That's because the SER doesn't build proteins , so it won't ever need to use ribosomes. The space inside the ER is called the cisternal space or the lumen. The hydrophilic parts of the fats built by the SER point out into the lumen. It also describes the structure and functions of the SER. The endoplasmic reticulum is a network of membranes found in eukaryotic cells. There are actually two parts to it: the rough and the smooth part.
It is studded with ribosomes. The ribosomes help build proteins through a process called translation , in which they convert a strand of mRNA into a chain of amino acids. The smooth part of the endoplasmic reticulum SER is called such because it lacks ribosomes.
The SER contains many enzymes with different roles depending on what type of cell they are in. Unlike the RER, it does not have a role in protein synthesis. There is usually not a lot of SER in cells except for a few special cases, like muscle cells. They have something called a sarcoplasmic reticulum, which is a specialized SER that stores calcium ions.
This includes things like hormones and phospholipids that make up the cell membrane. Biology Endoplasmic Reticulum ER. Explanations 5 Sylvia Freeman. The Cellular Synthesizer In eukaryotic cells there are a lot of different organelles , so it can be hard to remember what all of them do. Image source: By Sylvia Freeman.
Related Lessons. 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.
Examples of cells with abundant sER are sebaceous glands, gonadal cells involved in producing steroid hormones such as Leydig cells in the testis and follicular cells in the ovary , hepatocytes in the liver, and cells of striated muscles.
The diverse metabolic processes where sER is involved varies according to the cell type and they are found abundant in certain cell types that rely heavily on sER functions.
The major functions of sER include lipid synthesis, carbohydrate metabolism, regulation of intracellular calcium concentration, and drug detoxification. The cells involved in the secretion of these products, such as those in the testes, ovaries, and skin oil glands have a great amount of sER. Even though the cholesterol synthesized by the endoplasmic reticulum is very low, most of the molecular machinery that regulates the cellular cholesterol homeostasis resides within it.
The smooth endoplasmic reticulum is also involved in the transfer of molecules produced in the rough ER to the Golgi complex. The smooth endoplasmic reticulum is the major site for lipid synthesis, particularly at the membrane contact sites MCS. MCS are areas where ER membranes make close contact with other cytoplasmic organelles, such as Golgi apparatus , mitochondria , lysosomes , peroxisomes , endosomes, chloroplasts , and plasma membrane, and allow the transfer of substances.
The sER, therefore, has a major role to play in the balancing of different categories and classes of lipids and thereby influencing the cellular lipid biomass. They also help to maintain the membrane homeostasis across the cell. The membrane-contact site between ER and mitochondria is involved with the synthesis of phospholipids.
Phospholipids primarily create barriers in cellular membranes to protect the cell and the organelles.
They are also involved in the regulation of cellular processes related to growth, immune surveillance, and synaptic transmission. Phospholipids also play a major role in the nonvesicular lipid transportation between the ER and other organelles.
Ceramides are a family of waxy lipids that play a key role as a structural element of the cell. They are also crucial in cellular signaling, cell cycle, cell senescence, cell migration, and adhesion. Ceramides are synthesized in the sER. They are transported from sER to Golgi as a component of the transport vesicle membrane. Steroid hormones synthesized in the smooth endoplasmic reticulum are lipophilic and cannot be stored in vesicles from which they would diffuse easily, and hence they are synthesized when needed as precursors.
Among the steroids produced by the SER in animal cells are the sex hormones of vertebrates and the various steroids secreted by the adrenal glands.
The cells that synthesize and secrete these hormones, for example, the cells in the testes and ovaries are rich in the smooth endoplasmic reticulum. Glucose is the main source of energy in eukaryotes. Organisms have evolved capabilities to metabolize glucose from other non-carbohydrate precursors in the absence of glucose. The synthesis of glucose from non-carbohydrate precursors like pyruvate, oxaloacetate, succinate, lactate, etc.
These non-carbohydrate precursors undergo several processes where they are converted from one intermediate compound to another, in the presence of several enzymes in multiple steps. In most tissues, the final compound formed is glucose 6-phosphate and free glucose is not generated. Glucose 6-phosphate cannot diffuse out of the cell, therefore it is stored inside the cell. This in turn paves a way for the regulation of glucose effectively.
A special enzyme called glucose 6-phosphatase which is produced by smooth endoplasmic reticulum is required to hydrolyze glucose 6-phosphate into glucose. This enzyme is only present in tissues of the liver and kidney, where they are involved in the maintenance of blood glucose homeostasis. In these tissues, glucosephosphate is transported to the lumen of the smooth endoplasmic reticulum and finally converted into glucose by the action of the glucosephosphatase enzyme.
The smooth endoplasmic reticulum also stores calcium ions. In muscle cells, for example, a special type of smooth endoplasmic reticulum known as sarcoplasmic reticulum serves as an important intracellular calcium buffer zone.
When the muscle cells are stimulated by a nerve impulse, calcium ions rush across the membrane of the endoplasmic reticulum into the cytosol and trigger the contraction of muscle cells. In other cell types, calcium ions released from the smooth endoplasmic reticulum triggers responses, such as the secretion of vesicles carrying newly synthesized proteins. Following protein synthesis and translocation into the ER lumen, a protein destined for secretion must undergo proper folding and modifications, with the aid of chaperones and folding enzymes.
These modifications include N-linked glycosylation, disulfide bond formation and oligomerization [ 3 ]. At this point the fate of the secretory proteins is determined. If the protein functions in the ER, for example as a chaperone, then proper folding will commence. If the protein is destined for secretion, it will be released by the chaperones and packaged for travel through the Golgi on to a final destination such as the plasma membrane or secreted or move into peroxisomes [ 21 ].
Additionally, the cytosolic regions of the transmembrane protein may interact with cytosolic proteins or chaperones to properly fold these domains. On the other hand, even with several proteins and complexes dedicated to folding proteins properly, a fraction of proteins do not achieve native and functional form and are either misfolded or aggregated [ 22 ].
These proteins can either remain in the ER or enter the ER-associated degradation ERAD pathway mediated by the proteasome, assuring that aberrant polypeptides do not inadvertently enter the secretory pathway [ 23 ]. Recognition of misfolded proteins, followed by clearing of these aggregates through the ERAD pathway, needs to be tightly controlled so as not to affect cellular function [ 23 ]. Interestingly, there are several connections to activation of ER stress response pathways and pathological human conditions.
Additionally, activation of the ER stress response pathway is observed in diabetes, inflammatory bowel disease, and various cancers. How ER stress response pathways play a role in these pathologies is an active area of research and various components of the stress response pathways are being investigated as potential therapeutic targets [ 24 ].
While the ER is a major site of protein synthesis, it is also a site of bulk membrane lipid biogenesis [ 4 ], 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 [ 25 ].
Once lipids are mobilized to the ERGIC they are distributed throughout the cell through organelle contacts or secretory vesicles [ 26 ]. 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 [ 4 ].
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 [ 27 ]. Calcium is a widespread signaling molecule that can affect diverse processes including localization, function and association of proteins, either with other proteins, organelles or nucleic acids. 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. 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. Various ER structural morphologies. This view highlights the relationship of the ER to the nuclear envelope red arrow.
This highlights the complexity of the peripheral ER. Three-way junctions, ER tubules and small ER sheets are highlighted red arrows. Reprinted with permission from James Jamieson.
Scale bar is 0. Reprinted with permission from Fig. Here, we will discuss what is known about how the structures of ER are formed, how the dynamics of the ER are regulated, and how these dynamics change in response to cell cycle state and cellular cues. In addition, we provide examples of how the proteins that are involved in contributing to ER shape are influenced by these cellular cues, such as calcium release, and how this is reflected in the dynamics of the ER and ultimately the function of specialized cells that display varying ratios of sheets to tubules.
There have been several excellent, recent reviews that cover the topic of general ER structure in detail [ 7 , 44 — 48 ], 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 flat in nature consisting of two lipid bilayers with an intervening lumen, with curved regions located only at the membrane edges. Peripheral ER Sheets may vary in size, but the luminal spacing is very consistent, usually about 50 nm in mammals and 30 nm in yeast [ 49 ] Fig.
Sheets are usually observed in a stacked conformation and are connected via regions of twisted membranes with helical edges [ 50 ]. These rough sheets, as defined by the high density of ribosomes on the cytosolic surface [ 51 , 52 ], are the main site of synthesis, folding and post-translational modifications for secreted or membrane-bound proteins.
In turn, far fewer ribosomes are present on the membrane surface of ER tubules [ 52 ], which is highly curved and smooth and may not accommodate the binding of large polysomes Fig. The tubular network is dynamic, continually rearranging and growing, and is defined by three-way junctions that connect individual tubules Fig. While tubules and sheets possess very different structural features, and hence play a role in different cellular processes, the luminal spacing of both tubules and sheets is similar [ 49 , 52 ].
Structure of ER sheets and tubules. Eukaryotic ribosomes are 25—30 nm and localize to the flat regions of ER sheets, giving the sheets a rough appearance rough ER. Ribosomes are present in much lower numbers on tubules, giving the tubules a more smooth appearance smooth ER.
Interestingly, ER tubules and sheets are found in all eukaryotic cells [ 53 ], though the ratio of sheets to tubules varies in different cell types and reflects the different functions of these cells. For example, the ER architecture of specialized cells that synthesize vast amounts of secreted proteins, such as pancreatic secretory cells and B cells, is largely made up of sheets Fig.
In turn, cells that are involved in processes including lipid synthesis, calcium signaling and sites of contact for other organelles possess an ER composed of primarily tubules Fig.
Adrenal, liver and muscle cells are all examples of specialized cells with a predominantly tubular network and reflects the function of these cells [ 54 ]. An additional configuration of the peripheral ER includes cortical ER, which abuts the plasma membrane and displays an intermediate phenotype between sheets and tubules with membranes that are both highly curved as well as regions that are flat in nature.
Calcium signaling occurs at the contact sites between the plasma membrane and the abutting cortical ER and is necessary for muscle contraction [ 55 , 56 ]. Therefore, the morphology and intracellular location of the ER subdomains contribute to the function of these structures and hence the role of the specialized cell in which they are located.
Improved microscopy techniques have allowed for the characterization of different ER structures, and the ratios of these structures to one another, in specialized cell types. When comparing the roles of these cells in the organism, it is clear that the type and amount of peripheral ER present reflects the function of that particular cell type.
It is still unclear how these ratios are generated and what cellular signaling pathways play a role in designating which ER type will be most prominent in a particular cell type.
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 [ 51 , 57 ].
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 [ 57 ]. These proteins tend to form oligomers and are much less mobile than other ER-resident proteins [ 58 ]. Overexpression of some reticulon isoforms leads to formation of long ER tubules at the expense of sheets [ 58 ].
In turn, depletion of reticulons, and hence the ability to bend membranes, leads to a reduction in the number of ER tubules, leading to an expansion of peripheral sheets [ 57 , 59 , 60 ]. Therefore, the level of reticulons within a cell determines the abundance and fine structure of ER tubules.
Reticulons do not act alone in shaping ER tubules. Atlastins, members of the dynamin-like GTPase family, mediate these homotypic fusion events. Depletion by RNAi or expression of dominant-negative atlastin in cells results in a lack of fusion events leading to an abundance of long, unbranched tubules [ 61 ].
When a dominant-negative cytoplasmic fragment from Xenopus , which contains the GTPase domain but lacks the transmembrane domain and cytoplasmic tail [ 64 ], are introduced into Xenopus interphase extracts ER network formation was blocked [ 65 ].
Comparable point mutations that prevent dimerization of the cytoplasmic fragment of human atlastin [ 66 ] were made in the Xenopus cytoplasmic atlastin protein, added into interphase extract and had no effect on ER network formation [ 65 ]. Furthermore, antibodies directed against atlastin inhibit ER network formation when introduced into Xenopus egg extracts [ 61 ].
In Drosophila , atlastin depletion leads to ER fragmentation and purified atlastin is sufficient to catalyze GTP-dependent fusion of proteoliposomes [ 64 , 66 , 67 ]. Therefore, studies from multiple organisms, extracts and purified components indicate that atlastin is likely required for catalyzing homotypic vesicle fusion between ER membranes, which is important for proper network formation. Recently, a few new key players have been identified that are involved in ER dynamics.
Work using purified ER vesicles derived from Xenopus eggs has demonstrated that GTP is required for homotypic ER vesicle fusion in the absence of cytosolic factors [ 57 , 68 ]. Previous studies indicated that GTPases are required for ER fusion events [ 69 , 70 ], and a recent study utilized a proteomics approach to identify Rab10 as a factor required for ER assembly [ 71 ].
Knock-down of Rab10, or overexpression of a GDP-locked dominant-negative point mutant, in cultured human cells caused an increase in ER sheets and a decrease in tubules [ 71 ]. ER—ER fusion events occurred at regions where Rab10 was enriched. It is currently not clear what role Rab10 plays in the ER vesicle fusion reaction or how homotypic ER vesicle fusions are coupled to lipid synthesis.
Depletion of Rab18 leads to a phenotype similar to that observed following Rab10 inhibition [ 72 ]. Additionally, when Rab10 is depleted, Rab18 redistributes to peripheral sheets [ 72 ]. Therefore, it appears that depletion of either Rab10 or Rab18 prevents the stabilization of ER tubule fusion, reducing the density of tubules resulting in an increase in ER sheets.
In addition to the role RAB-5 plays in peripheral ER formation, kinetics of nuclear envelope disassembly is affected in these mutants [ 70 ]. In addition to GTPases that may play a direct role in homotypic membrane fusion of vesicles, recent work has demonstrated a role for lipid synthesizing enzymes in controlling the shape and organization of the ER. Inhibition of C-terminal domain CTD nuclear envelope phosphatase-1 CNEP-1 , which is enriched on the nuclear envelope and promotes the synthesis of membrane phospholipids, led to the appearance of ectopic sheets that encased the nuclear envelope, interfering with nuclear envelope breakdown [ 74 ].
These results reflect the interconnected network of proteins and functions that play a role in shaping the structures of the ER.
The ER is a very dynamic network that is constantly undergoing rearrangements and remodeling [ 75 ]. ER tubules are continually fusing and branching resulting in the creation of new three-way junctions. In a competing process, junction sliding and tubule ring closure leads to loss of three-way junctions and the characteristic polygonal structure [ 76 ].
Very little is known about the complexes controlling this process, but it was recently discovered that Lunapark Lnp1 localizes to and stabilizes three-way junctions [ 77 , 78 ]. Lnp1 binds to reticulons and Yop1, and localization of Lnp1 to junctions is regulated by Sey1p, the yeast homolog of atlastin [ 78 ].
Loss of Lnp1 leads to a collapsed and densely reticulated ER network in yeast and human cultured cells [ 77 , 78 ], though only half of the junctions are bound to Lnp1 [ 77 ], which reflects the fluidity of the ER network. If Lnp1 is overexpressed, the protein localizes to the peripheral ER and induces the formation of a large polygonal tubular network [ 79 ]. Additionally, formation of this network was inhibited by Lnp1 mutations that blocked N -myristoylation [ 79 ], an attachment of myristic acid a carbon saturated fatty acid , indicating that this modification plays a critical role in Lnp1-induced effects on ER morphology.
N -myristoylation is not required for membrane translocation, topology formation, or protein localization to the ER but may play a role in protein—protein or protein-lipid interactions that are required for morphological changes in the ER, though the exact molecular mechanism of action remains to be elucidated [ 79 ]. The actual mechanism for Lnp1-mediated stabilization of three-way junctions is unknown, though recent studies and insights from the structure and domains within the protein shed light on how Lnp1 stabilizes junctions [ 77 , 78 ].
First, Lnp1 contains two transmembrane domains as well as a zinc finger domain, which is located on the cytoplasmic face of the ER membrane [ 77 ]. When cysteines were mutated within the zinc finger domain, the polygons became smaller and regions lacking cortical ER were more apparent as the number of cysteines mutated increased [ 78 ]. Therefore, mutations in the zinc finger domain may affect protein—protein interactions, complex formation or interfere with the distribution of resident lipids on the cytoplasmic face of the membrane causing deleterious effects on junction stabilization.
In addition, the transmembrane domains may be acting as an inverted wedge, adding to the local negative curvature characteristic of three-way junctions [ 77 ], and acting opposite to the positive curvature promoted by reticulons. Another possibility is that multiple Lnp1 proteins may also act cooperatively together to stabilize the junction, or Lnp1 may be acting transiently to stabilize or modify lipids or other proteins at junctions [ 77 ].
In addition to proteins that regulate membrane structure and dynamics, there is accumulating evidence that changing the nucleic acid content of the ER can also impact ER shape. Early experiments showed that brief treatment of tissue culture cells with the translation inhibitor puromycin, which dissociates mRNA:ribosome complexes, leads to loss of ribosomes from the ER and a loss of ER sheets [ 51 , 80 ].
Depletion of XendoU leads to the formation of long, unbranched tubules in Xenopus leavis egg extract, and rescue of this phenotype requires intact catalytic activity of the protein, indicating that the nuclease function is critical to proper ER network formation [ 82 ]. Furthermore, antibody addition to purified vesicles leads to a block in network formation, demonstrating that XendoU acts on the surface of ER membranes to regulate ER structure [ 82 ].
Depletion of XendoU also leads to a delay in replication and nuclear envelope closure [ 82 ], and BAPTA blocks nuclear envelope formation in Xenopus egg extract reconstitution experiments [ 85 ]. Upon vesicle fusion it was found that RNAs were degraded and released from the surface of membranes, suggesting that XendoU acts to degrade these RNAs, as well as release proteins, to clear patches of membrane to allow for vesicle formation leading to network formation [ 82 ].
Interestingly, when purified vesicles were treated with increasing concentrations of RNaseA and subjected to the same assay, an increasingly aberrant network formed with large vesicles that were unable to fuse [ 82 ]. Results from in vitro studies indicate that XendoU is activated on membranes in coordination with calcium release to locally degrade RNAs and clear patches of membranes leading to fusion in a controlled manner to fine tune network formation.
Lastly, similar to other proteins that play a role in tubule formation, knock-down of the human homolog EndoU in cultured human cells leads to an expansion of sheets [ 82 ]. Additionally, rescue of the expanded sheet phenotype depended on intact catalytic function as observed with recombinant protein in the extract system. Therefore, XendoU is an example of a protein that is activated in response to cellular cues to regulate proper ER formation, and further studies may reveal additional proteins that are regulated in this manner to fine tune organelle structure.
We have considered how tubules are formed and maintained, which leads the discussion to sheets, the other peripheral ER structure. First, we must consider how sheets are formed. Several mechanisms have been proposed, including the idea that integral membrane proteins can span the intraluminal space and form bridges, connecting the lipid bilayers [ 51 , 86 , 87 ].
These proteins may either stabilize the structure or define the distance between the two lipid layers based on the size of the proteins. Additionally, these proteins or protein complexes may form a scaffold that aids in the stabilization of the sheets or bring the two lipid membranes in closer proximity [ 86 ]. Several proteins including Climp63, p and kinectin have been implicated in the generation, maintenance and stabilization of ER sheets [ 51 ].
In addition to highly enriched membrane proteins and core components of the translocon, Climp63, a coiled—coiled protein with a single transmembrane domain, was identified along with kinectin and p in a mass spectrometry screen for abundant integral ER membrane proteins [ 51 ]. Through various techniques and in various cell types Climp63 was shown to be a highly abundant protein [ 88 — 90 ] that localizes to perinuclear ER and is absent from the nuclear envelope [ 91 , 92 ].
Very stable oligomers of Climp63 can form, restricting mobility of the protein along the membrane, promoting localization to the rough ER [ 92 ]. Overexpression of Climp63 leads to a massive proliferation of ER sheets while reduction in expression surprisingly does not lead to loss of sheets but instead a decrease in the distance between sheets [ 51 ].
Moreover, these sheets are spread diffusely throughout the cytoplasm, reminiscent of the phenotype of cells treated with the translation inhibitor puromycin [ 51 ]. This is interesting as the core components of the translocon, the protein channel that interacts with ribosomes and is responsible for translocating nascent peptides into the ER or anchoring transmembrane segments of newly synthesized proteins, were found to be enriched on sheets [ 93 ].
Therefore, these results suggest that the role of Climp63 in formation of sheets is likely to involve additional factors and acts as a part of an elaborate regulatory network that balances the production of sheets and tubules.
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