The Endoplasmic Reticulum

Endoplasmic reticulum

Endoplasmic reticulum (ER) is a continuous membrane system but consists of various domains that perform the different functions. Structurally distinct domains of this organelle include the nuclear envelope (NE), the rough and the smooth ER, and the regions that contact other organelles. The establishment of these domains and the targeting of the proteins to them are understood to varying degrees. Despite its complexity, the ER is a dynamic structure. In mitosis, it must be divided into the daughter cells and the domains must be re-established, and even in interphase, it is constantly rearranged as the tubules extend along the cytoskeleton. Throughout these rearrangements, the ER maintains its basic structure. This is accomplished remains the mysterious, but some insight has been gained from in vitro systems.

The endoplasmic reticulum (ER) has the different functions. These include the translocation of the proteins (such as secretory proteins) across the ER membrane and the integration of proteins into the membrane; the folding and the modification of proteins in the ER lumen; the synthesis of the phospholipids and the steroids on cytosolic side of the ER membrane; and the storage of the calcium ions in the ER lumen and their regulated release into the cytosol . These functions have been studied extensively. We concentrate on structural and other aspects of the ER that are less well understood. ER shape At the light microscopy level, when we stained by fluorescent dyes, or with antibodies, when marked with the GFP-tagged proteins, the interphase ER can be divided into the nuclear and peripheral ER . The nuclear ER, or nuclear envelope (NE), consists of the two sheets of membranes with a lumen. The NE surrounds the nucleus, with the inner and the outer membranes connecting only at the nuclear pores. It is underlaid by a network of lamins. The peripheral ER is a network of the interconnected tubules that extends throughout the cell cytoplasm . The cell types, such as sea urchin eggs, flat sheets are also abundant. The lumenal space of the peripheral ER is with the nuclear envelope and together they can comprise the >10% of the total cell volume. In the Saccharomyces cerevisiae, the peripheral, the tubular ER network is located exclusively underneath the plasma membrane, and the dozen large tubules connect it to the membrane sheets of the NE . The ultrastructure of the ER has been visualized by the electron microscopy in the number of cell types. The most obvious difference seen is between the rough, i.e. ribosome-studded, and smooth regions of the ER. The RER often has a tubular appearance, where is the SER is often more dilated and convoluted. The relative abundance of the RER and the SER found among different cell types correlates with their functions and for the example, cells that secrete the large percentage of their synthesized proteins that contain mostly RER. The morphological differences between the RER and the SER allow these two regions of the ER to be distinguished visually; for example, the SER is often more convoluted than RER, and the RER tends to be more granular in the texture. These differences in appearance may be directly related to the presence of the bound ribosomes on the RER as there is some evidence that this affects ER structure. Ultimately, however, the distinction between the two must have been explained by differences in membrane protein composition. Most membrane proteins are shared between the RER and SER, but several proteins involved in translocation or processing of the newly synthesized proteins are enriched in the RER, as shown by the fractionation of liver cells.

The ER contacts with other organelles

The ER is closely associated with the essentially all other organelles in the cell. These include the plasma membrane, the Golgi, the vacuoles, the mitochondria, peroxisomes, late endosomes, and the lysosomes. The contact sites may establish the separate ER domains. In skeletal muscle, the SR abuts either the plasma membrane or the T-tubules,the specialized extensions of the plasma membrane which invaginate into the muscle cell, thereby forming junctional membranes. The several proteins including the ryanodine receptors , which are ER calcium release channels to localize to these structures. The junctophilin family members contribute the formation of these structures; transfection of the cells with at least one of the junctophilin proteins establishes regions of proximity between the plasma membrane and SR.

Formation of ER tubules

The cytoskeleton is not necessary for the formation of a tubular network in vitro. In Xenopus egg extracts,the ER networks can form de novo and this process is not affected by the addition of inhibitors of microtubule polymerization, by the depletion of tubulin from the extractor by inhibitors of actin polymerization. If the ER network is not formed along a cytoskeleton, how is it generated? The answer is not known, but some properties of the ER formation have been elucidated using in vitro systems. ER network formation in extracts requires ATP and GTP and is the NEM sensitive. In a Xenopus in vitro system, incubation of membranes in the absence of cytosol leads to the formation of the large vesicles that cannot subsequently be converted into networks by the addition of cytosol, and it is thought that of cytosolic factors convert the basic fusion reaction into a regulated process that produces tubular networks.

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