Cell Biology - Endoplasmic Reticulum and Golgi
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Questions and Answers

Cholesterol acts to reduce permeability of the cell ______.

membrane

The Smooth ER is involved in calcium ______ and drug detoxification.

storage

Ribosomal sub-units complex together with mRNA in the cytoplasm to initiate ______ synthesis.

protein

The signal recognition particle (SRP) recognizes and binds to the signal ______.

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

Protein distribution to the ER begins with signal ______ particle recognition.

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

The carboxy or amino ______ can be located in the cytosol.

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

ER is a major ______ folding site.

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

BiP chaperone protein is responsible for mediating protein ______.

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

Protein disulfide isomerase catalyzes the oxidation of ______ groups to form disulfide bonds.

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

About half of the proteins processed in ER are ______.

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

N-linked glycosylation stabilises proteins by masking ______, proteolytic cleavage sites or immune recognition.

<p>hydrophobic stretches</p> Signup and view all the answers

An unfolded protein will undergo continuous cycles of glucose ______ and glucose addition until it has achieved its fully folded state.

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

The endomembrane system includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, and the ______.

<p>plasma membrane</p> Signup and view all the answers

Once 3 glucose and 1 mannose are ______, the protein can be transported to the Golgi.

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

The accumulation of unfolded, misfolded or damaged proteins leads to ER ______.

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

The endoplasmic reticulum (ER) is a labyrinth of branching tubules and sacs that is continuous with the outer nuclear ______.

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

When the misfolded proteins exceed ER degradation capacity, the unfolded protein ______ is activated.

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

The rough ER is characterized by flattened sheets with ______ bound to its cytosolic surface.

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

Smooth ER is involved in lipid biosynthesis, carbohydrate metabolism, calcium storage, and ______ of drugs and poisons.

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

The internal space of the endoplasmic reticulum is called the ______.

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

The primary function of the rough ER is protein ______.

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

Each step in lipid synthesis in the smooth ER is catalyzed by ______ within the organelle.

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

Rough ER also contributes to protein folding and ______ of incorrect proteins.

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

The SRP-ribosome complex binds to the SRP ______ and the translocation complex on ER membrane.

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

During protein synthesis, the signal sequence enters the ______ in the ER membrane.

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

For secretory proteins, the protein is completely translocated across the ______.

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

The amino acid signal sequence is cleaved by a signal ______.

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

Transmembrane proteins can have a second hydrophobic region known as a ‘Stop Transfer ______.

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

Types II and III transmembrane proteins have an internal amino acid signal ______.

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

In multipass transmembrane proteins, there are alternating internal signal sequences and Stop ______ signals.

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

Examples of transmembrane proteins include receptor tyrosine kinases, integrins, or cytokine ______.

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

The rough ER is primarily responsible for protein ______.

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

The smooth ER performs diverse metabolic functions, including lipid ______.

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

Endocytosis is the ______ of nutrients or removal of plasma membrane components.

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

COPII-coated vesicles are involved in transportation from the ER to the ______.

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

The Golgi apparatus consists of multiple discrete compartments called ______.

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

The trans Golgi network acts as a ______ center for sorting and distribution.

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

N-linked glycosylation is a signaling molecule in the ______ process.

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

Transport vesicles are membrane-enclosed ______ to move cargo between compartments.

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

The movement of cargo from the cis Golgi network to the ______ Golgi network is an essential step in processing.

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

Chaperone proteins in the ER lumen mediate protein ______.

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

The Smooth ER has tubular structure with no _____.

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

Protein biosynthesis in the Rough ER involves water soluble secretory proteins and ______ proteins.

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

Water soluble proteins in the RER are ______ across the ER membrane.

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

Transmembrane proteins in the RER are _____ within the ER membrane.

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

Cholesterol is involved in _____ synthesis and steroid synthesis.

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

Membranes largely consist of phospholipids, ______ and cholesterol.

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

Ubiquitin-proteosome system: translocated into the cytosol, tagged with ubiquitin, and degraded by ______.

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

Autophagic-lysosomal system: aberrant protein fragments degraded by ______.

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

Cholesterol plays a role in membrane structure and acts to reduce ______ of the cell membrane.

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

The Smooth ER is a site for drug ______, aiding in the detoxification of harmful substances.

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

Translation always begins in the ______, except for a few proteins made in certain organelles.

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

Calcium is stored in the ER lumen, where it binds to specific ______ proteins.

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

The primary function of the rough ER is protein ______, particularly for secretory proteins.

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

N-linked glycosylation stabilises proteins by masking ______ stretches.

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

Once 3 glucose and 1 mannose are trimmed, the protein can be transported to the ______.

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

Chaperone proteins recognise and bind to ______ molecules.

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

The _____ apparatus is part of the endomembrane system and plays a key role in modifying proteins.

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

The rough endoplasmic reticulum (RER) is characterized by flattened sheets with _____ on its cytosolic surface.

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

The internal space of the endoplasmic reticulum is referred to as the _____ lumen.

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

The endoplasmic reticulum (ER) constitutes more than _____ of the total membrane of an average animal cell.

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

The _____ endoplasmic reticulum (SER) has a tubular structure and is involved in various metabolic functions.

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

Each step in lipid synthesis is catalyzed by _____ within the smooth ER.

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

The transmembrane protein moves along the membrane ______ using the signal sequence.

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

The signal sequence enters the ______ in the ER membrane during protein synthesis.

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

Molecular chaperones and folding enzymes in the ER __________ assist in folding and control subsequent release from the ER.

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

BiP is an ATP-driven protein that mediates protein __________ and prevents protein aggregation.

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

N-linked glycosylation involves the addition of a pre-formed precursor __________ onto the side chain of asparagine.

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

Protein disulfide isomerase catalyzes the oxidation of sulfhydryl groups on cysteines to form __________ bonds.

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

About half of the proteins processed in the ER are __________.

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

The rough ER is primarily responsible for ______ biosynthesis.

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

Smooth ER is involved in lipid ______ and drug detoxification.

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

The ______ Golgi network acts as a sorting center for distribution.

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

N-linked glycosylation is a signaling molecule in the folding ______.

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

Vesicular tubular clusters move along ______ towards the Golgi apparatus.

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

What is the primary function of the Smooth ER?

<p>Calcium storage and drug detoxification</p> Signup and view all the answers

Which statement correctly describes the role of ribosomal sub-units during protein biosynthesis?

<p>They initiate protein synthesis in the cytosol.</p> Signup and view all the answers

What role does cholesterol play in cellular membranes?

<p>It acts to reduce permeability of the cell membrane.</p> Signup and view all the answers

What is the function of the signal recognition particle (SRP)?

<p>To recognize and bind to the signal sequence on proteins.</p> Signup and view all the answers

What process is enhanced by the presence of cyclic enzymes like cytochrome P450 in the Smooth ER?

<p>Oxidation of lipid-soluble drugs for detoxification.</p> Signup and view all the answers

What role does BiP chaperone protein primarily serve in the endoplasmic reticulum (ER)?

<p>It helps in the folding and prevents aggregation of proteins.</p> Signup and view all the answers

Which of the following statements about protein disulfide isomerase is true?

<p>It catalyzes the oxidation of cysteines to form disulfide bonds.</p> Signup and view all the answers

What percentage of glycoproteins undergo N-linked glycosylation in the ER lumen?

<p>90%</p> Signup and view all the answers

What is a primary role of N-linked glycosylation in proteins?

<p>To prevent early degradation of proteins</p> Signup and view all the answers

What is the primary function of molecular chaperones in the ER?

<p>To aid in proper protein folding and prevent aggregation.</p> Signup and view all the answers

What happens to an unfolded protein undergoing quality control in the endoplasmic reticulum?

<p>It undergoes cycles of glucose trimming and addition</p> Signup and view all the answers

Which feature of ER resident proteins is crucial for their retention in the ER lumen?

<p>ER retention signal at the C terminus.</p> Signup and view all the answers

What consequence occurs when misfolded proteins accumulate in the endoplasmic reticulum?

<p>Activation of the unfolded protein response</p> Signup and view all the answers

What is the fate of proteins that do not achieve their fully folded state in the endoplasmic reticulum?

<p>They are tagged with ubiquitin and degraded</p> Signup and view all the answers

How does the endoplasmic reticulum quality control (ERQC) system ensure the integrity of glycoproteins?

<p>It allows only fully folded proteins to be transported to the Golgi.</p> Signup and view all the answers

What is the primary role of the smooth endoplasmic reticulum?

<p>Lipid and steroid hormone synthesis</p> Signup and view all the answers

Which process primarily occurs in the rough endoplasmic reticulum?

<p>Biosynthesis of water-soluble secretory proteins</p> Signup and view all the answers

Which statement best describes the Golgi apparatus's function within the endomembrane system?

<p>It modifies and sorts proteins received from the ER.</p> Signup and view all the answers

What is one characteristic feature of the rough endoplasmic reticulum?

<p>It is studded with ribosomes on its cytosolic surface.</p> Signup and view all the answers

What type of proteins are synthesized in the rough endoplasmic reticulum?

<p>Transmembrane proteins and secretory proteins</p> Signup and view all the answers

How do transport vesicles function in the endomembrane system?

<p>By moving proteins and lipids between organelles</p> Signup and view all the answers

What is the internal space of the endoplasmic reticulum called?

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

Which organelle is continuous with the outer nuclear membrane?

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

What happens to the signal sequence in secretory proteins once they are fully translocated across the membrane?

<p>It is cleaved by a signal peptidase.</p> Signup and view all the answers

In Type I transmembrane proteins, what is the role of the secondary hydrophobic region?

<p>It functions as a stop transfer signal.</p> Signup and view all the answers

Which statement correctly describes the orientation of internal signal sequences in Types II and III transmembrane proteins?

<p>They can be oriented either way within the translocon.</p> Signup and view all the answers

What is the main function of the translocon during protein translocation?

<p>It allows the signal sequence to enter the ER membrane.</p> Signup and view all the answers

How do multipass transmembrane proteins differ from single-pass proteins?

<p>They contain alternating internal signal and stop transfer sequences.</p> Signup and view all the answers

Which type of protein is characterized by having a signal sequence targeted to the ER lumen?

<p>Type I transmembrane proteins.</p> Signup and view all the answers

What characterizes the movement of transmembrane proteins within the ER membrane?

<p>They can move laterally along the membrane.</p> Signup and view all the answers

What type of proteins are typically involved in receptor functions within the cell membrane?

<p>Transmembrane proteins.</p> Signup and view all the answers

What is the primary role of the Rough ER in the cell?

<p>Protein biosynthesis</p> Signup and view all the answers

Which type of coated vesicle is responsible for transporting materials from the ER to the Golgi apparatus?

<p>COPII-coated vesicles</p> Signup and view all the answers

What process allows for the packaging and transport of proteins and lipids received from the ER?

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

Which statement describes the function of the Golgi apparatus?

<p>It modifies, sorts, and packages proteins and lipids.</p> Signup and view all the answers

During which process do COPI-coated vesicles bud off from vesicular tubular clusters?

<p>Retrieval pathway</p> Signup and view all the answers

What role do chaperone proteins play in the endoplasmic reticulum?

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

What initiates the transport of a ribosome and polypeptide chain to the ER membrane?

<p>Protein signal sequence</p> Signup and view all the answers

Which network in the Golgi apparatus is tasked with the sorting and distribution of proteins?

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

What is the primary role of endocytosis in cellular function?

<p>Internalization of nutrients</p> Signup and view all the answers

What occurs during the modification of oligosaccharides within the Golgi apparatus?

<p>Sugars are added or removed.</p> Signup and view all the answers

What is the primary function of Binding immunoglobulin protein (BiP) in the endoplasmic reticulum?

<p>To bind with polypeptide chains and mediate their folding</p> Signup and view all the answers

Which statement about N-linked glycosylation in the ER is accurate?

<p>It involves the addition of a pre-formed oligosaccharide of 14 sugars.</p> Signup and view all the answers

What role does protein disulfide isomerase perform in the ER?

<p>It catalyzes the formation of disulfide bonds between cysteine residues.</p> Signup and view all the answers

How does BiP respond to incorrectly folded proteins?

<p>It prevents these proteins from leaving the ER by binding to exposed amino acids.</p> Signup and view all the answers

What is the significance of the ER retention signal found in resident proteins in the ER lumen?

<p>It ensures proteins remain in the ER by preventing their transport to the Golgi.</p> Signup and view all the answers

What is the primary role of N-linked glycosylation in protein stability?

<p>To mask hydrophobic stretches and prevent early degradation</p> Signup and view all the answers

What process occurs when an unfolded protein is not properly folded in the endoplasmic reticulum?

<p>Continuous cycles of glucose addition and trimming</p> Signup and view all the answers

What happens to proteins that exceed the degradation capacity of the endoplasmic reticulum?

<p>They activate the unfolded protein response</p> Signup and view all the answers

Which system is responsible for degrading misfolded proteins that cannot be handled by the endoplasmic reticulum?

<p>Ubiquitin-proteasome system</p> Signup and view all the answers

What is the consequence of ineffective protein folding in the ER?

<p>Accumulation of unfolded proteins leading to ER stress</p> Signup and view all the answers

What is the main role of cholesterol in the cell membrane?

<p>Decreases permeability of the cell membrane</p> Signup and view all the answers

Which enzyme in the smooth ER is critical for detoxifying lipid-soluble drugs?

<p>Cytochrome p450</p> Signup and view all the answers

How does the signal recognition particle (SRP) initiate protein distribution to the ER?

<p>It binds to the signal sequence of a nascent protein</p> Signup and view all the answers

What is the specialized function of the sarcoplasmic reticulum in muscle cells?

<p>Calcium storage and release</p> Signup and view all the answers

What initiates the process of protein synthesis in the rough ER?

<p>Binding of ribosomal subunits to mRNA</p> Signup and view all the answers

What primary function does the smooth endoplasmic reticulum not perform?

<p>Protein biosynthesis</p> Signup and view all the answers

Which correctly describes the internal structure of the endoplasmic reticulum?

<p>An interconnected network of tubules and sacs</p> Signup and view all the answers

In what way does the rough endoplasmic reticulum assist in protein processing?

<p>Facilitates protein folding and modification</p> Signup and view all the answers

What role do ribosomes play in the structure of the rough endoplasmic reticulum?

<p>They are attached to the cytosolic surface.</p> Signup and view all the answers

How do transport vesicles facilitate movement between the endoplasmic reticulum and Golgi apparatus?

<p>By carrying cargo in membrane-enclosed sacs</p> Signup and view all the answers

Which characteristic distinguishes smooth endoplasmic reticulum from rough endoplasmic reticulum?

<p>Function in detoxifying drugs</p> Signup and view all the answers

What is a key function of the Golgi apparatus in relation to proteins?

<p>Modification and sorting of proteins</p> Signup and view all the answers

What property of membranes in the endomembrane system is highlighted?

<p>They are dynamic and not identical.</p> Signup and view all the answers

What role does the Rough ER primarily serve in the cell?

<p>Protein biosynthesis</p> Signup and view all the answers

Which of the following functions is NOT associated with the Smooth ER?

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

What is the role of the signal sequence in transmembrane proteins?

<p>It is responsible for the translocation of proteins across the membrane.</p> Signup and view all the answers

What type of coated vesicle is chiefly responsible for transport from the ER to the Golgi apparatus?

<p>COPII-coated</p> Signup and view all the answers

In the context of transmembrane proteins, what is a 'Stop Transfer Signal'?

<p>A hydrophobic region that halts the translocation process.</p> Signup and view all the answers

In the Golgi apparatus, what is the primary function of the Trans Golgi network?

<p>Sorting and distribution of cargo</p> Signup and view all the answers

Which component aids in the transport of vesicles along microtubules towards the Golgi apparatus?

<p>Motor proteins</p> Signup and view all the answers

What distinguishes Types II and III transmembrane proteins from Type I?

<p>They possess an internal signal sequence.</p> Signup and view all the answers

How are multipass transmembrane proteins structured?

<p>By alternating internal signal sequences and stop transfer signals.</p> Signup and view all the answers

What is the process by which nutrients are internalized by the cell called?

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

What does the term 'cis Golgi network' refer to in the context of the Golgi apparatus?

<p>Initial entry site for ER-derived vesicles</p> Signup and view all the answers

Which process includes the fusion of ER-derived vesicles to form vesicular tubular clusters?

<p>Cisternal maturation</p> Signup and view all the answers

What initiates the binding of the SRP-ribosome complex to the ER membrane?

<p>The interaction between the signal sequence and the SRP receptor.</p> Signup and view all the answers

Which of the following statements is TRUE regarding internal signal sequences?

<p>They can orientate either terminus in the cytosol depending on their positioning.</p> Signup and view all the answers

What modification process occurs to proteins within the Golgi apparatus?

<p>Oligosaccharide processing</p> Signup and view all the answers

How do transmembrane proteins move laterally within the membrane?

<p>By following the directional cues from the signal sequence.</p> Signup and view all the answers

What is one of the roles of cholesterol in cellular membranes?

<p>Reduces membrane fluidity</p> Signup and view all the answers

Which enzyme is associated with drug detoxification in the Smooth ER?

<p>Cytochrome P450 enzymes</p> Signup and view all the answers

What characterizes the rough endoplasmic reticulum?

<p>It contains ribosomes on its cytosolic surface.</p> Signup and view all the answers

What does the signal recognition particle (SRP) bind to during protein distribution to the ER?

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

Which of the following is a function of the smooth endoplasmic reticulum?

<p>Calcium storage</p> Signup and view all the answers

How are transport vesicles primarily involved in the endomembrane system?

<p>They move cargo between organelles.</p> Signup and view all the answers

What is the primary internal space of the endoplasmic reticulum known as?

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

Which of these membrane-bound organelles is part of the endomembrane system?

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

What is one of the metabolic functions of the rough endoplasmic reticulum?

<p>Removal of incorrect proteins</p> Signup and view all the answers

What do the membranes within the endomembrane system have in common?

<p>They compartmentalize various organelles.</p> Signup and view all the answers

What form of proteins does the rough endoplasmic reticulum primarily synthesize?

<p>Transmembrane proteins</p> Signup and view all the answers

What role does BiP chaperone protein play in the endoplasmic reticulum?

<p>It mediates protein folding and prevents aggregation.</p> Signup and view all the answers

What is the primary function of protein disulfide isomerase in the endoplasmic reticulum?

<p>To catalyze oxidation of sulfhydryl groups to form disulfide bonds.</p> Signup and view all the answers

What percentage of glycoproteins undergo N-linked glycosylation in the endoplasmic reticulum?

<p>90%</p> Signup and view all the answers

How do molecular chaperones in the ER assist with protein folding?

<p>By binding to exposed amino acids to prevent misfolding.</p> Signup and view all the answers

What is the primary signal for retention of resident proteins in the ER lumen?

<p>An ER retention signal at the C terminus.</p> Signup and view all the answers

What role does N-linked glycosylation play in protein stability?

<p>Masks hydrophobic stretches to stabilize proteins</p> Signup and view all the answers

What happens to proteins that do not achieve their fully folded state in the ER?

<p>They undergo degradation through the ubiquitin-proteosome system</p> Signup and view all the answers

What is the function of chaperone proteins in the context of N-linked glycosylation?

<p>To recognize and bind to glucose molecules</p> Signup and view all the answers

What initiates the unfolded protein response in the ER?

<p>An accumulation of misfolded proteins</p> Signup and view all the answers

During the glycosylation process, which sugars are trimmed before the protein is transported to the Golgi?

<p>3 glucose and 1 mannose</p> Signup and view all the answers

What directs the ribosome and polypeptide chain to the ER membrane?

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

What type of coated vesicle mediates transport from the ER to the Golgi?

<p>COPII-coated vesicles</p> Signup and view all the answers

Which function is NOT associated with the Smooth ER?

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

What is the purpose of vesicular tubular clusters?

<p>To move cargo from the ER to the Golgi apparatus</p> Signup and view all the answers

What is one of the roles of the Golgi apparatus?

<p>Modification of proteins, lipids, and polysaccharides</p> Signup and view all the answers

What is the role of the trans Golgi network?

<p>Sorting and distribution of proteins and lipids</p> Signup and view all the answers

Which process is responsible for internalization of nutrients?

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

Which of the following is true regarding N-linked glycosylation?

<p>It is involved in protein folding</p> Signup and view all the answers

How do COPI-coated vesicles function within the Golgi?

<p>They transport proteins from the Golgi back to the ER</p> Signup and view all the answers

What happens to the signal sequence after protein synthesis for secretory proteins?

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

Which characteristic differentiates Type I transmembrane proteins from Types II and III?

<p>Type I proteins have a N-terminus signal sequence</p> Signup and view all the answers

What does the 'Stop Transfer Signal' do during protein translocation?

<p>Closes the translocon to prevent further translocation</p> Signup and view all the answers

Which statement is true about the orientation of internal signal sequences in Types II and III transmembrane proteins?

<p>They can orient either way through the translocon</p> Signup and view all the answers

What is the function of the translocon during protein synthesis?

<p>It allows polypeptides to translocate across the ER membrane</p> Signup and view all the answers

In multipass transmembrane proteins, what is the role of alternating internal signal sequences and stop transfer signals?

<p>To create multiple transmembrane domains</p> Signup and view all the answers

Which type of proteins are characterized by having an internal signal sequence that can orient either way through the translocon?

<p>Type II and III transmembrane proteins</p> Signup and view all the answers

What role does the SRP-ribosome complex play in protein synthesis?

<p>It binds to the SRP receptor and translocation complex on the ER membrane</p> Signup and view all the answers

Study Notes

Endoplasmic Reticulum (ER) and Golgi Apparatus

  • The endoplasmic reticulum (ER) and Golgi apparatus are part of the endomembrane system.
  • The ER and Golgi are involved in protein and lipid synthesis, modification, and transport.
  • Membranes within these organelles are not identical and are dynamic in nature.
  • The ER constitutes more than half of the total membrane in an average animal cell.

Endoplasmic Reticulum (ER)

  • Structure: The ER is a labyrinth of branching interconnected tubules and sacs continuous with the outer nuclear membrane. The single internal space is the ER lumen. There are two types: Rough ER and Smooth ER.

  • Rough ER: Composed of flattened sheets with ribosomes attached to the cytosolic surface. These ribosomes synthesize proteins.

  • Smooth ER: Consists of tubular structures with no ribosomes. It plays a role in lipid synthesis, carbohydrate metabolism, calcium storage, and detoxification.

ER Function

  • Smooth ER: Lipid biosynthesis (oils, steroids, phospholipids), carbohydrate metabolism, calcium storage, detoxification of drugs and poisons. Cholesterol is an important component in membrane structure and plays a role in bile and steroid synthesis.
  • Rough ER: Protein biosynthesis (water-soluble secretory proteins, transmembrane proteins), protein folding and modification, protein quality control (removal of incorrect proteins).

Rough ER: Protein Biosynthesis

  • Ribosomes in the cytoplasm bind to mRNA at initiation of protein synthesis.
  • Proteins with an amino acid signal sequence are directed to the ER.
  • Signal recognition particle (SRP) binds to the signal sequence, stopping translation temporarily.
  • SRP-ribosome complex binds to SRP receptor (on ER membrane) and the translocation complex (Sec61).
  • Polypeptide chain enters the translocon, and translation resumes.
  • Signal sequence is cleaved, and the protein is completely translocated across the membrane.
  • For transmembrane proteins, stopping transfer signals determine the location and orientation of the polypeptide chain in the membrane.

Transmembrane Proteins

  • Type I: Single pass – N-terminus signal sequence targeted to ER lumen, and a second hydrophobic region (Stop transfer signal) halts insertion and forms a single-pass transmembrane protein.
  • Types II/III: Single pass – internal signal sequences can be oriented either carboxyl or amino terminus in the cytosol (No signal sequence at N-terminus); one internal hydrophobic region insertion/orientation determines formation of a single-pass protein.
  • Multipass: Multiple alternating internal signal sequences determine the locations and orientations to form multipass transmembrane proteins.

Protein Folding

  • ER: The ER plays a crucial role in protein folding.
  • Molecular chaperones and folding enzymes in the ER lumen assist in protein folding and control subsequent release.

Protein Folding by ER Chaperones

  • BIP (Binding immunoglobulin protein) is a chaperone protein that binds to polypeptide chains and drives unidirectional translocation, aiding protein folding and preventing aggregation of proteins.
  • It detects incorrectly folded proteins by binding to exposed amino acids and prevents them from exiting the ER.

Protein Folding: Disulfide Bond Formation

  • Protein disulfide isomerase within the ER lumen catalyzes the oxidation of sulfhydryl groups on cysteine residues to form disulfide bonds, which are important for stabilizing protein structure.

N-linked glycosylation

  • N-linked glycosylation is the addition of a pre-formed oligosaccharide (14 sugars) to asparagine side chain NH₂ groups on around 90% of ER-processed glycoproteins.
  • This process stabilizes proteins by masking hydrophobic regions, preventing degradation, and increasing protein solubility.

ER Quality Control

  • The ER is responsible for protein quality control. Very few proteins achieve their fully folded state.
  • The build-up of misfolded, damaged proteins causes ER stress.
  • Two systems help with degradation of proteins:
    • Ubiquitin–proteasome system: misfolded proteins are translocated to the cytosol and tagged with ubiquitin, then degraded.
    • Autophagic–lysosomal system: aberrant protein fragments are degraded by lysosomes.

Golgi Apparatus

  • The Golgi apparatus is a series of membranous sacs (cisternae). The Golgi receives proteins and lipids from the ER, modifies them further, and sorts them for eventual destinations (e.g., plasma membrane, lysosomes, secretory vesicles).
  • The Golgi has different sections that perform different tasks.
  • Cis Golgi network: Beginning of modification (sorting and receiving); protein, lipid, polysaccharide modification begins.
  • Golgi stack: Modification continues.
  • Trans Golgi network: Final modifications and sorting for different destinations; protein sorting to different destinations.

Processing in Golgi

  • Enzymes in the Golgi cisternae (in specific steps) perform specific modifications of protein and lipids. This may include: addition/removal of sugars (oligosaccharide processing), phosphorylation, or sulfation.

Transport Mechanisms

  • Endocytosis: Bringing materials into the cell.
  • Exocytosis: Moving materials out of the cell.
  • Transport vesicles: Membrane-enclosed structures used to transport cargo between compartments—COPI, COPII, and clathrin-coated vesicles.

Vesicular transport / Tubular clusters

  • ER-derived COPII vesicles bind to COPI coated transport vesicles and fuse to form Vesicular Tubular clusters, which move on microtubules toward the Golgi apparatus.
  • COPI coated transport vesicles then bud off, carrying resident proteins back to the ER.
  • Vesicular tubular clusters continually mature as they move.

ER Summary

  • ER is part of the endomembrane system and is continuous with the nuclear envelope.
  • It produces proteins and lipids.
  • Rough ER primarily synthesizes secretory or transmembrane proteins.
  • Smooth ER handles diverse metabolic functions (lipid synthesis, carbohydrate metabolism, calcium storage, and detoxification.)
  • Signal sequences and signal recognition particle guide proteins to the ER membrane.
  • Chaperone proteins and enzymes help with protein folding and ensure quality control within the ER lumen.
  • N-linked glycosylation plays a role in protein folding and stability.

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Test your knowledge on cell biology and the functions of the endoplasmic reticulum. This quiz covers key concepts including cholesterol's role in cellular permeability, the function of chaperones, protein synthesis, and the endomembrane system. Perfect for students studying cell biology at any level.

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