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bioRxiv · 10.64898/2026.06.03.729928

Nir1-Nir2 Heterodimerization Confers Robustness to the Phosphoinositide Cycle

Rahn, T. A.·Li, W.-T.·Lee, W.-R.·Wu, L.·Airola, M. V.·Liou, J.
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Abstract

The phosphatidylinositol (PI) cycle sustains phosphoinositide-calcium signaling by replenishing plasma membrane phosphatidylinositol 4,5-bisphosphate (PIP2) consumed during receptor activation. Recruitment of the lipid transfer protein Nir2 to endoplasmic reticulum (ER)-plasma membrane (PM) contact sites (also known as ER-PM junctions) is central to this process, enabling transfer of PI from the ER to the PM for PIP2 resynthesis. Although the disease-associated Nir2 paralog Nir1 is essential for Nir2 recruitment under physiological conditions, the molecular basis of this regulation has remained unresolved. Here, we identify a conserved Nir Dimerization (NirD) domain in both Nir1 and Nir2 and determine the crystal structures. We show that NirD mediates preferential Nir1-Nir2 heterodimerization, which promotes Nir2 recruitment to ER-PM junctions and enhances PIP2 replenishment in stimulated cells. Furthermore, Nir1-Nir2 heterodimerization confers stimulus-strength-dependent, graded recruitment of Nir2, thereby broadening the sensitivity and dynamic range of PI cycle activity. Together, our findings uncover the structural mechanism underlying Nir1-dependent regulation of Nir2 and reveal paralog heterodimerization as a key strategy for scaling lipid transport at membrane contact sites to signaling demand, ensuring robust phosphoinositide homeostasis.

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Rahn, T. A., Li, W.-T., Lee, W.-R., Wu, L., Airola, M. V., Liou, J.. 2026-06-06. Nir1-Nir2 Heterodimerization Confers Robustness to the Phosphoinositide Cycle. https://doi.org/10.64898/2026.06.03.729928

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