Plasma membrane curvature promotes ER-PM contact formation mediated by junctophilin.

Abstract

The endoplasmic reticulum (ER) forms close membrane-membrane contact with the plasma membrane (PM), which is essential for lipid biosynthesis and transfer, as well as calcium release and retrieval. The spatial distribution and the abundance of ER-PM contact vary greatly within cells both spatially and temporally, while the spatial determinant for contact formation is still unclear. Cardiomyocytes (CMs), form curvature-enriched PM structures named transverse tubules (T-Tubules), acting as a hub for ER-PM contact formation. Loss of T-tubule structures in vitro culture is accompanied by decreased ER-PM contact. This suggests a potential role of membrane curvatures in ER-PM contact formation. Using nanofabricated quartz nanopillars of the same dimension as T-tubules, we showed that nano-curvatures could recruit ER-PM contact mediator junctophilin (JPH) and subsequently establish ER-PM contact in both CMs and non-muscle cells. JPH interacts with the PM through a positively charged low complexity region and senses the curvature through its interaction with caveolae on the PM. Calcium depletion assay reveals that calcium release-activated channel (CRAC) gets recruited to the curvature-driven ER-PM contacts upon activation. The measurement of intracellular calcium propagation in CMs reveals that PM curvatures could also promote voltage-gated calcium channel (VGCC) mediated calcium transients. Thus, PM curvature recruits ER through the interaction between caveolae and JPH and promotes the formation of functional ER-PM contact in excitable cells.