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Questions and Answers
What is the primary function of the Golgi apparatus in protein trafficking?
What is the name of the enclosed space within the Endoplasmic Reticulum?
Which type of Endoplasmic Reticulum is involved in lipid biosynthesis?
What is the primary function of the Endosome in protein trafficking?
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What is the approximate percentage of total cell membrane that can be made up of Endoplasmic Reticulum?
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What is the role of resident ER biosynthetic enzymes in lipid biosynthesis?
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What is the primary function of Scrambolase in the lipid bilayer?
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What is the primary function of Cytochrome P-450 in the smooth ER of the liver?
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What is the primary function of the rough ER?
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What is the fate of non-endomembrane proteins during protein synthesis?
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What is the primary function of Glucose-6-phosphatase in the smooth ER?
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What is the primary function of membrane transporters in the context of calcium ions?
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What direction do kinesin motor proteins move vesicles?
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What is the primary function of the Golgi apparatus in protein trafficking?
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Which enzyme is predominantly localized to the medial cisternae of the Golgi apparatus?
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What is the role of the Trans Golgi Network in protein trafficking?
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What is the result of differential glycosylation patterns in secretory proteins?
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In which direction do dynein motor proteins move vesicles?
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What is the primary function of the ER signal sequence?
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What is the role of the Signal Recognition Particle (SRP) in protein synthesis?
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What is the main function of chaperones in the ER?
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What is the primary site of protein glycosylation initiation?
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What is the primary difference between N-linked and O-linked glycosylation?
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What is the function of the 'stop transfer' anchor signal in newly translated membrane-associated proteins?
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Where does O-linked glycosylation occur?
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What is the role of calnexin in glycosylation?
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What is the function of UDP-glucose:glycoprotein glucosyltransferase (UGGT) enzymes?
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What happens to misfolded proteins in the ER?
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What is the role of COPII vesicles in the secretory pathway?
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What is the final step of the secretory pathway?
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Study Notes
Endomembrane System and Protein Trafficking
- Multiple organelles are involved in moving macromolecules (proteins) in and out of the cell.
- The rough Endoplasmic Reticulum (rER) is responsible for membrane/secretory protein biosynthesis, glycosylation, and folding.
- The Golgi apparatus is involved in protein processing, packaging, and sorting.
- Plasma membrane endomembrane vesicles are part of the trafficking system.
- Endosomes are responsible for cargo sorting, and lysosomes are responsible for degradation.
Endoplasmic Reticulum (ER)
- ER is a continuous network of flattened sacs or cisternae.
- The ER can make up around 50% of the total cell membrane.
- Rough ER has ribosomes attached to its membrane and is involved in synthesizing membrane/secretory proteins.
- Smooth ER lacks ribosomes and is involved in lipid biosynthesis, Ca++ storage, and drug detoxification.
- The ER lumen is the site of initial protein glycosylation and folding.
- Rough and smooth ER membranes are contiguous with each other and the nuclear envelope.
Rough ER
- Rough ER is studded with ribosomes that synthesize secretory and endomembrane system resident proteins.
- All mRNAs initially associate with cytosolic ribosomes, but secretory proteins are immediately trafficked to the ER for translation.
Smooth ER
- Smooth ER synthesizes membrane lipids, such as phospholipids, from glycerol, fatty acid precursors, and various polar head groups.
- Scramblase scrambles lipids to spread out the lipid bilayer.
- Smooth ER also stores/sequesters Ca++ and contains enzymes that metabolize foreign chemicals, such as cytochrome P-450.
- Smooth ER aids in glycogen metabolism, breaking down stored glycogen into glucose.
Protein Trafficking
- Secretory proteins "traffic" from the ER to the Golgi and then out towards the plasma membrane via vesicles.
- Vesicle transport occurs along microtubule "train tracks" using ATP-dependent motor proteins, such as kinesin and dynein.
- The Golgi apparatus is a processing/sorting station for endomembrane and secretory proteins, modifying and directing cargo through post-translational modifications and packaging.
Golgi Apparatus
- The Golgi apparatus consists of an ordered series of compartments, including cis, medial, and trans cisternae.
- Each Golgi compartment is biochemically distinct and contains specific resident enzymes.
- The Golgi apparatus is responsible for post-translational modifications, such as differential glycosylation, phosphorylation, and sulfation.
- The Golgi apparatus also plays a role in trafficking lipids.
Post-Translational Modifications
- The Golgi apparatus terminally N-glycosylates secretory proteins, allowing for structure and function diversity.
- Trans Golgi network sorts vesicles/secretory protein cargo based on "signals/motifs" built into the cargo.
- ER signal sequence is required for cotranslational import of nascent secretory proteins into the ER.
- Signal recognition particle (SRP) recognizes the signal sequence and transports the translating ribosome to the ER.
Protein Glycosylation
- Protein glycosylation (polysaccharide) initiates in the ER and finishes in the Golgi.
- Chaperones assist with protein folding, and carbohydrate transferase enzymes add "core" sugar moieties onto the growing polypeptide chain.
- "N-linked" glycosylation occurs on asparagine residues, while "O-linked" glycosylation occurs on serine or threonine residues.
- Core glycosylation promotes proper protein folding in the ER via calnexin.
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Description
Learn about the process of protein trafficking in the endomembrane system, including the roles of different organelles such as the rER, Golgi, plasma membrane, endosomes, and lysosomes. Understand how proteins are synthesized, processed, and transported in and out of the cell.
