Name: sec61 and srp in protein targeting
Uploaded: 2023-04-30T10:07:08.000Z
Duration: 1 min 20 s
Channel: JoVE

Most proteins enter the ER lumen while still under synthesis &#8212; a process called cotranslational translocation.
First, the SRP-SRP receptor complex carrying the ribosome-nascent chain complex unloads it on the Sec61 channel.
Once the ribosome aligns with the channel, the signal sequence at the N terminal of the polypeptide chain latches onto a signal sequence recognition site.
As the translation continues,&#160; the polypeptide chain forms a loop pushing open the channel plug for entry into the ER lumen.
While the polypeptide descends into the lumen, the signal sequence exits through the lateral gate of the Sec61 channel to enter the adjacent signal peptidase complex, where it is cleaved off.
The channel plug remains pushed open as the protein passively descends into the ER lumen.
After complete protein translocation into the lumen, the channel plug bounces back to block the protein from returning to the cytosol.
Inside the ER lumen, resident ER chaperones like BiP immediately bind the translocated protein and assist in protein folding.

cotranslational protein translocation

Cotranslational Protein Translocation

Translocation of proteins across membranes is an ancient process that occurs even in bacteria and archaebacteria. In fact, the components of the translocation machinery are still conserved between prokaryotes and eukaryotes.
Sec61 channel partners for cotranslational translocation
During cotranslational translocation, the Sec61 channel partners with the signal recognition particle (SRP), the signal recognition particle receptor (SR), and the ribosomes to transport the nascent polypeptide chain into the lumen.
SRP mediates ribosome attachment to the Sec61 complex via the polypeptide exit tunnel of the large ribosomal subunit. The GTPase domains on both SRP and SR carry out GTP hydrolysis, which supports the transfer of the target polypeptide to the Sec61 channel. Subsequently, the ribosomes act as primary translocation motors and push the protein towards the ER lumen. Except for the GTPs spent during peptide chain elongation, no additional energy is expended for the protein to descend through the channel.&#160;
Accessory translocon complex proteins involved in cotranslational translocation
The translocating chain-associating membrane (TRAM) protein and translocon-associated protein (TRAP) complex are auxiliary components associated with the Sec61 channel during SRP-mediated translocation. The TRAP complex is a substrate-specific complex essential for secure anchoring of the signal sequences having low hydrophobicity. Its proteins directly interact with Sec61 and keep the channel in open confirmation to compensate for the weak interaction with the signal sequence.
On the other hand, TRAM is a membrane protein that acts as a chaperone. It assists in folding and membrane integration of cotranslationally translocating proteins.

Translocation of proteins across membranes is an ancient process that occurs even in bacteria and archaebacteria. In fact, the components of the ...

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JoVE Core

Cell Biology

Unit 1

Transmembrane Transport in Endoplasmic Reticulum and Peroxisomes

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KEY TERMS AND CONCEPTS

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 ribosomal subunit. This attachment helps guide the growing polypeptide chain to the Sec61 complex and into the lumen of the endoplasmic reticulum, or ER.

Both SRP and SR have GTPase domains, which use GTP hydrolysis. This step supports transfer of the target polypeptide to the Sec61 channel. After that, the ribosome acts as the main translocation motor and pushes the protein toward the ER lumen. Aside from the GTP used during peptide chain elongation, no extra energy is needed for the protein to pass through the channel.

Two accessory proteins, TRAM and the TRAP complex, also support this pathway. TRAP is a substrate-specific complex that is essential for securing signal sequences with low hydrophobicity. Its proteins interact directly with Sec61 and help keep the channel open when the signal sequence binds weakly. TRAM is a membrane protein that acts as a chaperone and helps with folding and membrane integration of proteins that are being translocated.

Sec61 and SRP in Protein Targeting

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...

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...

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Rough ER Protein Folding Changes

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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 unfolded protein response

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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

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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 Sec61, SRP, SR, ribosomes, TRAM, and TRAP move new proteins into the ER during cotranslational translocation in cells.

Sec61 and SRP in Protein Targeting | 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.

Sec61 and SRP in Protein Targeting

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