The endoplasmic reticulum connects to the nucleus by constricted junctions that mature after mitosis

Junctions between the endoplasmic reticulum (ER) and the outer membrane of the nuclear envelope (NE) physically connect both organelles. These ER-NE junctions are essential for supplying the NE with lipids and proteins synthesized in the ER. However, little is known about the structure of these ER-NE junctions. Here, we systematically study the ultrastructure of ER-NE junctions in cryo-fixed mammalian cells staged in anaphase, telophase, and interphase by correlating live cell imaging with three-dimensional electron microscopy. Our results show that ER-NE junctions in interphase cells have a pronounced hourglass shape with a constricted neck of 7-20 nm width. This morphology is significantly distinct from that of junctions within the ER network, and their morphology emerges as early as telophase. The highly constricted ER-NE junctions are seen in several mammalian cell types, but not in budding yeast. We speculate that the unique and highly constricted ER-NE junctions are regulated via novel mechanisms that contribute to ER-to-NE lipid and protein traffic in higher eukaryotes. Junctions between the endoplasmic reticulum (ER) and the nuclear envelope (NE) form a narrow and constricted hourglass shape, suggesting that these junctions are formed and maintained by novel mechanisms that might be crucial for nuclear function.ER-NE junctions exhibit an hourglass shape with a highly curved and constricted neck of 7-20 nm width in interphase. The morphology of ER-NE junctions is significantly distinct from that of junctions within the ER. ER-NE junctions start to become specialized in early telophase. The specialized ER-NE junctions occur in several mammalian cells but not in budding yeast. Junctions between the endoplasmic reticulum (ER) and the nuclear envelope (NE) form a narrow and constricted hourglass shape, distinct from junctions within the ER.