Robustness to the Phosphoinositide Cycle

The phosphatidylinositol (PI) cycle maintains plasma membrane (PM) phosphatidylinositol 4,5-bisphosphate (PIP2) levels to sustain signaling and cellular homeostasis in receptor-stimulated cells. This process requires recruitment of the PI transfer protein Nir2 to endoplasmic reticulum (ER)-PM junctions where PI and phosphatidic acid are exchanged between the two membranes. Nir2 recruitment is promoted by its paralog Nir1, which lacks PI transfer activity but is constitutively localized to ER-PM junctions; however, the mechanism underlying this regulation has remained unclear. Here, we identify and determine crystal structures of a conserved Nir Dimerization (NirD) domain in Nir1 and Nir2. Mutations that disrupt NirD domain dimerization abolish Nir1-mediated recruitment of Nir2 and impair PIP2 replenishment driven by Nir1-Nir2 dimerization in receptor-stimulated cells. Mechanistically, Nir1-Nir2 dimerization enables graded recruitment of Nir2 in proportion to stimulus strength, thereby expanding both the sensitivity and dynamic range of PI cycle responses. Together, this defines the structural basis of Nir1-dependent Nir2 recruitment and reveals a mechanism that confers robustness to the PI cycle, enabling precise homeostatic signaling across a wide range of stimulus intensities.