The Endoplasmic Reticulum Flashcards

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1
Q

What is the function of the endoplasmic reticulum

A

Function focused on proteins (and lipids)
Most membrane proteins assembled in the ER membrane
Starting point of all secreted proteins
Starting point for most proteins destined for lumen of ER, Golgi and lysosomes
Proteins are folded here and modified
Major site of quality control
Cellular lipids also made in ER membrane

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2
Q

What is the rough endoplasmic reticulum

A

Ribosomes on rER
Correlation of rER with capacity of cells to secrete proteins
Membrane-found ribosomes synthesise secretory proteins

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3
Q

How does a cell make a protein to be secreted

A

The signal hypothesis- prediction that newly synthesised proteins have a built in signal which directs them to the ER and through a channel in the membrane, onwards to their destination
Mechanism where proteins insert into or across a membrane are synthesised by membrane-bound ribosomes
Signal peptide guides positioning of the ribosome to the membrane

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4
Q

What are the three options for a new protein

A

If it has no targetting/signal sequence= released form ribosome to the cytosol (default pathway)
If it has organelle-specific targetting sequence= released then imported (e.g mitochondria, chloroplasts, peroxisomes or nucleus - NLS)
If it has an ER signal sequence = will enter ER and join the secretory pathway

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5
Q

What are signal peptides (or signal patches)

A

Sequence of amino acid that are exposed in the folded protein
Regions of aa (separated from each other in the unfolded protein) that need to be juxtaposed to function as a transport signal (harder to predict)

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6
Q

What are the different types of signals and their typical sequence peptide

A

Some proteins have more than one
Typically at the N-terminus

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7
Q

Where does the synthesis of all proteins begin

A

Begins in the cytoplasm
Signal for ER translocation is N-terminal positive , hydrophobic, polar

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8
Q

What is co-translation translocation

A

Most general mode
Utilises signal-recognition particle (SRP) and SRP receptor
Energy comes from translational (GTP hydrolysis)
SRPs move between ER membrane and cytosol binding to any ER signal sequence
That ER signal sequence= ‘start transfer sequence’
SRP binds this in a cleft (and the ribosome) and pauses translation dual recognition of signal peptide by SRP in cytosol and by translocator at ER membrane (in pore) allows selective proteins into ER

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9
Q

What happens during the translocation of protein into ER

A

After engagement with translocator:
1. Polypeptide translocates through pore in translocator
2. Signal peptide see clips off the signal peptide
3. Mature protein released into ER lumen
4. Translocator closed

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10
Q

What is sec translocon

A

Used in both co and post translational translocation
Passive pore- associating partners provide energy. Depending on partner there are different ways the channel can work
Aqueous pore
Closed by a plug when unused= gated
Polypeptide fed unfolded through cores hydrophobic lining
Core can also open along side to let cleaved signal sequence out and to integrate membrane proteins into plasma membrane

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11
Q

What is the sec translocon pore and plug

A

Helical plug blocks channel in closed state
Pore lined by hydrophobic residues
Cross linking experiments confirm movement of the plug during translocation

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12
Q

What is the role of the smooth endoplasmic reticulum

A
  • makes and metabolises fats such as those involved in steroid hormone synthesis from cholesterol
  • in liver SER makes lipoprotein lipid carriers and detoxifies lipid-soluble drugs, poisons and metabolites, makes them water soluble for excretion
  • SER also a major calcium ion store
    Regions of SER specialised for calcium ion storage
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13
Q

What happens to proteins that are misfolded in the ER

A

Detection by chaperones: Misfolded proteins are detected by ER chaperones.
UPR activation: The UPR is triggered to restore ER homeostasis.
ERAD: Misfolded proteins are ubiquitinated and degraded by the proteasome.
ER-phagy: Autophagy selectively degrades ER portions containing misfolded proteins.
Apoptosis: If stress is unmanageable, the cell may initiate programmed cell death

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14
Q

How are soluble proteins modified in the ER

A

Assembly of multi-subunit/multimeric proteins
Cleavage of some specific sites
Addition of GPE anchors
Synthesising of cell membrane lipids
There is also N-linked oligosaccharide addition that starts in the rough ER with addition of a large preformed oligosachharise precursor - N- linked glycosylation

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15
Q

What is meant by topogenic sequence

A

Collective term used for a peptide sequence that isessential for insertion and orienting protein in cellular membranes

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16
Q

How can proteins be inserted into the ER membrane

A

Can happen co translationally or post translationally

17
Q

What is the difference between co and post translational translocation

A

Co-translational translocation occurs while the protein is still being translated whereas post translational translocation occurs after the polypeptide has been translated by cytosolic ribosomes

18
Q

What is post translational translocation

A

Proteins are fully synthesized by ribosomes in the cytosol before they are translocated to their target organelles.
Targeting Signals: Proteins destined for specific organelles have targeting signals, usually short amino acid sequences, that direct them to their correct location.
Chaperones: Cytosolic chaperone proteins often assist in keeping the newly synthesized proteins in an unfolded state, which is necessary for their translocation across membranes.
Translocation Channels: The proteins pass through translocation channels or complexes in the membranes of organelles

19
Q

What is a translocon

A

Translocons are composed of several ER membrane proteins that associate to form an aqueous pore through which secretory proteins and lumenal domains of membrane proteins pass from the cytoplasm to the ER lumen. These sites are not passive holes in the bilayer, but instead are quite dynamic

The channel is gated by a short alpha helix that is through to keep translocator closed when it isn’t being used and to move aside when a polypeptide is going through

20
Q

What is the role of a translocator/translocon

A
21
Q

What happens to water soluble proteins during translocation

A

They are completely translocated through the ER membrane and released into the lumen

22
Q

What happens to transmembrane proteins during translocation

A

They become embedded in the ER membrane

23
Q

Describe the process of translocation through the ER membrane for water soluble proteins

A

N-terminal signal sequence initiates translocation

24
Q

What is a stop transfer sequence

A
25
Q

Describe how transmembrane proteins are translocated through the membrane

A
26
Q

What are membrane bound ribosomes

A

Attached to cytosolic side of ER membrane
Involved in synthesis of proteins that are being translocated into the ER membrane at the same time

27
Q

What are free ribosomes

A

They aren’t attached to any membranes
Are in the cytosol
Synthesise all other proteins coded for by the genome

28
Q

What is the function of the ER

A
29
Q

What is the basic structure of the ER

A
30
Q

What is the ER lumen

A

formed because the ER and nuclear membranes form a continuous sheet enclosing a single internal space

31
Q

What is the smooth ER

A

• Smooth ER = where vesicles which transport newly made proteins to the Golgi - these smooth parts of the ER are called transitional ER
• Smooth ER in abundant in cells which specialise in lipid metabolism
• release of calcium ions from cytosol from ER and reuptake of calcium ions cures in response to extracellular signals
• Calcium ion pumps transport calcium ions from the cytosol to the ER lumen
• High conc of calcium ion binding proteins in ER facilitate calcium ion storage