case 6 cellular transport
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Questions and Answers

What is the role of disulfide bridges in the structure of insulin?

  • They facilitate hydrolysis of peptide bonds.
  • They direct insulin to its cellular destination.
  • They hold together the pre-form of insulin. (correct)
  • They allow for the degradation of insulin.

    • What is the primary outcome of proteolysis in relation to proteins?

  • It enhances the structural integrity of proteins.
  • It determines the secondary structure of proteins.
  • It removes larger segments to form smaller peptides or amino acids. (correct)
  • It facilitates the entry of proteins into the nucleus.

    • In the context of intracellular protein transport, what describes gated transport?

  • Transport of proteins without any size limitation.
  • Transport between compartments with different environments.
  • Transport that requires energy regardless of molecule size.
  • Transport between compartments with same environments (correct)

    • Which mechanism is involved in the sorting of proteins based on their amino acid sequence?

    <p>Signal sequence recognition</p> Signup and view all the answers

    What characterizes transmembrane transport in intracellular protein movement?

    <p>It requires protein translocators for movement.</p> Signup and view all the answers

    What is the primary function of the TOM-complex in mitochondria?

    <p>Transporting proteins through the outer membrane</p> Signup and view all the answers

    Which component is responsible for translocating proteins through the inner mitochondrial membrane?

    <p>TIM-complex</p> Signup and view all the answers

    What happens to the signal peptide during mitochondrial protein import?

    <p>It is cleaved off by signal peptidase after translocation</p> Signup and view all the answers

    Which type of protein is destined to be secreted from the cell?

    <p>Proteins in the ER-lumen</p> Signup and view all the answers

    What initiates the transport of free ribosomes to the ER membrane?

    <p>Recognition of a signal sequence for ER-specific proteins</p> Signup and view all the answers

    How are membrane proteins formed during translation?

    <p>They undergo a series of stop-transfer and start-transfer sequences</p> Signup and view all the answers

    What role does the signal recognition particle (SRP) play in protein synthesis?

    <p>Binding and pausing ribosome translation</p> Signup and view all the answers

    What is the outcome for transmembrane proteins during their synthesis?

    <p>Half of the protein is embedded in the membrane while the other half is outside</p> Signup and view all the answers

    What role do RAB-effector proteins play in vesicular transport between the ER and Golgi?

    <p>They help in capturing vesicles that carry specific proteins.</p> Signup and view all the answers

    In the context of the Golgi apparatus, what distinguishes the consecutive secretory pathway from the regulated secretory pathway?

    <p>Regulated pathways depend on hormonal or neurotransmitter signals.</p> Signup and view all the answers

    What is the function of ubiquitin in protein degradation?

    <p>It provides a degradation signal for targeted proteins.</p> Signup and view all the answers

    Which of the following best describes the function of proteasomes?

    <p>They shred proteins into amino acids for recycling.</p> Signup and view all the answers

    What is the primary effect of mutations in the GBA gene related to Gaucher's disease?

    <p>Defective enzyme folding and stability affecting transport.</p> Signup and view all the answers

    Which protein structure is primarily maintained by hydrogen bonds between backbone atoms?

    <p>Secondary structure</p> Signup and view all the answers

    How do post-translational modifications affect proteins?

    <p>They change the three-dimensional structure and function of proteins.</p> Signup and view all the answers

    What do V-SNARE proteins do during vesicular transport?

    <p>They facilitate the fusion of vesicles with target membranes.</p> Signup and view all the answers

    What primarily directs proteins to their appropriate destination within the cell?

    <p>Amino acid sequence</p> Signup and view all the answers

    Which process allows for the transport of larger molecules from the nucleus to the cytosol?

    <p>Active transport through nucleopores</p> Signup and view all the answers

    In intracellular protein sorting, what is the significance of the signal sequence?

    <p>It guides proteins to their specific cellular locations.</p> Signup and view all the answers

    Which characteristic differentiates transmembrane transport from other types of intracellular protein transport?

    <p>It involves protein translocators.</p> Signup and view all the answers

    How do small molecules typically cross nuclear pores during gated transport?

    <p>Through free diffusion without additional transport.</p> Signup and view all the answers

    How does a protein intended for the mitochondria get initiated for transport into the mitochondria?

    <p>It binds to import receptors in the TOM-complex.</p> Signup and view all the answers

    What stops the translation of a protein destined for the endoplasmic reticulum (ER) during its synthesis?

    <p>The signal sequence becomes recognized by the SRP.</p> Signup and view all the answers

    Which characteristic specifically describes secretory proteins produced by ribosomes bound to the ER?

    <p>They are packaged into vesicles for transport to the plasma membrane.</p> Signup and view all the answers

    What mechanism mediates the entry of polypeptides into the mitochondrial matrix?

    <p>Transport via the TIM-complex after passing the TOM-complex.</p> Signup and view all the answers

    How do free ribosomes ensure a protein is directed to the ER after synthesis?

    <p>They recognize the presence of an ER-specific signal sequence.</p> Signup and view all the answers

    What happens to the signal peptide during the import of proteins into the mitochondria?

    <p>It is cleaved by signal peptidase after translocation into the matrix.</p> Signup and view all the answers

    What primarily distinguishes membrane proteins from secretory proteins during synthesis?

    <p>The presence of stop-transfer sequences during translation.</p> Signup and view all the answers

    Which of the following accurately describes the process for a protein becoming a transmembrane protein?

    <p>It is produced with alternating stop- and start-transfer sequences.</p> Signup and view all the answers

    What mechanism allows vesicles to be specifically recognized by their target membranes during intracellular transport?

    <p>V-SNARE interactions with RAB-effector proteins</p> Signup and view all the answers

    In the context of intracellular protein transport, what primarily dictates the sorting of proteins within the Golgi apparatus?

    <p>The protein's folding and post-translational modifications</p> Signup and view all the answers

    What is the role of RAB-proteins in the process of vesicular transport?

    <p>To recruit specific motor proteins for vesicle movement</p> Signup and view all the answers

    Which pathway is characterized by the random release of proteins as observed in the Golgi apparatus?

    <p>Consecutive secretory pathway</p> Signup and view all the answers

    What impact do mutations in the GBA gene have on protein transport in the case of Gaucher's disease?

    <p>They impair the folding and transport of the enzyme β-glucocerebrosidase</p> Signup and view all the answers

    Which factor is crucial for the proper signaling and targeting of proteins destined for degradation?

    <p>The attachment of ubiquitin to the protein</p> Signup and view all the answers

    What differentiates the regulated secretory pathway from the consecutive secretory pathway?

    <p>The regulated pathway depends on hormonal or neurotransmitter signals</p> Signup and view all the answers

    How do vesicles ensure a successful fusion process with target membranes during transport?

    <p>Through the interaction of V-SNARE and T-SNARE proteins</p> Signup and view all the answers

    Study Notes

    Protein Transport

    • Protein transport occurs from the cytosol to different organelles like mitochondria and ER.
    • TIM- and TOM-complex are crucial for protein transport into mitochondria.
    • TOM-complex facilitates transport across the outer mitochondrial membrane.
    • TIM-complex assists in transporting proteins across the inner mitochondrial membrane.
    • Proteins bind to the import receptor of the TOM-complex, initiating translocation through the outer membrane.
    • The TIM-complex and signal peptide take over, guiding the protein into the mitochondrial matrix.
    • Signal peptide is cleaved by signal peptidase upon protein entry into the matrix.

    ER-Bound Ribosomes and Protein Synthesis

    • Ribosomes attached to the ER are involved in protein synthesis.
    • Signal sequence emerges during translation, triggering the binding of SRP (signal recognition particle) to the ribosome.
    • SRP halts translation and brings the ribosome-SRP complex to the SRP receptor protein on the ER membrane.
    • SRP detaches from the ribosome as it attaches to the SRP receptor.
    • Translation resumes, and protein translocation through the ER membrane starts, mediated by a protein translocator.
    • The signal peptide is cleaved by signal peptidase, and the protein is inserted into the ER lumen.

    Secretion & Membrane Proteins

    • Secretory proteins are synthesized and released from the cell.
    • Secretory proteins are packaged into vesicles in the ER lumen, transported to the plasma membrane, and released outside the cell.
    • Membrane proteins have a stop-transfer sequence that halts translation and leads to an embedded protein, with one part in the membrane and another outside.
    • Transmembrane proteins have alternating stop- and start-transfer sequences, resulting in multi-pass transmembrane proteins.

    Differences between ER-bound and Free Ribosomes

    • Free ribosomes translate proteins that lack ER-specific signal sequences.
    • Proteins destined for the ER are directed there by the signal sequence.
    • Removing the signal sequence deactivates the protein, while adding it activates the protein for ER transport.

    Protein Modifications

    • Proteolysis involves the removal of amino acids, like the breakdown of the pre-form of insulin into mature insulin.
    • Hydroxylation adds hydroxyl groups to certain amino acids, contributing to a protein's structural integrity.
    • Signal sequences are amino acid sequences that guide proteins to their target destinations, like organelles and membranes.

    Intracellular Protein Transport and Sorting

    • Gated transport involves movement between compartments with similar environments (e.g., nucleus to cytosol) via pores.
    • Transmembrane transport involves translocation across membranes, with the assistance of protein translocators, and occurs between compartments with distinct environments (e.g., ER to Golgi).
    • Vesicular transport involves transport of proteins in membrane-enclosed packages, often involving RAB-proteins for vesicle targeting.

    Vesicular Transport

    • V-snare proteins on vesicles interact with t-snare proteins on target membranes, enabling vesicle fusion.
    • Specific RAB-effector proteins capture vesicles with specific RAB-proteins, ensuring targeted delivery.

    Golgi Apparatus

    • The Golgi apparatus functions as a sorting station for proteins.
    • Different regions of the Golgi apparatus perform distinct modifications on proteins.

    Secretory Pathways

    • Consecutive secretory pathway involves random release of proteins.
    • Regulated secretory pathway involves controlled release of proteins based on specific signals, like hormones or neurotransmitters.

    Protein Functions in the Body

    • Structural: Hair, nails, etc.
    • Enzymes: Catalyze chemical reactions.
    • Movement: Muscles, flagella, etc.
    • Transport: Membrane transport of substances like oxygen (O2).
    • Hormonal: Signaling molecules like insulin.
    • Nutrition: Milk protein (casein).
    • Protection: Blood clotting, antibodies, etc.
    • Cell regulation: Signal transduction and gene expression.

    Protein Degradation

    • Proteases are enzymes responsible for protein breakdown.
    • Ubiquitin is a small protein that marks proteins for degradation.
    • Ubiquitin ligase recognizes degradation signals and attaches ubiquitin to target proteins.
    • Proteasome is a multi-protein complex that degrades ubiquitinated proteins into amino acids.
    • Degraded amino acids are reused for new protein synthesis.

    Disease Implications

    • Gaucher's disease is a lysosomal storage disorder caused by a deficiency in β-glucocerebrosidase, leading to an accumulation of glucocerebroside.
    • Mutations in the GBA gene can affect β-glucocerebrosidase folding, stability, and transport, contributing to disease development.

    Protein Transport

    • Protein transport involves moving proteins from one cellular compartment to another.

    • Cytosol to Mitochondria

      • Proteins destined for the mitochondria are transported through the TOM complex (outer membrane) and the TIM complex (inner membrane).
      • Protein binds to the import receptor in the TOM complex initiates translocation through the outer membrane.
      • The TIM complex and signal peptide take over, guiding the protein into the mitochondrial matrix.
      • The signal peptide is cleaved off by signal peptidase.

    • Cytosol to ER

      • Ribosomes bound to the ER translate proteins, initiating protein synthesis.
      • The signal sequence emerges during translation, attracting the SRP (signal recognition particle).
      • SRP binds to the ribosome, pausing translation and guiding the complex to the SRP receptor on the ER membrane.
      • SRP detaches from the ribosome as it binds to the receptor, allowing translation to resume.
      • The protein translocates through the ER membrane via a protein translocator.
      • The signal peptide is cleaved off by signal peptidase, and the protein is situated within the ER lumen.

    Secretory Proteins

    • Secretory proteins are synthesized in the ER lumen and released from the cell.
    • They travel in vesicles to the plasma membrane, where they are released through secretion.

    Membrane Proteins

    • Similar to secretory proteins, membrane proteins are initially synthesized in the ER lumen.
    • However, they encounter a stop-transfer sequence, halting translation, and anchoring the protein within the ER membrane.
    • The signal peptidase cleaves the signal peptide, resulting in a transmembrane protein with one portion in the membrane and the other in the cytoplasm.
    • Some proteins have alternating stop- and start-transfer sequences, creating multiple transmembrane domains.

    ER-Bound and Free Ribosomes: Differences

    • Free ribosomes can be directed to the ER membrane by the presence of a signal sequence for ER-specific proteins.

    Protein Modification

    • Proteolysis is the breakdown of proteins into smaller peptides or amino acids via hydrolysis of peptide bonds.
      • An example of proteolysis is the conversion of pre-insulin into insulin.
    • Hydroxylation is a modification that affects protein structure and stability.

    Protein Sorting and Transport: Overview

    • Signal sequences guide proteins to their correct destinations.

    • Intracellular protein transport can be categorized as:

      • Gated transport: For proteins moving between compartments with similar environments (e.g., nucleus to cytosol).
        • Transport occurs via pores, like those found in the nuclear envelope.
        • Small molecules move via free diffusion.
        • Large molecules (macromolecules) require active transport.
      • Transmembrane transport: For proteins moving between compartments with different environments (e.g., ER to Golgi).
        • Transport involves protein translocators.

    • Vesicular Transport*

    • Vesicles are small sacs involved in transporting materials throughout the cell.

    • RAB proteins act as guides for vesicle transport, marking the vesicle's destination.

    • V-snare proteins on the vesicle interact with t-snare proteins on the target membrane, facilitating vesicle fusion.

    Golgi Apparatus

    • The Golgi apparatus functions as a sorting station for proteins.
    • Consecutive secretory pathway involves the random release of proteins.
    • Regulated secretory pathway involves additional controls, including signals from hormones or neurotransmitters.

    Protein Functions

    • Structural: Hair, nails, etc.
    • Enzymes: Catalyze chemical reactions.
    • Movement: Muscles.
    • Transport: O2 membrane transport, etc.
    • Hormones: E.g., Insulin.
    • Nutrition: E.g., Casein in milk.
    • Protection: Blood clotting factors, antibodies, etc.
    • Cell regulation: Receptors, signal transduction, gene expression.

    Protein Degradation

    • Proteases are enzymes that breakdown proteins into smaller peptides or amino acids.
    • Ubiquitin is a protein tag that marks proteins for degradation.
      • Ubiquitin ligase recognizes the "death sentence" signal on the protein and attaches ubiquitin.
    • Proteasomes are protein complexes that breakdown ubiquitinated proteins into amino acids.
    • The amino acids are recycled for reuse in the cell.

    Disease Implications

    • Lysosomal Storage Disorders (LSDs), such as Gaucher's disease, are caused by deficiencies in lysosomal enzymes.
    • Defects in protein folding, stability, and transport can contribute to the development of Gaucher's disease.

    Gaucher's Disease

    • Gaucher’s disease is a lysosomal storage disorder (LSD) caused by a deficiency of the enzyme β-glucocerebrosidase.
    • This enzyme is responsible for breaking down glucocerebroside, a specific lipid.
    • Accumulation of glucocerebroside in lysosomes leads to enlarged liver and spleen, bone abnormalities, and neurological issues.

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