Cell Biology Chapter: Endoplasmic Reticulum Quiz
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What is the primary function of the rough endoplasmic reticulum (RER)?

  • Storage of calcium ions
  • Synthesis and folding of proteins (correct)
  • Synthesis of lipids and cholesterol
  • Detoxification of drugs

    • The smooth endoplasmic reticulum (SER) is more abundant near the nucleus compared to the rough endoplasmic reticulum (RER).

    False

    Where in the cell are most lipids synthesized?

    Smooth endoplasmic reticulum

    The ______ is the only gateway to the endomembrane system.

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

    Match the following functions to the correct type of endoplasmic reticulum:

    <p>Rough ER = Addition of disulphide bonds Smooth ER = Detoxification of hydrophobic drugs</p> Signup and view all the answers

    What function does Lumacaftor serve in the treatment of cystic fibrosis?

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

    The median age of people with cystic fibrosis is now below 30.

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

    What is the next destination for lipids and proteins after they exit the ER?

    <p>Golgi complex</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

    Match the following terms with their corresponding descriptions:

    <p>Lumacaftor = Chaperone for protein folding Golgi complex = Processing of lipids and proteins Vesicle transport = Transportation within the endomembrane system Cystic fibrosis = A genetic disorder affecting lung function</p> Signup and view all the answers

    What happens if a protein is unfolded for too long in the ER?

    <p>It is degraded by mannosidase</p> Signup and view all the answers

    N-glycosylation marks improperly folded proteins for exit from the ER.

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

    What is a leading cause of cystic fibrosis disease?

    <p>Defects in the stringency of the ER quality control mechanism</p> Signup and view all the answers

    If defects occur in the Cl- channel, it triggers an ______ protein response in the ER.

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

    Match the following terms with their descriptions:

    <p>Calnexin = Assists in the folding of glycoproteins Mannosidase = Removes mannoses from misfolded proteins N-glycosylation = Tags protein folding status Correctors = Developed to block unfolded protein response</p> Signup and view all the answers

    What role does the Signal Recognition Particle (SRP) play in the process of protein translation?

    <p>It pauses translation and facilitates docking onto the ER.</p> Signup and view all the answers

    The SRP consists of only an RNA molecule and has no polypeptides.

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

    How does GTP hydrolysis affect the SRP?

    <p>It allows the SRP to release from the SRP receptor when cargo is delivered.</p> Signup and view all the answers

    The pause domain of SRP blocks _____ from binding, thus pausing translation.

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

    Match the following components with their functions:

    <p>Signal Recognition Particle (SRP) = Pauses translation SRP receptor = Docks with SRP on ER membrane Translocation channel = Facilitates protein passage into the ER GTP = Triggers SRP release from receptor</p> Signup and view all the answers

    What maintains the permeability barrier of the ER membrane during ribosome docking?

    <p>The closed state of the translocation channel</p> Signup and view all the answers

    The ribosome must be properly docked before GTP cleavage can occur.

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

    What happens to the translocation channel after the signal sequence binds?

    <p>It opens by displacing a loop that acts as a plug.</p> Signup and view all the answers

    What is the primary function of the signal sequence in protein translocation into the ER?

    <p>Starts the transfer of the polypeptide into the ER</p> Signup and view all the answers

    Integral membrane proteins are completely translocated into the ER lumen.

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

    What is the role of the signal recognition particle (SRP) during protein synthesis?

    <p>It binds the ER import signal and pauses translation.</p> Signup and view all the answers

    Proteins enter the endoplasmic reticulum via __________ import.

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

    Match the following processes or components with their functions:

    <p>Signal Recognition Particle = Binds the ER import signal Translocation Channel = Facilitates entry into the ER Polyribosome = Multi-ribosomal complex for protein synthesis Hydrophobic Amino Acids = Part of signal sequences that initiate translocation</p> Signup and view all the answers

    Which of the following accurately describes how ribosomes are docked at translocation channels?

    <p>Docking relies on a signal sequence containing 6-15 hydrophobic amino acids.</p> Signup and view all the answers

    The ER import signal is recognized by a __________ that docks the ribosome.

    <p>co-receptor</p> Signup and view all the answers

    What happens when there are conflicting signal sequences for ER and mitochondrial import?

    <p>The stronger import signal will dictate the final destination.</p> Signup and view all the answers

    What is typically towards the N-terminus of a protein with an internal start-transfer signal sequence?

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

    Translocated polypeptides are folded in the cytosol.

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

    What is the function of the 'KDEL' signal in the ER?

    <p>To retain resident proteins in the ER.</p> Signup and view all the answers

    The addition of a common N-linked complex oligosaccharide is known as __________.

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

    Match the types of membrane proteins with their characteristics:

    <p>Single pass transmembrane protein = One start and one stop transfer signal Multi pass transmembrane protein = Multiple start and stop transfer signals Peripheral membrane protein = Not embedded in the lipid bilayer Integral membrane protein = Embedded within the lipid bilayer</p> Signup and view all the answers

    What role do ER chaperones like calnexin play in protein folding?

    <p>Assist in protein folding</p> Signup and view all the answers

    Disulfide bridges are formed in the rough ER.

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

    What happens to translocated proteins after the terminal glucose is trimmed off during glycosylation?

    <p>They are released from calnexin for a round of folding.</p> Signup and view all the answers

    Pairs of start and stop transfer signals determine how __________ proteins integrate into the lipid bilayer.

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

    What is the main purpose of glycosylation in the rough ER?

    <p>To serve as a marker of protein folding</p> Signup and view all the answers

    Study Notes

    Protein Sorting II

    • Proteins enter the endomembrane system via the endoplasmic reticulum (ER).
    • mRNA encoding a protein with an ER signal sequence is targeted to the ER.
    • The ER is the sole "gateway" to the endomembrane system.
    • Transport pathways are categorized as protein translocation, gated transport, and vesicular transport.

    ER Structure

    • The ER is an extensive network of flattened sacs and branching tubules.
    • The tubule network extends to the edges of the cell.
    • The ER network is densest around the nucleus.
    • The nuclear envelope is part of the ER, with the lumen continuous.
    • The membrane layers of the ER are interconnected.

    Rough ER (RER)

    • RER is defined by its ribosome coating.
    • It's most abundant near the nucleus and throughout the cell.
    • It's the site where proteins are synthesized, folded, and glycosylated.
    • The size of RER is controlled by the demand for protein.

    Smooth ER (SER)

    • SER is more abundant near cell edges and expanded in certain cell types.
    • It's the site for lipid and cholesterol synthesis.
    • It's for intracellular Ca²⁺ storage.
    • It's where transport vesicles carrying newly synthesized proteins and lipids travel to the Golgi apparatus.
    • Expanded in liver, muscle, and endocrine cells synthesizing steroid hormones.

    Co-translational Import

    • Most proteins entering the ER use a co-translational import mechanism.
    • The process involves ribosomes, signal sequence, protein translocator, and mRNA.
    • Proteins enter the ER while being synthesized.

    Ribosomes at ER Translocation Channels

    • Docking relies on a signal sequence—6-15 hydrophobic amino acids usually at the protein N-terminus.
    • This signal sequence is called the start transfer sequence.
    • Soluble proteins are synthesized through a channel into the ER lumen.
    • Integral membrane proteins are partly translocated and become embedded in the membrane.

    ER Signal Recognition Particle (SRP)

    • SRP is an ancient ribonucleoprotein co-receptor recognizing ER import signals.
    • The SRP binding pocket is rich in methionine and accommodates hydrophobic ER import signals.
    • GTP hydrolysis by SRP and SRP receptor releases SRP from the SRP receptor when cargo is delivered, allowing it to return to its original shape.
    • In eukaryotes, SRP is six polypeptides plus one RNA molecule.
    • SRP is conserved in prokaryotes, implying early evolutionary origins.
    • SRP has a hinge-like mechanism; binding triggers a conformation change exposing a binding site for SRP receptor.
    • The pause domain binds to the interface between the large and small ribosomal subunits.
    • The entire complex docks with SRP receptor on ER membrane.
    • SRP serves as a "molecular matchmaker," binding to signal sequence and causing a pause in translation.
    • Proper docking results in GTP cleavage, SRP and SRP receptor release, and translocation channel closure. The ER membrane permeability is maintained.

    Signal Sequence Binding

    • Signal sequence binding opens the translocation channel displacing a loop that acts as a "plug."
    • The pore has four identical complexes, open sideways at the seam and allows diffusion of signal sequence or release of transmembrane proteins into the bilayer.

    Soluble Protein Import

    • Signal is cleaved and released, and the pore closes.
    • Signal peptide is rapidly degraded.
    • Soluble protein is inside the ER.

    Internal Signal Sequences

    • Start-transfer signal sequence located elsewhere than at the N-terminus of the protein.
    • The basic end of the start-transfer sequence is typically toward the N-terminus, binding in the pore.
    • Internal signal sequences are not cleaved, forming a transmembrane domain.

    Transmembrane Proteins

    • Pairs of start and stop transfer signals determine how transmembrane proteins integrate into the lipid bilayer.
    • Start signal initiates transfer, stop signal stops transfer.
    • Single pass proteins have one signal sequence.
    • Multi-pass proteins have multiple signal sequences in addition to stop signals, resulting in multiple transmembrane segments.

    Protein Modifications inside ER Lumen

    • Translocated polypeptide chains are folded and glycosylated in the rough ER lumen.
    • Enzymes add lipid anchors to proteins.
    • Chaperones assist protein folding.
    • Glycosyltransferase adds sugars.
    • Protein disulfide isomerase forms disulfide bridges.
    • GPI anchors are added to some proteins.
    • Lipid-linked integral membrane proteins are anchored.

    N-linked Glycosylation

    • Most proteins synthesized in the rough ER are glycosylated.
    • A common N-linked complex oligosaccharide is added.
    • N-glycosylation can promote protein folding and activity.

    ER Protein Quality Control

    • Only properly folded proteins are allowed to leave the ER.
    • Up to 30% of proteins are destroyed if not properly folded.
    • Defective ER folding causes cystic fibrosis.

    ER to Golgi Transport

    • Lipids and proteins exiting the ER move to the Golgi complex for further processing.
    • Vesicle transport is the method of ER-to-Golgi protein and lipid transportation.

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