The ER in 3D: a multifunctional dynamic membrane network

Abstract

The endoplasmic reticulum (ER) is a large, singular, membrane-bound organelle that has an elaborate 3D structure with a diversity of structural domains. It contains regions that are flat and cisternal, ones that are highly curved and tubular, and others adapted to form contacts with nearly every other organelle and with the plasma membrane. The 3D structure of the ER is determined by both integral ER membrane proteins and by interactions with the cytoskeleton. In this review, we describe some of the factors that are known to regulate ER structure and discuss how this structural organization and the dynamic nature of the ER membrane network allow it to perform its many different functions.

Figure 1. ER morphological domains in mammalian cells and yeast

(a)Cos-7 cell expressing the ER membrane marker GFP-Sec61β. The ER in mammalian cells is a continuous membrane network that shares a lumen. This network consists of the nuclear envelope (NE) and peripheral ER cisternae and tubules. (b) EM tomography reveals the 3-D organization of the ER in budding yeast cells at nanometer resolution. The NE, central cisternal ER (CecER), tubular ER (TubER) and PM-associated ER (PmaER) are shown. Note that the NE is continuous with the pmaER through domains that are both tubular and cisternal. Scale bars: (a) 10 μm; (b) 100 nm. Image in (b) adapted with permission from J. Cell Biol. [4].

Figure 2. Two types of microtubule-dependent ER dynamics occur in mammalian cells

(a) Cos-7 cell imaged every 30 seconds apart and merged to demonstrate peripheral ER structure dynamics (expressing GFP-Sec61β at t=0 seconds in green and t=30 seconds in red). Arrows indicate unchanged ER (yellow), new ER tubule growth (white), and ER rearrangement (blue). (b) Model illustrating tip attachment complex (TAC) and sliding dynamics. TAC occurs on dynamic MTs; the ER extends along with the plus-end of a growing MT via an interaction between the ER protein STIM1 and the MT plus-end protein EB1. Sliding occurs on acetylated, curved, nocodazole-resistant MTs in a MT motor-dependent manner. The protein(s) that attach the ER to these motors are unknown. (c) Examples of a TAC event (left) or an ER sliding event (right). Cos-7 cells are expressing either GFP-Sec61β or YFP-STIM1 to visualize ER dynamics (green) and mCherry-α-tubulin to visualize microtubules (red) at the times indicated. Arrows indicate position of the ER tubule. Scale bars = (a) 10 μm; (c) 1 μm. Images in (a) and (c) adapted with permission from J. Cell Biol. [25].

Figure 3. The ER has many different subdomains that can interact with other membrane bound compartments

(a) Model depicting known ER domains and organelle contact sites in eukaryotic cells. Many of these contacts are maintained despite the dynamics of both membrane-bound compartments. (b–d) Coupled dynamics of the ER with other organelles. Cos-7 cells expressing GFP-Sec61β (ER) and (b) mito-dsRed (mitochondria), (c) mCherry-Rab5 (early endosomes), (d) mCherry-PTS1 (peroxisomes) at times indicated. Arrows indicate position of the ER-organelle contact. (e) EM tomography of wild type yeast cells reveals close apposition between the PmaER and PM membranes (black and white arrows mark ER and PM membrane bilayer, respectively). The distance between the two opposing membranes is maintained and the intervening region is ribosome-excluded. (f) EM tomograph of HeLa cells treated with thapsigargin (Tg, 1 μm) to deplete ER Ca²⁺ stores. Note induction of extensive ER-PM ribosome-free contact sites. Asterisk marks ER and closed circle marks PM. Scale bars: (b, c, d): 1 μm; (e) 50 nm. Images in (b, c, e) adapted with permission from J. Cell Biol. [25, 4]. Image in (f) adapted from [52].

Figure 4. The ER gets redistributed as cell shape changes and completely changes morphology during mammalian mitosis

(a) 3-D EM tomograms of wild type yeast cells reveal how ER domains are inherited into the daughter bud. Models are ordered from left to right with increasing bud size. ER tubules are inherited first into the bud (left and middle panel, tubER in green). The inherited tubular ER establishes contacts with the PM to form new PM-associated ER domains (middle and right panels, pmaER in blue). CecER in yellow. (b) A 3-D EM reconstruction of the ER network within the dendrite of a hippocampal neuron. Note the enrichment of tubular ER. (c) Fluorescent images of HeLa cells expressing H2B-mRFP (chromatin) and GFP-Sec61β (ER) demonstrate that 3-D ER structure changes dramatically between interphase and the indicated stages of mitosis. The NE breaks down, ER tubules are converted to cisternae, and the ER moves to the periphery of the cell away from the mitotic spindle. Scale bars: (a) 200 nm. Images in (a) adapted with permission from J. Cell Biol. [4]. Images in (b) adapted with permission from [73]. Images in (c) adapted with permission from [62].

Box 1, Fig. I. Models of how reticulon proteins shape regions of high membrane curvature in the peripheral ER

(a) Schematic of reticulon topology in the outer leaflet of the ER. Long transmembrane domains increase outer leaflet area relative to inner leaflet area, generating membrane curvature. (b) Schematic of ER cisterna and tubules (blue) indicating regions where reticulons (orange) have been observed to localize and shown to regulate membrane curvature, including ER tubules and the edges of cisternae and fenestra. Model in (a) adapted from [5]. Model in (b) adapted from [8].