Name: single-pass membrane protein topology in the er
Uploaded: 2023-04-30T10:07:15.000Z
Duration: 1 min 26 s
Channel: JoVE

The ER signal sequence of a transmembrane protein acts as a start-transfer signal for translocation through the Sec61 channel on the ER membrane.
As translocation continues, the looped signal sequence uses the lateral gate of the Sec61 channel to move out.
It is then cleaved off by the adjacent signal peptidase complex, releasing the N terminal into the lumen.
A hydrophobic domain in the polypeptide chain cannot cross the lipid bilayer and acts as a stop-transfer signal.
Therefore, when the Sec61 channel encounters such a domain, it opens laterally to release the hydrophobic domain into the lipid bilayer, forming a transmembrane domain.
The ribosome then continues the synthesis of the cytosolic domain.
After translation terminates, the dissociating ribosome leaves behind a type I signal transmembrane protein embedded in the ER membrane with its N terminal in the lumen and C terminal in the cytosol.
If the hydrophobic domain is preceded by positively charged residues, the N terminal remains out in the cytosol.
The resultant transmembrane protein, a type II protein, is an upside-down type I protein.

insertion of single-pass transmembrane proteins in the rer

Insertion of Single-pass Transmembrane Proteins in the RER

Integral membrane proteins are proteins adhered to the lipid bilayer of a cell organelle or membrane. They can be of two types: transmembrane integral proteins that span the lipid bilayer and monotopic proteins that are attached to either side of the membrane but do not pass through it.
Integral transmembrane proteins possess transmembrane and extra membrane domains. The transmembrane domains are primarily made of 20-25 hydrophobic amino acids arranged in a helical secondary confirmation. These domains are stable enough to be embedded in the phospholipid interior of the membrane and are critical in determining the protein&#39;s topology. Such proteins are inserted into the ER membrane co-translationally and are broadly categorized as single-pass and multipass transmembrane proteins. There are three types of single-pass transmembrane proteins with a single domain traversing through the membrane.
Type I membrane proteins have a cleavable ER-signal sequence, a single transmembrane domain, the N terminal placed in the ER lumen. It has two distinct sequences to start and stop the polypeptide transfer through the translocon on the ER membrane. The human growth hormone receptor is a type I transmembrane protein.
Type II membrane proteins also have a single transmembrane domain, but it acts as a non-cleavable signal sequence and a membrane anchor, unlike type I membrane proteins. The transmembrane domain of the type II proteins is also called the signal anchor sequence. This sequence is always preceded by positive amino acid residues that prevent the N terminal of the polypeptide chain from slipping down into the translocon. Hence, the C terminal of type II proteins is oriented inside the ER lumen. The transferrin receptor and the Golgi galactosyltransferase are examples of a type II membrane protein.
Type III membrane proteins are similar to type II proteins in terms of the transmembrane domain structure; however, the positive amino acid residues are placed after the signal-anchor sequence. Thus, their N terminal is inserted into the translocon, and the C terminal is left out in the cytosol. Cytochrome P40 is a type III membrane protein.

Integral membrane proteins are proteins adhered to the lipid bilayer of a cell organelle or membrane. They can be of two types: transmembrane integral ...

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

Education

JoVE Core

Cell Biology

Unit 1

Transmembrane Transport in Endoplasmic Reticulum and Peroxisomes

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

KEY TERMS AND CONCEPTS

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. These regions fit into the phospholipid interior of the membrane and help determine the protein’s topology, or final orientation in the membrane.

These proteins are inserted into the ER membrane co-translationally, which means during protein synthesis. They are grouped as single-pass or multipass transmembrane proteins. Single-pass transmembrane proteins cross the membrane once, and they are divided into type I, type II, and type III proteins.

Type I membrane proteins have a cleavable ER signal sequence and one transmembrane domain. Their N terminus is placed in the ER lumen. They use two separate sequences to start and stop polypeptide transfer through the translocon in the ER membrane. The human growth hormone receptor is an example of a type I transmembrane protein.

Type II membrane proteins also have one transmembrane domain, but it works as a non-cleavable signal sequence and a membrane anchor. This domain is called a signal anchor sequence. It is preceded by positive amino acid residues that keep the N terminus from slipping into the translocon. As a result, the C terminus is oriented inside the ER lumen. The transferrin receptor and the Golgi galactosyltransferase are examples of type II membrane proteins.

Type III membrane proteins have a similar transmembrane domain, but the positive amino acid residues are located after the signal-anchor sequence. This places the N terminus into the translocon and leaves the C terminus in the cytosol. Cytochrome P40 is an example of a type III membrane protein.

Single-pass Membrane Protein Topology in the ER

single-pass-membrane-protein-topology-in-the-er

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...

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 unfolded protein response

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...

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 single-pass membrane proteins insert into the ER membrane and how type I, II, and III proteins differ in orientation and signal sequences.

Single-pass Membrane Protein Topology in the ER | 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.

Single-pass Membrane Protein Topology in the ER

Description

Transcript

Tags