Name: er stress control by the upr
Uploaded: 2023-04-30T10:07:52.000Z
Duration: 1 min 37 s
Channel: JoVE

When the ER-associated degradation or ERAD pathways work inefficiently, misfolded proteins accumulate in the ER and trigger the unfolded protein response or UPR.
UPR uses three transmembrane proteins, IRE1, ATF6, and PERK, as stress sensors.
These sensors detect misfolded proteins through their luminal domains and are activated. They then accelerate the clearance of misfolded proteins by relaying distress signals to the nucleus and cytoplasm.
After activation, the dimerized IRE1 signals the nucleus to upregulate chaperone production to assist in accurate protein folding.
Additionally, piled-up misfolded proteins initiate vesicular transport of ATF6 from the ER membrane to the cisternal membrane of the Golgi complex, where its cytosolic N-terminal domain is cleaved.
The released protein domain acts as a transcription factor and binds the ER response elements responsible for activating cellular protein folding and ERAD pathways.
On the contrary, activated PERK dimers phosphorylate and inhibit the eukaryotic translation initiation factor 2 alpha or eIF2&#593;, stalling protein synthesis.
Together, the activated UPR sensors use different modes to balance the synthesis and refolding of prote

the unfolded protein response

The Unfolded Protein Response

The ER is the hub of protein synthesis in a cell. It has robust systems to quality control protein folding and also for degradation of terminally misfolded proteins. Under normal conditions, a small proportion of misfolded proteins that cannot be salvaged need to be transported to the cytoplasm by the ER-associated degradation or ERAD pathways. However, if the ERAD cannot handle the misfolded proteins, the cell activates the unfolded protein response or UPR to adjust the protein folding pathways in the cell.
UPR detectors for misfolded proteins
Each of the three UPR receptors, IRE1, PERK, and ATF6, sense misfolded proteins and manage cell processes that allow clearance of misfolded proteins from the ER. Active IRE1 is responsible for upregulating the synthesis of ERAD machinery to accelerate the export of misfolded proteins, whereas active PERK stalls ribosomes during protein synthesis. Thus, these two signaling arms manage to balance the clearance and synthesis of proteins to alleviate ER stress. At the same time, the N-terminal region of ATF6 acts as a transcription factor to signal the upregulation of ER chaperones, such as BiP, lectins, and protein disulfide isomerase. These proteins aid in either refolding of misfolded proteins or their transport back to the cytosol for degradation.
UPR and ERAD for protein homeostasis
Under normal conditions, the ERAD is active at low levels. However, when the concentration of misfolded proteins increases in the ER, the number of chaperone molecules and components of retrotranslocation machinery is insufficient to clear this protein pile up, resulting in proteotoxic stress. UPR upscales the clearance of misfolded proteins via ERAD or lysosomal degradation. If the UPR cannot restore cellular homeostasis, the cell uses JNK protein kinase and caspases to trigger apoptotic programs.

The ER is the hub of protein synthesis in a cell. It has robust systems to quality control protein folding and also for degradation of terminally ...

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JoVE Core

Cell Biology

Unit 1

Transmembrane Transport in Endoplasmic Reticulum and Peroxisomes

transmembrane-transport-in-endoplasmic-reticulum-and-peroxisomes

KEY TERMS AND CONCEPTS

er stress control by the upr

The endoplasmic reticulum, or ER, is a major site of protein synthesis in the cell. It also has strong quality control systems for protein folding and for breaking down misfolded proteins that cannot be fixed.

Under normal conditions, some misfolded proteins are sent from the ER to the cytoplasm through ER-associated degradation, or ERAD. If ERAD cannot keep up, the cell turns on the unfolded protein response, or UPR. This response helps the cell adjust its protein folding pathways and reduce ER stress.

The UPR uses three main receptors: IRE1, PERK, and ATF6. These receptors sense misfolded proteins and help clear them from the ER. IRE1 increases the production of ERAD machinery, which speeds the export of misfolded proteins. PERK slows ribosomes during protein synthesis, which helps balance protein-making with protein clearance. ATF6 uses its N-terminal region as a transcription factor to increase ER chaperones such as BiP, lectins, and protein disulfide isomerase.

These chaperones help misfolded proteins refold or move back to the cytoplasm for degradation. When misfolded proteins build up, the ER can enter proteotoxic stress because there are not enough chaperones or retrotranslocation components to clear the load. The UPR then increases removal of misfolded proteins through ERAD or lysosomal degradation. If homeostasis still cannot be restored, JNK protein kinase and caspases can trigger apoptotic programs.

ER Stress Control by the UPR

the endoplasmic reticulum

ER Membranes, Microsomes, and Cell Support

The endoplasmic reticulum, or ER, is a major membrane network in eukaryotic cells. It makes up more than half of a cell’s membranes and about 10% of total cell volume. The ER also serves as a main site for protein and lipid synthesis for organelles such as the Golgi apparatus, lysosomes, secretory vesicles, and the plasma membrane. The ER was also the last major cell organelle to be discovered. In 1954, Keith Porter and George Palade produced the first high-resolution electron microscope...

smooth endoplasmic reticulum

Smooth ER in Lipids, Sugar, and Detox

Smooth endoplasmic reticulum, or smooth ER, is a cell organelle with specialized jobs in both animal and plant cells. It is often linked to the tubular shape of the endoplasmic reticulum. Its main role is to support processes that need the right internal conditions inside the cell. Smooth ER is an important site for making steroid hormones and lipids. These lipids include phospholipids and triglycerides. It also makes major membrane lipids such as phosphatidylcholine, phosphatidylethanolamine,...

role of er in the secretory pathway

ER Quality Control in Protein Secretion

The endoplasmic reticulum (ER) helps control protein secretion in eukaryotic cells. It acts as the first checkpoint in the secretory pathway, where many proteins are folded, modified, and tested for quality before they move on. The secretory pathway links membrane-bound compartments inside the cell and also connects cells to the outside environment. About one-third of the proteins made in a cell travel this route. These proteins move in membrane-bound vesicles between the ER, the ER-Golgi...

directing proteins to the rough endoplasmic reticulum

directing-proteins-to-the-rough-endoplasmic-reticulum

How SRP Targets Proteins to Rough ER

Proteins made in the cytosol often need a signal sequence to reach the rough endoplasmic reticulum (ER). These organelle-specific signals act like address tags and guide proteins to places such as the ER, mitochondria, and peroxisomes. Proteins sent to the ER may later move in vesicles through the Golgi complex to other organelles or leave the cell. The rough ER makes soluble proteins that can be sent to lysosomes or secreted outside the cell. It also makes transmembrane proteins, which become...

protein translocation machinery on the er membrane

protein-translocation-machinery-on-the-er-membrane

ER Membrane Translocon and Protein Entry

The translocon complex on the ER membrane acts as the main gateway for the protein secretory pathway. It moves new peptides into the ER lumen and also helps insert proteins into the ER membrane. In eukaryotes, the core of this complex is the Sec61 channel. Sec61 is a heterotrimer made of Sec61α, Sec61β, and Sec61γ, and it is the largest part of the translocon. In higher eukaryotes such as mammals, the channel is capped when it is resting. This helps prevent ions such as calcium from leaking...

cotranslational protein translocation

Sec61 and SRP in Protein Targeting

Sec61 and SRP help move newly made proteins across membranes during cotranslational translocation. This process is ancient and has been found in bacteria, archaebacteria, and eukaryotes. The machinery is conserved across these groups, which shows how important it is for cells. During cotranslational translocation, the Sec61 channel works with the signal recognition particle, or SRP, the SRP receptor, or SR, and ribosomes. SRP binds the ribosome through the polypeptide exit tunnel in the large...

post-translational translocation of proteins to the rer

post-translational-translocation-of-proteins-to-the-rer

Sec62-Sec63 Pathway for ER Protein Import

Post-translational translocation moves some proteins from the cytosol into the endoplasmic reticulum (ER) after they are made. These proteins still use the Sec translocon complex to cross the ER membrane and enter the ER lumen. During this process, Hsp40 and Hsp70 chaperones bind the new protein in the cytosol and help stop it from folding too soon. That chaperone binding also keeps the signal sequence exposed so it can contact the Sec61 channel. The ER signal sequence has features that help...

insertion of single-pass transmembrane proteins in the rer

insertion-of-single-pass-transmembrane-proteins-in-the-rer

Single-pass Membrane Protein Topology in the ER

Single-pass membrane protein topology in the ER explains how certain proteins are built into a cell membrane. Integral membrane proteins are attached to the lipid bilayer of a cell organelle or membrane. They may span the membrane as transmembrane proteins, or they may sit on one side only as monotopic proteins. Transmembrane proteins have both membrane-spanning and extra-membrane domains. Their transmembrane regions usually contain 20-25 hydrophobic amino acids arranged in a helical shape.

insertion of multi-pass transmembrane proteins in the rer

insertion-of-multi-pass-transmembrane-proteins-in-the-rer

How Multipass Proteins Enter the ER Membrane

Multipass transmembrane proteins enter the rough ER membrane through a stepwise insertion process. These proteins are type IV integral membrane proteins, and they use multiple topogenic sequences to set their final position in the membrane. They can stay in the ER or later move to the Golgi complex, lysosome, or cell membrane. Most multipass proteins do not have a cleavable signal sequence. Instead, their first hydrophobic segment, also called a transmembrane domain, acts as the ER signal...

tail-anchoring of proteins in the er membrane

tail-anchoring-of-proteins-in-the-er-membrane

ER Membrane Targeting of Tail-Anchored Proteins

Tail-anchored proteins are a group of membrane proteins that reach the endoplasmic reticulum, or ER, after they are made. They make up about 3% to 5% of membrane proteins in eukaryotic cells. These proteins have one transmembrane domain near the C-terminal end, which is the last part of the protein chain. Because the transmembrane segment is so close to the end, the signal recognition particle, or SRP, cannot bring these proteins to the ER for cotranslational insertion. Cotranslational...

gpi anchoring of proteins in the er membrane

gpi-anchoring-of-proteins-in-the-er-membrane

How GPI-Linked Proteins Reach the Membrane

GPI-linked proteins are made through a post-translational modification that helps place them on the outer side of the plasma membrane in eukaryotic cells. The GPI, or glycosylphosphatidylinositol, anchor is reversible and is built through 11 enzyme-driven steps. It includes a hydrophobic part and a hydrophilic part. The hydrophobic region contains phosphatidylinositol. The hydrophilic region includes phosphoethanolamine, glucosamine, mannose residues, and phosphate groups. Phosphoethanolamine...

protein modifications in the rer

Rough ER Protein Folding Changes

Rough ER protein folding changes help secretory and transmembrane proteins mature inside the ER lumen. These changes support folding and help stabilize the protein’s final three-dimensional shape. They happen at different stages as the protein folds. The main rough ER modifications fall into four groups: glycosylation, disulfide bond formation, assembly of protein subunits, and specific proteolytic cleavages, such as removal of signal sequences. N-linked glycosylation is the most common. In...

protein folding quality check in the rer

ER Glycoprotein Folding Checkpoint

The endoplasmic reticulum, or ER, is the main site where soluble and transmembrane secretory proteins mature and fold. A key part of this process is the calnexin cycle, a chaperone system that folds and checks N-glycosylated proteins before they leave the ER lumen. If this system cannot rescue a misfolded protein, the protein is sent to the cytoplasm for degradation. Calnexin and calreticulin are lectin chaperones. Lectins are glycoprotein-binding proteins, and these two are unique because...

export of misfolded proteins out of the er

export-of-misfolded-proteins-out-of-the-er

How the ER Clears Misfolded Proteins

The endoplasmic reticulum (ER) checks secretory and membrane proteins after they fold. Correctly folded proteins move through the calnexin cycle and continue to their final destination. Misfolded proteins are kept in the ER lumen, where ER chaperones try to unfold and refold them. When refolding fails, the protein becomes terminally misfolded. The ER then sends it to the cytosol through ER-associated degradation, or ERAD, for destruction. In this pathway, Hrd1 acts as both a ubiquitin ligase...

The endoplasmic reticulum, or ER, is a major site of protein synthesis in the cell. It also has strong quality control systems for protein folding and for breaking down misfolded proteins that cannot be fixed. Under normal conditions, some misfolded proteins are sent from the ER to the cytoplasm through ER-associated degradation, or ERAD. If ERAD cannot keep up, the cell turns on the unfolded protein response, or UPR. This response helps the cell adjust its protein folding pathways and reduce...

regulation of the unfolded protein response

regulation-of-the-unfolded-protein-response

IRE1 Signaling in ER Stress

IRE1 signaling helps cells respond to stress in the endoplasmic reticulum, or ER. IRE1, short for inositol-requiring kinase one, is the most conserved eukaryotic unfolded protein response receptor. It is a type I transmembrane protein kinase receptor with a site-specific RNase, or RNA-cutting, activity. The way misfolded proteins bind to the N-terminal domain of IRE1 is still not fully clear. Three activation models have been proposed: direct, indirect, and allosteric. Once a misfolded protein...

synthesis of phosphatidylcholine in the er membrane

synthesis-of-phosphatidylcholine-in-the-er-membrane

Phosphatidylcholine’s Roles in Cell Membranes

Phosphatidylcholine is a major glycerophospholipid in cell membranes. The endoplasmic reticulum, or ER, makes lipids that help build membranes and support cell functions such as energy storage and signaling. The lipid-making machinery sits in the ER membrane and gathers its reactants from the cytosol, the fluid inside the cell. After phosphatidylcholine and other lipids are made, the cell moves them to other organelles through the secretory pathway and through ER contact sites. Energy-rich...

assembly of the lipid bilayer in the er

How the ER Balances Membrane Lipids

The endoplasmic reticulum (ER) helps build and balance membrane lipids in cells. Biological membranes are not just barriers. They also keep cells and organelles stable and support communication between and within cells. Many membranes are made largely of phospholipids, which are a major type of membrane lipid. These lipids are not spread evenly. They differ from one organelle to another and even between the two layers, or leaflets, of a membrane. The ER is the starting point of the secretory...

peroxisomes

Peroxisome Functions in Cells

Peroxisomes are specialized organelles found in fungi, plant, and animal cells. Their number, size, shape, and activity can change with the tissue type and the cell’s nutritional state. Cells with active lipid metabolism, such as adipocytes, neurons, and hepatocytes, often contain more peroxisomes than other cells. Peroxisomes help break down complex organic molecules. They also synthesize certain macromolecules and take part in redox signaling, which is a way cells use oxidation and reduction...

protein import into the peroxisomes

How Peroxisomes Receive Their Proteins

Peroxisomes receive many of their proteins from the cytoplasm and the endoplasmic reticulum. These membrane-bound organelles oxidize organic molecules by transferring hydrogen atoms to oxygen. This process produces hydrogen peroxide, which peroxisomes then break down into water and oxygen. Peroxisomes do not have the genetic machinery to make their own proteins. Most peroxisomal membrane, lumenal, and transmembrane proteins are made in the cytoplasm or the ER and then moved to the peroxisome.

key terms and concepts

Learn how the unfolded protein response helps the ER manage misfolded proteins through IRE1, PERK, and ATF6 when ERAD is overwhelmed.

ER Stress Control by the UPR | JoVE K12

cell biology

JoVE Core Cell Biology illustrates complex cellular processes in easy-to-understand yet comprehensive animated video lessons. This collection focuses on the essential components and functions of cells, including organelles and membranes, cellular transport, and cell energetics. It is complemented by scientist-in-action videos that demonstrate the application of cell biology in real-world research experiments.

ER Stress Control by the UPR

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