Custom Photos. Site Info. Contact Us. The Galleries:. Photo Gallery. Silicon Zoo. Chip Shots. DNA Gallery. Amino Acids. Religion Collection. Cocktail Collection. Screen Savers. Win Wallpaper. 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.
It is clear that proteins involved in the promotion, maintenance or stabilization of peripheral ER structures function through interactions with additional proteins or structures, and these interactions are key to proper formation of the ER network.
Interestingly, several of the proteins discussed above have been shown to interact with microtubules, including Climp63 [ 91 ], p [ 94 ], kinectin [ 95 ] and STIM1 discussed below. One important interaction discussed below is with microtubules.
The ER network exhibits several dynamic interactions with microtubules that are important for determining the distribution of the ER within the cell. The two main types of interactions between the ER and microtubules are Tip Attachment Complexes TACs and sliding along preformed microtubules by the action of kinesin and dynein motors [ 96 — ].
In cultured cells treated with nocodazole to depolymerize microtubules, the ER retracts from the periphery [ ], though the retraction does not occur immediately. Further investigation revealed that sliding events occurred mainly on a small subset of microtubules, modified by acetylation, that are more resistant to nocodazole treatment [ 76 ].
Furthermore, ER tubules can form in the absence of microtubules [ 57 , 65 , 68 ], raising many questions and leading several groups to study the interaction between ER and microtubules more in-depth. In the past 10 years we have learned a great deal about what proteins are responsible for the intrinsic shape of the ER and how these proteins are connected to specific ER subdomains.
However, we know very little about how cellular signals communicate with ER shaping proteins to change the shape of the ER in response to cellular signals. During mitosis many cellular structures are dramatically remodeled to facilitate chromosome segregation. One of the most dramatic examples is changes to the microtubule cytoskeleton that occur as a result of increased microtubule dynamics caused by the action of cyclin-dependent kinases.
The increase in microtubule dynamics during mitosis is important for the bipolar attachment of chromosomes to the mitotic spindle and accurate segregation to daughter cells during anaphase [ ]. In addition to changes to the microtubule cytoskeleton, essentially all organelles change shape and function during mitosis to facilitate accurate organelle inheritance and orderly chromosome segregation. The ER undergoes dramatic shape changes during mitosis and recent studies are beginning to uncover the mechanisms linked to these structural changes.
Smooth ER — the detox stop Smooth ER also plays a large part in detoxifying a number of organic chemicals converting them to safer water-soluble products. Large amounts of smooth ER are found in liver cells where one of its main functions is to detoxify products of natural metabolism and to endeavour to detoxify overloads of ethanol derived from excess alcoholic drinking and also barbiturates from drug overdose.
To assist with this, smooth ER can double its surface area within a few days, returning to its normal size when the assault has subsided. The contraction of muscle cells is triggered by the orderly release of calcium ions.
These ions are released from the smooth endoplasmic reticulum. Cytoskeleton — the movers and shapers in the cell. Extracellular Matrix and Cell Adhesion Molecules. Endoplasmic reticulum is an organelle found in both eukaryotic animal and plant cells. It often appears as two interconnected sub-compartments, namely rough ER and smooth ER.
Both types consist of membrane enclosed, interconnected flattened tubes. The rough ER, studded with millions of membrane bound ribosomes, is involved with the production, folding, quality control and despatch of some proteins. Smooth ER is largely associated with lipid fat manufacture and metabolism and steroid production hormone production.
They also help to detoxify the harmful drugs. In some cells SER is responsible for transmission of impulses, e. It also plays an important role in transport of materials from one part of the cell to the other.
Endoplasmic reticulum also provides mechanical support to the cell so that its shape is maintained. What is the importance of Endoplasmic reticulum? May 26,
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