Nuclear Transportation through NPCs
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Questions and Answers

What is the main function of the nuclear lamina?

  • To act as an anchoring site for chromosomes and nuclear pore complexes (correct)
  • To transport molecules between the nucleus and cytosol
  • To regulate protein synthesis in the ER
  • To provide structural support for the cytoskeleton

    • What is the outer nuclear membrane continuous with?

  • The endoplasmic reticulum (ER) (correct)
  • The cytoskeleton
  • The plasma membrane
  • The Golgi apparatus

    • What is the purpose of the nuclear pore complexes?

  • To conduct bidirectional traffic between the cytosol and the nucleus (correct)
  • To synthesize proteins in the ER
  • To provide structural support for the nuclear envelope
  • To regulate gene expression

    • Where are the ribosomes that synthesize proteins in the ER located?

    <p>On the surface of the ER membrane</p> Signup and view all the answers

    What is transported into the perinuclear space?

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

    What are the proteins in the inner nuclear membrane part of?

    <p>The nuclear lamina</p> Signup and view all the answers

    What is the main function of the nuclear envelope?

    <p>To separate the nucleus from the cytosol</p> Signup and view all the answers

    What is the purpose of the protein complexes that span the nuclear envelope?

    <p>To provide structural links between the DNA, nuclear envelope, and cytoskeleton</p> Signup and view all the answers

    What is the primary function of a nuclear import receptor?

    <p>To recognize and bind to nuclear localization signals</p> Signup and view all the answers

    What happens to the nuclear localization signal on a cargo protein when it is loaded with a cargo protein?

    <p>It becomes exposed</p> Signup and view all the answers

    What is the role of adaptor proteins in nuclear import?

    <p>To form a bridge between import receptors and nuclear localization signals</p> Signup and view all the answers

    What is the function of the FG repeats in the unstructured domains of nucleoporins?

    <p>To initially recruit import receptors and their bound cargo proteins to NPCs</p> Signup and view all the answers

    What is the result of the import receptor binding to the FG repeats in the NPC pore?

    <p>The receptor-–cargo complex locally dissolves the gel-like mesh and can diffuse into and within the NPC pore</p> Signup and view all the answers

    What is the mechanism by which the import receptor accelerates cargo transport into the nucleus?

    <p>Facilitated diffusion</p> Signup and view all the answers

    What confers directionality to the nuclear import process?

    <p>The selective dissociation of cargo only on the nuclear side of the NPC</p> Signup and view all the answers

    What happens to the import receptor after it has transported cargo into the nucleus?

    <p>It returns back to the cytosol for transport of the next cargo</p> Signup and view all the answers

    What triggers the disassembly of the nuclear lamina during mitosis?

    <p>Phosphorylation of the lamins</p> Signup and view all the answers

    What is a common feature of the breakdown of the nuclear envelope during mitosis in metazoan cells?

    <p>The lamina network breaks down</p> Signup and view all the answers

    What occurs as decondensation of daughter chromosomes progresses?

    <p>The membrane structures fuse</p> Signup and view all the answers

    What is involved in the reassembly process of the nuclear envelope?

    <p>Dephosphorylation of nucleoporins</p> Signup and view all the answers

    In which organism does the nuclear envelope remain intact during mitosis?

    <p>Yeast cells</p> Signup and view all the answers

    What triggers the re-formation of the lamin network around the decondensing daughter chromosomes?

    <p>Recruitment of membranes with interacting lamin receptors</p> Signup and view all the answers

    In which cellular compartment is Ran GAP located?

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

    What happens to SREBP when the cholesterol-binding site on SCAP is empty?

    <p>The SCAP-SREBP complex is packaged into transport vesicles</p> Signup and view all the answers

    Where does the exchange of GDP for GTP occur to convert Ran-GDP to Ran-GTP?

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

    What is the result of SREBP binding to the promoters of genes involved in cholesterol biosynthesis?

    <p>Activation of gene expression</p> Signup and view all the answers

    What is the consequence of low cholesterol concentrations on SCAP?

    <p>It changes conformation and dissociates from INSIG</p> Signup and view all the answers

    Where is SREBP initially synthesized?

    <p>Endoplasmic Reticulum</p> Signup and view all the answers

    What is the purpose of the regulation of cholesterol biosynthesis through SREBP?

    <p>To maintain a stable cholesterol concentration</p> Signup and view all the answers

    What is the role of the SCAP-INSIG complex in cholesterol biosynthesis?

    <p>It anchors SREBP in the ER when cholesterol levels are sufficient</p> Signup and view all the answers

    What is the primary function of FG repeat–binding sites on the surface of nuclear import receptors?

    <p>To rapidly partition into the FG repeat mesh</p> Signup and view all the answers

    What is the result of the interaction between FG repeats and cargo receptors?

    <p>The mesh is locally melted, accelerating diffusion through the NPC</p> Signup and view all the answers

    What determines the directionality of nuclear import and export?

    <p>The differential localization of Ran-GTP in the nucleus and Ran-GDP in the cytosol</p> Signup and view all the answers

    What is the role of Ran GAP in the Ran GTPase cycle?

    <p>To trigger the hydrolysis of GTP to produce Ran-GDP</p> Signup and view all the answers

    What is the critical difference between Ran-mediated nuclear import and nuclear export?

    <p>The nature of cargo binding by the cargo receptor</p> Signup and view all the answers

    What is the characteristic of Ran GTPase?

    <p>A molecular switch that can exist in two conformational states</p> Signup and view all the answers

    What is the result of the interaction between cargo receptors and FG repeats in the NPC?

    <p>The mesh is locally melted, accelerating diffusion through the NPC</p> Signup and view all the answers

    What is the function of the mesh of FG repeat–containing proteins in the NPC?

    <p>To restrict nonspecific diffusion of proteins and other macromolecules through the pore</p> Signup and view all the answers

    Study Notes

    The Nuclear Envelope

    • The nuclear envelope is a double membrane structure penetrated by nuclear pore complexes.
    • Transmembrane proteins in the inner and outer nuclear membranes link the nuclear lamina to the cytosolic cytoskeleton.
    • The outer nuclear membrane is continuous with the endoplasmic reticulum (ER).

    The Lamina

    • The lamina provides structural support for the nuclear envelope and acts as an anchoring site for chromosomes and nuclear pore complexes.
    • The lamina is connected to the cytoplasmic cytoskeleton via protein complexes that span the nuclear envelope.

    Nuclear Pore Complexes

    • Nuclear pores conduct extensive bidirectional traffic between the cytosol and the nucleus.
    • The many proteins that function in the nucleus are selectively imported into the nuclear compartment from the cytosol.
    • These proteins contain a nuclear localization signal that binds them to an import receptor.

    Nuclear Import Receptors

    • Nuclear localization signals are recognized by nuclear transport receptors (karyopherin family members).
    • Each import receptor can bind and transport the subset of cargo proteins containing the appropriate nuclear localization signal.
    • Import receptors use adaptor proteins to form a bridge between the import receptors and the nuclear localization signals on the proteins to be transported.

    Nuclear Import Mechanism

    • Import receptors contain multiple low-affinity binding sites for the FG repeats found in the unstructured domains of several nucleoporins.
    • The FG repeats in the fibrils of cytosol-facing nucleoporins serve to initially recruit import receptors and their bound cargo proteins to NPCs.
    • The import receptor-cargo complex diffuses into and within the NPC pore, allowing the cargo to traverse the distance across an NPC in a few milliseconds.

    Ran GTPase Cycle

    • The Ran GTPase cycle provides directionality to both nuclear import and nuclear export.
    • The differential localization of Ran-GTP in the nucleus and Ran-GDP in the cytosol provides directionality.

    Regulation of Nuclear Transport

    • Feedback regulation of cholesterol biosynthesis is an example of how transcription through NPCs can be regulated by controlling access to the transport machinery.
    • The breakdown and re-formation of the nuclear envelope and lamina occur during mitosis, with phosphorylation and dephosphorylation of the lamins triggering the disassembly and reassembly of the nuclear lamina.

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    Related Documents

    Nuclear Transport PDF

    Description

    Learn about the role of nuclear import receptors and cargo receptors in facilitating nuclear transportation through nuclear pore complexes (NPCs). Understand how FG repeat-binding sites and interactions with cytosolic fibrils enable this process.

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