Endoplasmic reticulum architecture: structures in flux

Abstract

The endoplasmic reticulum (ER) is a dynamic pleiomorphic organelle containing continuous but distinct subdomains. The diversity of ER structures parallels its many functions, including secretory protein biogenesis, lipid synthesis, drug metabolism and Ca2+ signaling. Recent studies are revealing how elaborate ER structures arise in response to subtle changes in protein levels, dynamics, and interactions as well as in response to alterations in cytosolic ion concentrations. Subdomain formation appears to be governed by principles of self-organization. Once formed, ER subdomains remain malleable and can be rapidly transformed into alternative structures in response to altered conditions. The mechanisms that modulate ER structure are likely to be important for the generation of the characteristic shapes of other organelles.

Figure 1

Electron micrographs illustrating different forms of OSER. Shown are: (a) a karmella, (b) a lamella, (c) a whorl, (d,e) sinusoidal arrays with cubic symmetry, and (f) a bundle of packed tubules with hexagonal symmetry. In (a), N indicates the nucleus; at the distal side of the karmella is a forming cisterna that appears to be generated by the fusion of tubules. In (b) and (d), the arrows indicate continuity with the cytosolic space; note that the electron-dense constant width space present in all the OSER structures corresponds to the cytosolic space bridging the cisternae. In (e) and (f), the asterisk indicates the lumenal space. Note, in (b) and (c), the continuity with sinusoidal ER of the lamella and whorl, respectively. Note also, in (c) and (d), the close association of mitochondria (M) with OSER structures. Structures shown in (a–e) were induced in cells transfected with a GFP–cytochrome b5 fusion protein [44•]. ‘Crystalloid’ ER with hexagonal symmetry (f) is expressed in compactin-resistant UT-1 cells expressing high levels of HMG-CoA reductase [63]. Scale bars: (a,b,c,f) 200 nm; (c,d) 600 nm. (f) is reproduced with permission from the Company of Biologists from [59].