Protein Sorting II: Endomembrane System
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What happens to proteins that are improperly folded and fail to exit the ER?

  • They are routed for degradation. (correct)
  • They remain in the ER indefinitely.
  • They are restored to their original state.
  • They are exported to the Golgi apparatus.

    • Up to 50% of proteins are estimated to be destroyed due to improper folding.

    False

    What role does N-glycosylation play in protein quality control?

    It serves as a tag to mark the state of protein folding.

    What is the primary role of Lumacaftor?

    <p>It acts as a chaperone during protein folding.</p> Signup and view all the answers

    Median age for people with cystic fibrosis has increased to 40+ years due to new treatments.

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

    Defects in the stringency of the ER quality control mechanism lead to __________ disease.

    <p>cystic fibrosis</p> Signup and view all the answers

    Match the following proteins or processes with their descriptions:

    <p>Calnexin = Helps fold newly synthesized proteins Mannosidase = Removes mannoses from misfolded proteins Correctors = Target partial function mutant channels Unfolded Protein Response = Triggers degradation of misfolded proteins</p> Signup and view all the answers

    What structure do lipids and proteins move to after the endoplasmic reticulum (ER) for further processing?

    <p>Golgi complex</p> Signup and view all the answers

    What is the primary function of Rough ER?

    <p>Protein synthesis, folding, and glycosylation</p> Signup and view all the answers

    Vesicle transport is the method of transportation within the ______ system.

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

    The nuclear envelope is separate from the Endoplasmic Reticulum.

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

    Match the following processes to their descriptions:

    <p>Vesicle Transport = Method of transportation within the endomembrane system Lumacaftor = Corrector that aids in protein folding Golgi complex = Further processes lipids and proteins Cystic fibrosis treatment advancements = Increased median age for affected individuals</p> Signup and view all the answers

    What synthesizes most lipids and cholesterol within the cell?

    <p>Smooth ER</p> Signup and view all the answers

    The tubular structure of the ER is more abundant near the __________ and is involved in steroid hormone synthesis.

    <p>cell edges</p> Signup and view all the answers

    Match the following functions with the correct type of ER:

    <p>a) Synthesis of phospholipids and cholesterol = Smooth ER b) Synthesis of secreted proteins and proteins of the endomembrane system = Rough ER c) Protein folding and modifications: glycosylation, addition of disulfide bonds = Rough ER d) Synthesis of steroid hormones = Smooth ER e) Detoxification of hydrophobic drugs = Smooth ER</p> Signup and view all the answers

    What is the primary mechanism for proteins entering the endoplasmic reticulum?

    <p>Co-translational import</p> Signup and view all the answers

    The ER import signal is recognized by a signal recognition particle that initiates protein synthesis.

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

    What is the role of the signal sequence in protein targeting to the ER?

    <p>It starts the transfer of the polypeptide into the ER.</p> Signup and view all the answers

    Proteins synthesized for the ER include soluble proteins and ________ membrane proteins.

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

    What is the main function of the Signal Recognition Particle (SRP)?

    <p>Recognizing ER import signals</p> Signup and view all the answers

    The SRP system is not conserved in prokaryotes.

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

    Match the following components with their corresponding functions:

    <p>Signal Recognition Particle (SRP) = Binds to the ER import signal Ribosome = Synthesizes polypeptides Synthesis Channel = Translocates proteins into the ER Translocation Channel = Facilitates protein entry into the ER</p> Signup and view all the answers

    If a protein has conflicting signal sequences for import into the nucleus and the ER, which signal is likely to prevail?

    <p>ER import signal</p> Signup and view all the answers

    What triggers the conformation change in SRP when it binds to the ER import signal?

    <p>Binding to the ER import signal sequence</p> Signup and view all the answers

    The pause domain of SRP blocks __________ from binding to the ribosome.

    <p>elongation factor</p> Signup and view all the answers

    Soluble proteins are synthesized and directly released into the cytoplasm.

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

    Match the following components with their functions:

    <p>SRP = Pauses translation SRP receptor = Binds to the ribosome Translocation channel = Facilitates protein entry into the ER Signal sequence = Opens the translocation channel</p> Signup and view all the answers

    What happens to the translation process while the SRP brings the ribosome to the ER membrane?

    <p>Translation pauses.</p> Signup and view all the answers

    What happens to SRP after cargo is delivered and GTP hydrolysis occurs?

    <p>It is released and returns to its original shape</p> Signup and view all the answers

    The entire SRP complex docks with SRP receptor on the mitochondria.

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

    SRP is composed of six polypeptides and one __________ molecule.

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

    What is the role of the start-transfer signal sequence?

    <p>To anchor the protein in the lipid bilayer</p> Signup and view all the answers

    The basic part of internal start-transfer sequences is always oriented towards the C-terminus.

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

    What modification do most proteins synthesized in the rough ER undergo?

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

    The signal sequence that helps retain proteins in the ER is known as ______.

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

    Match the following roles with their corresponding proteins or enzymes:

    <p>Chaperones = Help with protein folding Glycosyl transferase = Adds sugars to proteins Protein disulphide isomerase = Forms disulphide bonds Calnexin = Binds to N-glycosylated proteins</p> Signup and view all the answers

    What is the effect of N-glycosylation on proteins?

    <p>It promotes folding and serves as a marker of proper folding</p> Signup and view all the answers

    Single pass transmembrane proteins have only start-transfer signals.

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

    What role do ER chaperones like calnexin play in relation to N-glycosylated proteins?

    <p>They assist in protein folding.</p> Signup and view all the answers

    Translocated polypeptide chains are folded and glycosylated in the ______ of the rough ER.

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

    What is a basic characteristic of pairs of start and stop transfer signals?

    <p>They dictate the integration of transmembrane proteins into the lipid bilayer.</p> Signup and view all the answers

    What is the main function of Smooth ER?

    <p>Synthesis of lipids and cholesterol</p> Signup and view all the answers

    Rough ER is characterized by its tubular structure.

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

    What is the role of the nuclear envelope in relation to the endoplasmic reticulum?

    <p>It is part of the endoplasmic reticulum and its lumen is continuous with the ER lumen.</p> Signup and view all the answers

    Most intracellular Ca2+ is stored in the ______ ER.

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

    What sequence is crucial for docking ribosomes at translocation channels in the ER membrane?

    <p>Start transfer sequence</p> Signup and view all the answers

    Ribosomes are docked at the ER membrane before protein synthesis begins.

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

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

    <p>The SRP binds to the ER import signal and pauses translation to bring the ribosome to the ER membrane.</p> Signup and view all the answers

    Which signal sequence is likely to prevail when conflicting signals for ER import and mitochondrial import are present?

    <p>ER import signal</p> Signup and view all the answers

    Integral membrane proteins are only partially translocated and embedded in the ER membrane.

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

    What happens to translation when the SRP binds to the ER import signal?

    <p>Translation pauses.</p> Signup and view all the answers

    The GTP hydrolysis process of SRP does not affect its ability to release the cargo once delivered.

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

    What triggers the conformational change in the SRP when it binds to the ER import signal?

    <p>Binding to the ER import signal sequence</p> Signup and view all the answers

    Match the following components of the SRP system with their functions:

    <p>SRP receptor = Facilitates docking with ribosome GTP hydrolysis = Releases SRP and SRP receptor Pause domain = Blocks elongation factor binding Binding pocket = Accommodates hydrophobic ER signals</p> Signup and view all the answers

    What happens to the translocation channel when the ribosome is properly docked?

    <p>It opens for signal sequence release</p> Signup and view all the answers

    The SRP system is conserved across prokaryotes and eukaryotes.

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

    What is the result of the SRP binding to the signal sequence?

    <p>Translation is paused</p> Signup and view all the answers

    What is the consequence of proteins that are misfolded and left unresolved in the ER for too long?

    <p>They are routed for degradation.</p> Signup and view all the answers

    Only properly folded proteins are allowed to leave the ER.

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

    What role does mannosidase play in the lifecycle of misfolded proteins?

    <p>Mannosidase removes mannoses from proteins that remain unfolded for too long.</p> Signup and view all the answers

    Defects in the ER quality control mechanism are a leading cause of __________ disease.

    <p>cystic fibrosis</p> Signup and view all the answers

    Match the roles with their corresponding entities:

    <p>Calnexin = Assists in the folding of N-glycosylated proteins Correctors = Block unfolded protein response Mannosidase = Removes mannoses from improperly folded proteins N-glycosylation = Marks the state of protein folding</p> Signup and view all the answers

    What is the primary function of Lumacaftor in the treatment of cystic fibrosis?

    <p>Acts as a corrector to assist in proper protein folding.</p> Signup and view all the answers

    Vesicle transport is used to move lipids and proteins from the Golgi complex back to the ER.

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

    What is the median age now for people living with cystic fibrosis due to new treatments?

    <p>40+ years</p> Signup and view all the answers

    Lipids and proteins exit the ER to move to the ______ complex for further processing.

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

    Match the following components to their descriptions:

    <p>Vesicle transport = Method of transportation within the endomembrane system Lumacaftor = Corrector acting during protein folding Golgi complex = Processing and packaging center for proteins Endoplasmic Reticulum = Site of protein and lipid synthesis</p> Signup and view all the answers

    What is the role of start-transfer sequences in proteins?

    <p>They anchor proteins in the lipid bilayer.</p> Signup and view all the answers

    Internal signal sequences are cleaved after the protein is integrated into the membrane.

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

    What modification helps in the folding of proteins in the rough ER?

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

    Proteins that need to remain in the ER contain the retention signal ______.

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

    Which part of an internal start-transfer sequence is typically oriented toward the N-terminus?

    <p>Basic part</p> Signup and view all the answers

    Proteins synthesized in the rough ER are typically not glycosylated.

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

    What structure is associated with multi-pass transmembrane proteins?

    <p>Pairs of start and stop transfer signals</p> Signup and view all the answers

    The process by which sugars are added to proteins in the ER is known as ______.

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

    Match the types of transmembrane proteins:

    <p>Single pass transmembrane protein = Contains one start-transfer signal Multi pass transmembrane protein = Contains multiple start and stop transfer signals Glycosylated protein = Has sugars attached Resident proteins of the ER = Require KDEL signal for retention</p> Signup and view all the answers

    Study Notes

    Protein Sorting II: How Proteins Enter the Endomembrane System

    • The endoplasmic reticulum (ER) is the primary gateway to the endomembrane system.
    • Proteins destined for the ER possess a signal sequence (6-15 hydrophobic amino acids, typically at the N-terminus)
    • mRNA encoding these proteins is targeted to the ER, and ribosomes bind to the ER membrane during translation
    • The process where ribosomes attach and protein synthesis occurs at the ER membrane is called co-translational import.
    • Soluble proteins are synthesized and directed into the ER lumen through a channel.
    • Membrane proteins are partly translocated and integrated into the ER membrane.
    • The SRP (signal recognition particle) recognizes and binds the signal sequence, causing translation to pause.
    • The SRP-ribosome complex then docks with the SRP receptor on the ER membrane, and the ribosome and protein are now attached to translocator on the ER membrane.
    • Translation resumes, and the growing polypeptide chain is threaded through the translocator into the ER lumen.
    • The signal sequence is cleaved by signal peptidase in the ER lumen.
    • The signal sequence is rich in methionine, and accommodates hydrophobic ER import signals.
    • GTP hydrolysis by SRP and SRP receptor releases the SRP and restarts translation.
    • The complete complex docks with the SRP receptor on the ER membrane, and translation resumes.
    • The translocation channel opens in a plug-like fashion permitting proteins to enter the lumen.
    • Disulfide bridges are formed in the ER by PDI.
    • Proteins are glycosylated.
    • GPI anchors can be added.
    • Proper folding of proteins in the ER is monitored.
    • Misfolded proteins are exported to the cytosol for degradation.
    • The ER quality control mechanism ensures that only properly folded proteins are allowed to exit and enter the Golgi apparatus.
    • Proteins needing to remain in the ER have an ER retention signal (KDEL).
    • The ER quality control mechanism estimates that, up to 30% of proteins, are destroyed if the folding is not proper.
    • Defects in the ER quality control are a cause of cystic fibrosis.
    • Lipids and proteins exit the ER, moving to the Golgi apparatus for further processing via vesicle transport.
    • Vesicle transport is used inside the endomembrane system, including from the ER to Golgi.

    Rough ER vs. Smooth ER

    • Rough ER:
      • Flattened sheets
      • Coated with ribosomes
      • Synthesizes, folds, and glycosylates proteins.
    • Smooth ER:
      • Tubular
      • Less abundant in most cell types
      • Synthesizes lipids and cholesterol
      • Stores intracellular Ca2+.
      • Detoxifies compounds.
      • Involved in steroid hormone synthesis.

    ER Import Signal

    • The ER import signal is recognized by a co-receptor.
    • Co-receptor docks the ribosome at translocators in the ER membrane.
    • The ribosome binds to the ER membrane, and the mRNA is translated.

    Signal Recognition Particle (SRP)

    • SRP is an ancient rbonucleoprotein co-receptor.
    • SRP in eukaryotes has 6 polypeptides and 1 RNA molecule.
    • The system evolved early in evolution.
    • SRP has a hinge mechanism.
    • SRP binds to the signal ER import sequence and causes a change in conformation of this sequence, thus triggering a conformational change that exposes a binding site.
    • The SRP receptor is involved in binding to the ribosome complex
    • The pause domain helps block the elongation of translation.
    • SRP serves as a molecular matchmaker.
    • SRP binding pauses translation, so that peptide synthesis does not occur until the protein and ribosome complex are docked properly into the ER.

    Modification of Proteins in the rough ER

    • GPI anchors are added to some proteins.
    • Disulfide bridges are formed by PDI, establishing a critical part of the protein structure.
    • Some proteins are glycosylated (N-linked complex oligosaccharides).

    N-glycosylation

    • N-glycosylation can promote the folding of certain proteins and thus is used as a "tag" that marks the state of protein folding, as properly folded proteins are able to exit the ER.

    Export and Degradation of Misfolded ER Proteins

    • Misfolded proteins are exported to be degraded by proteasomes in the cytosol or other organelles.
    • A signaling chain of ubiquitin marks misfolded proteins for degradation.

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    Lecture 14. Protein Sorting II 2024 PDF

    Description

    Explore how proteins enter the endomembrane system, focusing on the role of the endoplasmic reticulum (ER) and various mechanisms involved, including co-translational import. Understand the complexities of signal sequences, SRP functionality, and the translocation process of proteins. Test your knowledge with this quiz covering key concepts of protein sorting.

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