The Endoplasmic Reticulum as an Active Liquid Network
The peripheral endoplasmic reticulum (ER) forms a dense, interconnected, and constantly evolving network of membrane-bound tubules in eukaryotic cells. While individual structural elements and the morphogens that stabilize them have been described, a quantitative understanding of the dynamic large-scale network topology remains elusive. We develop a physical model of the ER as an active liquid network, governed by a balance of tension-driven shrinking and new tubule growth. This minimalist model gives rise to steady-state network structures with density and rearrangement timescales predicted from the junction mobility and tubule spawning rate. Several parameter-independent geometric features of the liquid network model are shown to be representative of ER architecture in live mammalian cells. The liquid network model connects the time-scales of distinct dynamic features such as ring closure and new tubule growth in the ER. Furthermore, it demonstrates how the steady-state network morphology on a cellular scale arises from the balance of microscopic dynamic rearrangements. SIGNIFICANCEThe peripheral endoplasmic reticulum (ER) forms a continuous, dynamic network of tubules that plays an important role in protein sorting, export, and quality control, as well as cellular signaling and stress response. Elucidating how the unique morphology of the ER arises and supports its function is critical to developing a mechanistic understanding of the many neurological diseases associated with ER structural perturbations. We develop a physical model of the ER as an active liquid network to understand how its cellular-scale structure emerges from small-scale dynamic rearrangements. The model demon-strates how key features of ER architecture can arise from a balance of tubule growth and tension-driven sliding. This work provides insight into the fundamental physical mechanisms underlying the emergent morphology of the ER.