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Questions and Answers
What is the primary function of the smooth endoplasmic reticulum?
What is the primary function of the smooth endoplasmic reticulum?
Which type of protein is synthesized in the rough endoplasmic reticulum?
Which type of protein is synthesized in the rough endoplasmic reticulum?
What is the site of protein folding, processing, and modification?
What is the site of protein folding, processing, and modification?
What is the purpose of ribosomes attached to the rough endoplasmic reticulum?
What is the purpose of ribosomes attached to the rough endoplasmic reticulum?
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What is the name of the pathway that involves the movement of proteins from the ER to the Golgi to the outside of the cell?
What is the name of the pathway that involves the movement of proteins from the ER to the Golgi to the outside of the cell?
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Where are soluble proteins translated that are found in the cytosol of the cell?
Where are soluble proteins translated that are found in the cytosol of the cell?
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What is the function of SRP in co-translational ER protein translocation?
What is the function of SRP in co-translational ER protein translocation?
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What type of sequence is required for ER translocation of soluble proteins?
What type of sequence is required for ER translocation of soluble proteins?
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What is the difference between co-translational and post-translational ER protein translocation?
What is the difference between co-translational and post-translational ER protein translocation?
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What is the function of chaperones in the ER?
What is the function of chaperones in the ER?
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What is the purpose of protein disulphide isomerase (PDI) in the ER?
What is the purpose of protein disulphide isomerase (PDI) in the ER?
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What is the result of protein modification in the ER?
What is the result of protein modification in the ER?
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What is the function of the signal peptidase in ER protein translocation?
What is the function of the signal peptidase in ER protein translocation?
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What is the difference between membrane proteins and soluble secretory pathway proteins in the ER?
What is the difference between membrane proteins and soluble secretory pathway proteins in the ER?
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What is the importance of Sec61 in ER protein translocation?
What is the importance of Sec61 in ER protein translocation?
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Study Notes
Endomembrane System
- Also known as the secretory pathway, where proteins move from the ER to the Golgi to the outside of the cell.
Endoplasmic Reticulum (ER)
- The earliest secretory pathway organelle, consisting of a system of membranes that enclose a space (lumen) separated from the cytosol.
- ER has two sub-compartments: Smooth ER and Rough ER.
Smooth ER
- Synthesizes steroid hormones, not proteins.
- Stores calcium ions that trigger muscle contractions.
- Detoxifies alcohol and liver cells known as “barbiturates”.
Rough ER
- Studded with ribosomes.
- Synthesizes secretory pathway proteins and lipids.
- Site of protein folding, processing, and modification.
- Translates proteins made in the ER on ribosomes attached to the RER.
Protein Translation
- Proteins made in the ER are translated on ribosomes attached to the RER.
- Examples: secreted proteins (hormones and neurotransmitters), transmembrane proteins, and soluble proteins (golgi, lysosomal, and vacuolar proteins).
- Proteins translated in the cytosol are made on "free" ribosomes floating in the cytosol.
- Cytosolic proteins include enzymes, cytoskeletal proteins, and other soluble proteins.
ER Protein Co-translation and Translocation
- SRP (signal recognition particle) plays a crucial role in recognizing the signal sequence of the protein being translated.
- Translation begins on free ribosomes, then the protein complex moves to the ER membrane.
- Signal peptidase cleaves the signal sequence, allowing the protein to enter the ER lumen and fold with the help of chaperones.
ER Protein Post-translational Translocation
- Some proteins are fully synthesized in the cytosol and then transferred to the ER.
- These proteins still require a signal sequence to translocate.
ER Translocons
- Structurally similar across species, playing a crucial role in protein translocation.
- Examples: Sec61 (from chloroplast) is important for soluble and ER movement proteins.
Post-translational vs. Co-translational Translocation
- Protein translocation can occur either co-translationally (during protein synthesis) or post-translationally (after protein synthesis).
ER Membrane Protein Translocation
- Proteins have hydrophobic stop sequences that stay in the membrane.
- Proteins can be linked to the membrane with a GPI anchor.
Protein Folding and Modification
- Multitude of chaperones assist in folding proteins in the ER (e.g., BiP, GRP94, Calnexin, and calreticulin).
- PDI (protein disulphide isomerase) assists in forming disulphide bonds and is required for proper protein folding.
- Chaperones guide proteins to align correctly.
- Modification of proteins includes glycosylation.
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Description
Explore the secretory pathway and its components, including the Endoplasmic Reticulum, Smooth ER, and Golgi apparatus. Learn about their roles in protein synthesis and cellular functions.
