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endoplasmic reticulum (ER), in biology, a continuous membrane system that forms a series of flattened sacs within the cytoplasm of eukaryotic cells and serves multiple functions, being important particularly in the synthesis, folding, modification, and transport of proteins .
All eukaryotic cells contain an endoplasmic reticulum (ER). In animal cells, the ER usually constitutes more than half of the membranous content of the cell. Differences in certain physical and functional characteristics distinguish the two types of ER, known as rough ER and smooth ER.
Rough ER is named for its rough appearance, which is due to the ribosomes attached to its outer (cytoplasmic) surface.
Rough ER lies immediately adjacent to the cell nucleus, and its membrane is continuous with the outer membrane of the nuclear envelope. The ribosomes on rough ER specialize in the synthesis of proteins that possess a signal sequence that directs them specifically to the ER for processing.
(A number of other proteins in a cell, including those destined for the nucleus and mitochondria, are targeted for synthesis on free ribosomes, or those not attached to the ER membrane; see the article ribosome.) Proteins synthesized by the rough ER have specific final destinations.
Proteins secreted from the Golgi apparatus are directed to lysosomes or to the cell membrane; still others are destined for secretion to the cell exterior.
The rough endoplasmic reticulum has on it ribosomes, which are small, round organelles whose function it is to make those proteins. Sometimes, when those proteins are made improperly, the proteins stay within the endoplasmic reticulum. They’re retained and the endoplasmic reticulum becomes engorged because it seems to be constipated, in a way, and the proteins don’t get out where they’re suppose to go.
Then there’s the smooth endoplasmic reticulum, which doesn’t have those ribosomes on it. And that smooth endoplasmic reticulum produces other substances needed by the cell. So the endoplasmic reticulum is an organelle that’s really a workhorse in producing proteins and substances needed by the rest of the cell.
The ER is the largest organelle in the cell and is a major site of protein synthesis and transport, protein folding, lipid and steroid synthesis, carbohydrate metabolism and calcium storage . The multi-functional nature of this organelle requires a myriad of proteins, unique physical structures and coordination with and response to changes in the intracellular environment.
Work from a variety of systems has revealed that the ER is composed of multiple different structural domains, each of which is associated with a specific function or functions. However, it is not yet clear how these functional subdomains are organized and how different functional domains translate into different structures.
Protein synthesis and folding
One of the major functions of the ER is to serve as a site for protein synthesis for secreted and integral membrane proteins , as well as a subpopulation of cytosolic proteins . Protein synthesis requires localization of ribosomes to the cytosolic face of the ER, and the canonical pathway that regulates protein synthesis involves co-translational docking of the mRNA:ribosome complex on the ER membrane.
Translation of secretory or integral membrane proteins initiates in the cytosol, then ribosomes containing these mRNAs are recruited to the ER membrane via a signal sequence within the amino terminus of the nascent polypeptide that is recognized and bound by the signal recognition particle (SRP) ].
The complex of mRNA:ribosome:nascent polypeptide:SRP is targeted to the ER where it docks on the SRP receptor . Translation continues on the ER and the emerging polypeptide can co-translationally enter the ER through the translocon , which is a channel that contains several Sec proteins and spans the lipid bilayer .
While the ER is a major site of protein synthesis, it is also a site of bulk membrane lipid biogenesis , which occurs in the endomembrane compartment that includes the ER and Golgi apparatus. Proteins and phospholipids, which are the major lipid component of membranes, are transferred and biochemically modified in the region of the ER that is in close juxtaposition to the Golgi apparatus .
This region, known as the ER-Golgi intermediate compartment (ERGIC), is rich in tubules and vesicles . Once lipids are mobilized to the ERGIC they are distributed throughout the cell through organelle contacts or secretory vesicles .
The cis-Golgi, which is the closest structure to the ERGIC, leads to the trans-Golgi network where vesicles carrying newly synthesized secretory proteins from the ER form and bud . The trans-Golgi network has traditionally been viewed as the main sorting station in the cell where cytosolic cargo adaptors are recruited to bind, indirectly or directly, and transport proteins or lipids .
Calcium (Ca2+) metabolism
Finally, while the ER is a major site of synthesis and transport of a variety of biomolecules, it is also a major store of intracellular Ca2+ . The typical cytosolic concentration of Ca2+ is ~100 nM, while the Ca2+ concentration in the lumen of the ER is 100–800 μM, and the extracellular Ca2+ concentration is ~2 mM . The ER contains several calcium channels, ryanodine receptors and inositol 1,4,5-trisphosphate (IP3) receptors (IP3R) that are responsible for releasing Ca2+ from the ER into the cytosol when intracellular levels are low . Ca2+ release occurs when phospholipase C (PLC) is stimulated through G protein-coupled receptor (GPCR) activation and cleaves phosphatidylinositol 4,5 bisphosphate (PIP2) into diacyl-glycerol (DAG) and IP3, which can then bind the IP3R leading to Ca2+ release and transient increase in intracellular Ca2+ levels
. Ryanodine receptors (RyRs) act through Ca2+-induced Ca2+ release (CICR), when the receptors bind Ca2+ in response to increased cytoplasmic levels of Ca2+ . In addition, depolarization of t-tubule membranes can lead to conformational changes in voltage-dependent Ca2+ channels, such as dyhydropyridine receptors (DHPRs), which interact and activate RyRs leading to Ca2+ release .
Regulation of ER shape and function
The ER is a complex organelle, involved in protein and lipid synthesis, calcium regulation and interactions with other organelles. The complexity of the ER is reflected in an equally complex physical architecture.
The ER is composed of a continuous membrane system that includes the nuclear envelope (NE) and the peripheral ER, defined by flat sheets and branched tubules (Fig. 1). The shape and distribution of these ER domains is regulated by a variety of integral membrane proteins and interactions with other organelles and the cytoskeleton.
These interactions are dynamic in nature and reflect changes within the cell, either through cell cycle or developmental state, cell differentiation, intracellular signals or protein interactions. While it is generally known how the basic shapes of ER sheets and tubules are determined, it is relatively unclear how changes in shape or the ratio of sheets to tubules occur in response to specific cellular signals.
There have been several excellent, recent reviews that cover the topic of general ER structure in detail , so we will limit our review of the basic ER structure to only those factors that may play a role in changing the shape of ER in response to signaling. The ER consists of the nuclear envelope and the peripheral ER, which includes smooth tubules and rough sheets.
While the ER is defined as an interconnected network with a continuous membrane, the different structures that make up the ER perform very diverse and specialized functions within the cell.
The nuclear envelope is made up of two lipid bilayers, the inner nuclear membrane (INM) and outer nuclear membrane (ONM), and shares a common lumen with the peripheral ER.
Hundreds of nuclear pores spanning the ONM and INM of the nuclear envelope allow transport of molecules, including RNAs and proteins, at various rates of diffusion or regulated transport depending on the size of the molecule. The nuclear envelope is connected to sheets, or cisternae, that make up part of the peripheral ER . Sheets are usually observed in a stacked conformation and are connected via regions of twisted membranes with helical edges .
ER shaping proteins
Peripheral ER structures are just as distinct and diverse as the set of proteins that contribute to their shape. Several proteins have been identified that promote specific ER structures, but perhaps the most well-studied group of proteins include the reticulon family of proteins that localize to tubules and the highly curved edges of ER sheets .
These integral membrane proteins contribute to the bending of the membrane by forming a transmembrane hairpin topology that acts as a wedge, displacing lipids in the outer leaflet of the bilayer leading to curvature of the membranes . These proteins tend to form oligomers and are much less mobile than other ER-resident proteins .
Overexpression of some reticulon isoforms leads to formation of long ER tubules at the expense of sheets .Therefore, the level of reticulons within a cell determines the abundance and fine structure of ER tubules.