Questions and Answers
What role does the signal sequence play in protein synthesis?
What happens to the signal sequence once the protein is inside the ER lumen?
How are transmembrane proteins retained in the ER membrane?
What modification occurs to proteins that are synthesized in the RER?
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Which of the following proteins is crucial for packing lysosomal proteins in the trans Golgi network?
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What is the primary role of the translocon in the ER membrane?
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How are proteins targeted to specific compartments after leaving the trans Golgi?
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What is a key feature of the mitochondrial targeting signal?
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What transport method is primarily used by proteins to travel from the ER to the Golgi?
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What role do signal peptides play in protein targeting?
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Which of the following correctly describes a consequence of defects in protein targeting?
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Which type of proteins are directed to the endoplasmic reticulum using signal peptides?
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What best describes the role of 'postcode' sequences in protein targeting?
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Which organelle is primarily involved in the synthesis of membrane proteins?
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What is a characteristic of proteins targeted to mitochondria?
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In what way does cellular compartmentalization influence protein function?
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What distinguishes cytosolic proteins from secreted proteins in terms of targeting?
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What is the primary role of Nuclear Localisation Signal (NLS) in protein targeting?
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Which protein is specifically responsible for recognizing the Ser-Lys-Leu sequence during peroxisomal import?
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What is required for the dissociation of the protein import complex in the nucleus?
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How are secreted, membrane, and lysosomal proteins characterized in terms of their targeting sequences?
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Which sequence represents the Nuclear Localisation Signal (NLS) used for targeting proteins to the nucleus?
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What function does exportin serve in relation to nuclear proteins?
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What happens to proteins after the import complex dissociates in the nucleus?
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Which of the following statements about mitochondrial protein targeting is true?
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Study Notes
Protein Targeting Overview
- Proteins in eukaryotic cells must be directed to various locations: organelles, membrane, secretion, or remain cytosolic.
- Functional compartmentalization in cells necessitates specific targeting mechanisms.
Role of Signal Peptides and Postcodes
- Signal peptides act as “postcodes” that guide proteins to their specific cellular destinations.
- Correct localization is essential for cellular function; defects can lead to diseases.
Protein Targeting Mechanisms
- Secretory and membrane proteins utilize signal peptides to reach the endoplasmic reticulum (ER).
- Cytosolic proteins are targeted to various organelles through different signaling mechanisms.
Protein Synthesis in the ER
- Initiated by a cytosolic ribosome; signal sequence recognized by Signal Recognition Particle (SRP).
- SRP facilitates ribosome docking onto the ER membrane, allowing protein entry via the translocon.
- Signal sequences are cleaved by signal peptidase during this process.
Membrane Protein Integration
- Transmembrane proteins remain embedded in the ER membrane due to stop transfer sequences.
- Most ER-synthesized proteins are glycoproteins, often requiring correct folding to function properly.
Intracellular Transport via Vesicles
- Proteins are transported from the ER to the Golgi apparatus via vesicles.
- Golgi processing involves maturation and sorting of proteins for their final destinations.
Targeting to Specific Compartments
- Trans-Golgi vesicles utilize v-SNARE proteins for targeting, pairing with complementary t-SNARE proteins at the destination.
- Lysosomal proteins tagged with mannose-6-phosphate for correct vesicle transport to lysosomes.
Mitochondrial and Nuclear Targeting
- Mitochondrial proteins are targeted post-translation with help from chaperones; chaperones assist transit through outer membrane receptors.
- Nuclear proteins contain Nuclear Localisation Signals (NLS) for import through nuclear pores, involving G-protein Ran.
Peroxisome Targeting
- Proteins destined for peroxisomes have a specific three-amino acid C-terminal sequence recognized by PEX5 for import.
Summary of Protein Targeting
- Specific protein sequences determine the compartment destinies within the cell, with distinct pathways for secreted, membrane, and organelle proteins.
- Targeting mechanisms involve recognition of unique signal sequences and associated transport processes following protein synthesis.
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Description
Explore the intricacies of protein targeting in eukaryotic cells through this quiz. Understand the role of signal peptides as cellular postcodes and learn about the mechanisms that ensure proteins reach their specific destinations. A solid grasp of these concepts is crucial for appreciating cellular functionality and the implications of targeting defects.
