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bioRxiv · 10.1101/2025.06.18.660273

ERM proteins regulate the shape and number of Endoplasmic Reticulum-Plasma Membrane Junctions in neurons

Deng, H.·Cheng, J.·Fetter, R. D.·Qin, G.·Zhang, J.·Liang, X.·Taylor, C.·Zhang, M.·Wu, X.·Shen, K.
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Abstract

Endoplasmic Reticulum (ER) - Plasma Membrane (PM) Junctions (EPJs) are specialized contact sites between ER membrane and the inner leaflet of PM. These junctions are critical for lipid exchange and Ca2+ signaling. In excitable cells like neurons and muscle, EPJs further modulate membrane excitability by regulating Ca2+ homeostasis. The mechanisms controlling EPJ abundance and morphology remain poorly understood. Using in vivo fluorescence imaging and electron microscopy of C. elegans neurons, we showed that EPJs form discrete, patch-like structures distributed across the soma. Through a forward genetics screen, we identified two conserved ERM (Ezrin-Radixin-Moesin) proteins, FRM-4 and FRM-1, as key regulators of EPJ shape and abundance. Both proteins localize to EPJs and exhibit liquid-liquid phase separation properties (LLPS). in vitro, purified FRM-4 binds to FRM-1, and together bundle filamentous actin. However, their presence in LLPS condensates and actin-bundling activity are mutually exclusive. In vivo, F-actin cables surround--but do not penetrate--EPJs, where FRM proteins are enriched as phase-separated condensates. Loss of FRM-4, FRM-1, or disruption of F-actin led to increased mobility of EPJs that fused into fewer but enlarged junctions. Together, our findings demonstrate that FRM-4 and FRM-1 control EPJ morphology by organizing peri-junctional F-actin networks, thereby restricting EPJ mobility and fusion.

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Deng, H., Cheng, J., Fetter, R. D., Qin, G., Zhang, J., Liang, X., Taylor, C., Zhang, M., Wu, X., Shen, K.. 2025-06-18. ERM proteins regulate the shape and number of Endoplasmic Reticulum-Plasma Membrane Junctions in neurons. https://doi.org/10.1101/2025.06.18.660273

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