Lecture 13: Cell bio Protein Sorting in Eukaryotic CellL
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Questions and Answers

What mechanism do organelles use to import proteins?

  • Endocytosis only
  • Passive diffusion
  • Signal sequences only
  • One of three mechanisms (correct)

    • All nuclear-encoded proteins are synthesized within the nucleus.

    False

    What are signal sequences compared to in directing proteins to their destinations?

    Postal code

    Proteins lacking a signal sequence are likely to be __________.

    <p>degraded</p> Signup and view all the answers

    Match the signal sequences with their characteristics:

    <p>N-terminus = Rich in hydrophobic amino acids C-terminus KDEL = Retention in the endoplasmic reticulum N-terminus amphipathic helix = Basic amino acids on one face C-terminus SKL = Targeting for peroxisomes</p> Signup and view all the answers

    What role does the import receptor (R) play in peroxisome protein import?

    <p>It recognizes C-terminal SKL signals on proteins</p> Signup and view all the answers

    The SKL signal sequence on the cargo protein is cleaved off during import into the peroxisome.

    <p>False</p> Signup and view all the answers

    Where is catalase likely to be located in cells without peroxisomes?

    <p>In the cytosol</p> Signup and view all the answers

    Proteins are imported into peroxisomes in a __________ state.

    <p>folded</p> Signup and view all the answers

    Match the following features with their respective transport processes:

    <p>Mitochondrial import = Uptake of proteins in an unfolded state Nuclear transport = Requires nuclear localization signals Peroxisomal import = Involves import receptor recognizing SKL signals Endoplasmic reticulum import = Typically co-translational</p> Signup and view all the answers

    What does NLS stand for in the context of nuclear transport?

    <p>Nuclear Localization Sequence</p> Signup and view all the answers

    Proteins enter the nucleus in an unfolded state.

    <p>False</p> Signup and view all the answers

    What drives the nuclear transport cycle?

    <p>Energy supplied by GTP hydrolysis</p> Signup and view all the answers

    Nuclear export works like nuclear import, but in __________.

    <p>reverse</p> Signup and view all the answers

    What is the primary function of nuclear import receptors?

    <p>To bind to cargo with a nuclear localization signal</p> Signup and view all the answers

    Match the following terms with their descriptions:

    <p>Ran-GTP = Form that facilitates the release of cargo from import receptors Nuclear pores = The gateway between the nucleus and cytoplasm Active transport = Mechanism used for large proteins and complexes to enter the nucleus Size exclusion chromatography = Method used to separate small proteins for nuclear entry</p> Signup and view all the answers

    Small proteins, weighing less than or equal to 40 kDa, pass through nuclear pores by diffusion.

    <p>True</p> Signup and view all the answers

    What determines the directionality of nuclear transport?

    <p>The compartmentalization of Ran-GDP and Ran-GTP</p> Signup and view all the answers

    What does NFᴋB do in response to infection?

    <p>Regulates immune response</p> Signup and view all the answers

    The NLS of NFᴋB is unmasked after IᴋB is ubiquitinated and degraded.

    <p>True</p> Signup and view all the answers

    What happens to NFᴋB after it has activated gene expression in the nucleus?

    <p>It is transported back to the cytosol.</p> Signup and view all the answers

    The signal sequence for mitochondrial import is often found at the __________ of a polypeptide.

    <p>N-terminus</p> Signup and view all the answers

    What do TOM and TIM complexes form?

    <p>A continuous translocation channel</p> Signup and view all the answers

    Active import is required for protein import into mitochondria.

    <p>True</p> Signup and view all the answers

    Which modification leads to the inactivation of NFᴋB?

    <p>Ubiquitination of IκB</p> Signup and view all the answers

    Ran-GEF being kept in an active form in the nucleus would lead to decreased nuclear localization of NFᴋB.

    <p>False</p> Signup and view all the answers

    What does TOM stand for?

    <p>Translocator of the OUTER Membrane</p> Signup and view all the answers

    What must an endosymbiont be able to do to eventually evolve into an organelle?

    <p>Reproduce inside its host.</p> Signup and view all the answers

    Peroxisomes bud off from the ______.

    <p>endoplasmic reticulum</p> Signup and view all the answers

    Which enzyme is primarily involved in recycling hydrogen peroxide in peroxisomes?

    <p>Catalase</p> Signup and view all the answers

    Match the following mitochondrial components with their functions:

    <p>TOM = Transports proteins across the outer membrane TIM = Transports proteins across the inner membrane TIM22 = Inserts multi-pass transmembrane proteins into the IMM SAM = Inserts porins into the OMM</p> Signup and view all the answers

    Changing the hydrophobic amino acids in a mitochondrial signal sequence to other hydrophobic amino acids will affect protein import.

    <p>False</p> Signup and view all the answers

    What substances do peroxisomes help break down?

    <p>Lipid-soluble organic molecules, uric acid, and amino acids</p> Signup and view all the answers

    Study Notes

    Protein Sorting I: Nucleus, Mitochondria, and Peroxisomes

    • Eukaryotic cells contain many membrane-bound compartments, including the nucleus, mitochondria, chloroplasts (in plants), endoplasmic reticulum (ER), Golgi apparatus, endosomes, lysosomes, and peroxisomes.
    • Organelles occupy about half the volume of a cell.
    • Protein sorting mechanisms direct proteins to their appropriate cellular compartments.
    • There are three main mechanisms for protein import:
      1. Transport through nuclear pores
      2. Transport across membranes
      3. Transport by vesicles.
    • The cytosol is the main compartment of the cell, exclusive of membrane-bound organelles making up 54% of a typical hepatocyte.
    • Proteins that lack a signal sequence remain in the cytosol.
    • Signal sequences are amino-acid sequences used to direct proteins to their target location.
    • These sequences are often found at the N-terminus of a polypeptide.

    Signal Sequences and Protein Import

    • Signal sequences act like "postal codes", directing proteins to their destinations.

    • Signal sequences in proteins destined for the ER, mitochondria and peroxisomes are different in amino acid content. These sequences often contain hydrophobic amino acids.

    • Signal sequences for different organelle import are different in amino acid sequence.

      • N-terminus rich in hydrophobic amino acids (often at C-terminus)
      • C-terminus rich in hydrophobic amino acids
      • Often contains KDEL
      • Typically amphipathic helix

    • Many proteins enter and exit the nucleus multiple times.

    • Small proteins (less than 40 kDa) enter the nucleus by diffusion, while larger proteins use active transport to enter.

    • Nuclear import receptors recognize nuclear localization sequences (NLS).

    Nuclear Import

    • Nuclear import receptors bind cargo proteins with an NLS and transport them through nuclear pores.
    • Families of receptors can recognize a wide variety of different NLS sequences on proteins.
    • NLS is an encoded part of the protein.
    • Ran-GTP and Ran-GDP drive the movement of cargo. Energy supplied by GTP hydrolysis drives nuclear transport.

    Nuclear Export

    • Nuclear export is the reverse process of nuclear import.
    • Cargo proteins with nuclear export signals (NES) are exported from the nucleus.
    • Ran-GTP facilitates the binding of proteins to export receptors.

    Mitochondrial Import

    • Mitochondrial precursor proteins are unfolded during import to facilitate translocation across the mitochondrial membranes.
    • TOM and TIM23 complexes form a continuous channel across the double mitochondrial membrane.
    • Protein import into mitochondria requires energy from either the membrane potential or from ATP hydrolysis.

    Peroxisome Import

    • Peroxisomes import soluble proteins through a mechanism that combines features of mitochondrial and nuclear import.
    • Proteins are imported in a folded state through a translocation channel in peroxisomes.
    • An import receptor recognizes C-terminal SKL signal sequences.
    • SKL is recognized by the import receptor.
    • The receptor shuttles cargo into the peroxisome, unloads the protein, and returns to the cytosol.

    Subcellular locations and organelles

    • The synthesis of all nuclear-encoded proteins starts in the cytoplasm after which it can be directed to other organelles.
    • Cytosol is the primary site of protein synthesis.

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    Description

    This quiz explores the mechanisms of protein sorting within eukaryotic cells, focusing on the nucleus, mitochondria, and peroxisomes. Understand how signal sequences guide proteins to their cellular compartments and the various transport methods involved in this process.

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