MODULE 14

Questions and Answers

What happens to proteins synthesized on free ribosomes that lack sorting signals?

They are localized to the nucleolus.

They are exported from the cell.

They are sent to the rough ER.

They remain in the cytosol. (correct)

What role does the Signal Recognition Particle (SRP) play in protein synthesis?

It stops synthesis and relocates the ribosomal complex to the rER. (correct)

It catalyzes the formation of peptide bonds.

It binds to nucleotides to initiate transcription.

It facilitates the degradation of misfolded proteins.

How do integral membrane proteins differ from secretory proteins during synthesis?

Integral membrane proteins are processed differently in the Golgi apparatus.

Integral membrane proteins are shunted into the lipid bilayer instead of passing through. (correct)

Integral membrane proteins pass entirely through the rER membrane.

Integral membrane proteins are synthesized entirely in the cytosol.

What is the function of protein disulfide isomerase in the rER?

It catalyzes the formation of disulfide bonds in proteins. Signup and view all the answers

Which of the following best describes how oligosaccharides are added to proteins in the rER?

Sugars are added to dolichol-PP before they are flipped to the lumen. Signup and view all the answers

What type of proteins are synthesized on free ribosomes?

Mitochondrial proteins Signup and view all the answers

During the regulation of secretory protein synthesis, what triggers the release of the N-terminal signal sequence by the SRP?

GTP hydrolysis bound to SRP and its receptor. Signup and view all the answers

What characterizes the synthesis of lysosomal enzymes and secreted proteins?

They are synthesized on rER-bound ribosomes. Signup and view all the answers

What determines the final location of a protein synthesized in the cytosol?

The presence of a sorting signal. Signup and view all the answers

Which mechanism of transport is used for moving proteins from the cytosol into the ER?

Transmembrane transport. Signup and view all the answers

What is the primary function of the smooth ER in steroid-secreting cells?

Synthesis of steroid hormones. Signup and view all the answers

Which of the following proteins is synthesized on free ribosomes?

Proteins destined for peroxisomes. Signup and view all the answers

What role does glucose-6-phosphatase play in the liver?

It converts glucose-6-phosphate to glucose. Signup and view all the answers

What type of proteins are formed by ribosomes attached to the rough ER?

Integral membrane proteins. Signup and view all the answers

What facilitates the recognition of sorting signals on proteins?

Sorting receptors that are reutilized. Signup and view all the answers

Which cellular structure is primarily involved in detoxification and glycogen metabolism in liver cells?

Smooth ER. Signup and view all the answers

How do nascent proteins associate with the rough ER during synthesis?

Via an N-terminus signal sequence. Signup and view all the answers

What defines the ER lumen in relation to the cytosol?

It is separated from the cytosol by a membrane. Signup and view all the answers

What type of proteins are produced by ribosomes bound to the rough ER?

Secretory proteins and integral membrane proteins Signup and view all the answers

Which element of the signal recognition particle (SRP) is crucial for its interaction with the receptor?

Binding to GTP Signup and view all the answers

What facilitates the transition of a nascent protein from the translocator into the ER lumen?

Hydrolysis of GTP Signup and view all the answers

What are the three mechanisms of protein transport between compartments?

Gated, vesicular, and transmembrane transport Signup and view all the answers

Which function is primarily carried out by the smooth endoplasmic reticulum?

Synthesis of lipids and steroid hormones Signup and view all the answers

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

Binding to misfolded proteins to prevent aggregation Signup and view all the answers

How are hydrophobic transmembrane segments handled during integral membrane protein synthesis?

They are shunted into the lipid bilayer from the translocon. Signup and view all the answers

Where are proteins destined for secretion synthesized?

Ribosomes attached to the rough ER Signup and view all the answers

What distinguishes the components within the lumen of the ER from those in the cytosol?

They are separated by a membrane. Signup and view all the answers

What role does the smooth ER play in relation to carbohydrates?

It carries out carbohydrate metabolism. Signup and view all the answers

Which type of proteins are primarily synthesized on free ribosomes?

Cytosolic and mitochondrial proteins Signup and view all the answers

What initiates the synthesis of secretory proteins?

Binding of SRP to the ribosome Signup and view all the answers

What type of sorting signal is recognized during protein synthesis at the rough ER?

N-terminal signal sequence Signup and view all the answers

Where are N-linked oligosaccharides first assembled in the secretory pathway?

In the lumen of the rough ER Signup and view all the answers

In what way does the smooth ER contribute to muscle function?

By storing and releasing calcium ions Signup and view all the answers

What do signal sequences and patches do in the context of protein synthesis?

Guide proteins to their appropriate organelles Signup and view all the answers

How does glycogen metabolism occur in liver cells?

Glycogen is broken down by glucose-6-phosphatase. Signup and view all the answers

What are the characteristics of transmembrane transport?

Utilizes membrane-bound translocators to move proteins Signup and view all the answers

What is a defining feature of a signal sequence in proteins?

It is a continuous stretch of amino acids possibly removed after sorting. Signup and view all the answers

Which process involves the movement of proteins directly into the endoplasmic reticulum from the cytosol?

Transmembrane transport Signup and view all the answers

What role does the smooth endoplasmic reticulum primarily serve in muscle cells?

Calcium storage and release Signup and view all the answers

Which component is essential for proteins to be synthesized on ribosomes bound to the rough ER?

Signal sequences Signup and view all the answers

What is the consequence of having proteins synthesized with no sorting signals?

They remain in the cytosol Signup and view all the answers

Which process primarily involves the encapsulation of proteins into vesicles for transport?

Vesicular transport Signup and view all the answers

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

To halt protein synthesis and relocate the ribosomal complex Signup and view all the answers

Which statement best describes the function of the translocon during integral membrane protein synthesis?

It aids in the shunting of hydrophobic segments into the lipid bilayer Signup and view all the answers

What is a key function of the rough endoplasmic reticulum?

Protein folding and modification Signup and view all the answers

What distinguishes the ER lumen from the cytosol?

Separation by membranes Signup and view all the answers

What is the importance of GTP hydrolysis in the regulation of secretory protein synthesis?

It triggers the dissociation of SRP from its receptor and the release of the N-terminal signal Signup and view all the answers

Which occurs to signal sequences once a protein enters the lumen of the rER?

They are cleaved off by signal peptidase Signup and view all the answers

How does the structure of a signal patch differ from a signal sequence?

It is a three-dimensional arrangement on the protein's surface Signup and view all the answers

How does a protein with a hydrophobic transmembrane segment behave during its synthesis?

It is shunted into the lipid bilayer of the ER membrane. Signup and view all the answers

Which of the following organelles is responsible for the synthesis of steroid hormones?

Smooth endoplasmic reticulum Signup and view all the answers

What is the primary outcome of glycogen metabolism in liver cells?

Release of glucose into blood circulation Signup and view all the answers

What role do chaperones play in the rough ER?

They assist in the correct folding of newly synthesized proteins. Signup and view all the answers

What is the sequence for adding N-linked oligosaccharides to proteins in the rER?

Oligosaccharides are pre-assembled on dolichol-PP before transfer. Signup and view all the answers

Which aspect distinguishes secretory proteins from integral membrane proteins regarding their synthesis?

Integral membrane proteins do not enter the ER lumen. Signup and view all the answers

Which proteins are synthesized on free ribosomes and remain in the cytosol?

Cytosolic, mitochondrial, nuclear, and peroxisomal proteins Signup and view all the answers

What function do sorting receptors serve in the context of protein synthesis?

They deliver proteins to their appropriate organelles based on sorting signals. Signup and view all the answers

What happens to proteins synthesized on ribosomes bound to the rough endoplasmic reticulum (rER) that do not have a localization signal other than an N-terminal signal?

They are secreted from the cell. Signup and view all the answers

Which of the following components is responsible for recognizing the N-terminal signal during secretory protein synthesis?

Signal recognition particle (SRP) Signup and view all the answers

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

Integral membrane proteins Signup and view all the answers

What is the role of protein disulfide isomerase in the rough endoplasmic reticulum?

It catalyzes the formation of disulfide bonds. Signup and view all the answers

During the regulation of secretory protein synthesis, what condition allows the SRP and its receptor to interact?

Both are bound to GTP Signup and view all the answers

How are N-linked oligosaccharides added to proteins in the rough ER?

To an asparagine residue in the sequence N-X-S/T Signup and view all the answers

What distinguishes integral membrane proteins during their synthesis compared to secretory proteins?

They have hydrophobic transmembrane segments. Signup and view all the answers

What is the function of chaperone proteins in the rough endoplasmic reticulum?

To promote protein folding and prevent aggregation Signup and view all the answers

Which of the following accurately describes the role of the smooth endoplasmic reticulum?

It detoxifies metabolites and stores calcium ions. Signup and view all the answers

Which statement best describes the function of sorting receptors during protein synthesis?

They deliver proteins to their correct organelles. Signup and view all the answers

What characterizes gated transport in protein movement between compartments?

It occurs between the cytosol and the nucleus. Signup and view all the answers

Which statement best defines the role of the smooth endoplasmic reticulum (sER)?

It is responsible for lipid synthesis and calcium storage in muscles. Signup and view all the answers

What is the consequence of proteins synthesized on ribosomes attached to the rough ER?

They may be embedded in membranes or sent to the lysosomes. Signup and view all the answers

In which compartment does protein folding and quality control primarily occur?

Rough endoplasmic reticulum Signup and view all the answers

What is the main function of glucose-6-phosphatase in hepatocytes?

It converts glucose-6-phosphate to glucose for export to the blood. Signup and view all the answers

What distinguishes the lumen of the endoplasmic reticulum from the cytosol?

They are topologically equivalent but separated by membranes. Signup and view all the answers

What components are necessary for the assembly of N-linked oligosaccharides during protein synthesis?

Diverse sugar nucleotides and Golgi apparatus enzymes Signup and view all the answers

What process allows for proteins to be recognized and sorted during synthesis?

Signal sequences and patches on proteins Signup and view all the answers

Which of the following is NOT a function of the rough endoplasmic reticulum?

Synthesis of lipids Signup and view all the answers

Which type of ribosomes synthesizes proteins found in peroxisomes and mitochondria?

Free ribosomes in the cytosol Signup and view all the answers

Study Notes

Protein Movement Between Compartments

Proteins are synthesized on free ribosomes in the cytosol or on ribosomes attached to the endoplasmic reticulum (ER).
Proteins without sorting signals remain in the cytosol; those with signals are directed elsewhere.
Transport mechanisms include:
- Gated transport between the cytosol and nucleus.
- Transmembrane transport through membrane-bound translocators into distinct compartments.
- Vesicular transport, which involves vesicles moving proteins between compartments.

Endoplasmic Reticulum (ER)

Comprises smooth ER (sER) and rough ER (rER), organized in a network of tubules and sacs, with the rough ER having ribosomes attached.
Proteins are either co-translationally translocated, embedded, or fully translocated into the ER lumen, fated for various destinations (ER, Golgi, lysosomes, plasma membrane, secretion).
Polyribosomes form when multiple ribosomes bind to a single mRNA, allowing simultaneous synthesis of proteins.
The ER lumen and extracellular environment are topologically distinct from the cytosol but equivalent to each other.

Functions of the Smooth ER (sER)

Lacks ribosomes and is primarily involved in lipid synthesis.
Synthesizes steroid hormones, including cortisol, testosterone, and estrogens, in steroid-secreting cells.
Oxidizes long-chain fatty acids and plays a role in detoxification processes, especially in liver cells.
Stores and releases calcium ions in muscle cells, triggering muscle contraction.

Carbohydrate Metabolism in sER

Glycogen serves as a multi-branched glucose polymer, stored primarily in liver and muscle cells.
Synthesis is stimulated by insulin; breakdown occurs in response to glucagon.
In hepatocytes, glucose-6-phosphatase in sER membranes converts glucose-6-phosphate into glucose, allowing it to enter the bloodstream through GLUT2 transporters.

Functions of the Rough ER (rER)

Responsible for synthesizing secretory proteins, lysosomal proteins, integral membrane proteins, and resident proteins for the ER and Golgi.
Involved in the folding of proteins and quality control, removing misfolded proteins through ER-associated degradation.
Assemblies of multimeric proteins and performs co-translational modifications such as the formation of disulfide bonds and glycosylation.

Protein Synthesis on Membrane-bound Versus Free Ribosomes

Membrane-bound ribosomes on the rER synthesize proteins targeted for secretion, integral membranes, and the endomembrane system.
Free ribosomes produce proteins destined for the cytosol, peripheral membrane proteins, and proteins for peroxisomes, mitochondria, or the nucleus.

Signal Sequences and Patches

A signal sequence is a continuous stretch of amino acids, often removed post-sorting, while a signal patch involves a three-dimensional arrangement on protein surfaces.
Sorting receptors recognize these signals, facilitating protein transport to appropriate organelles.
Proteins synthesized on rER typically have an N-terminal signal sequence.

Secretory Proteins

Initial polypeptide translation includes a destination signal within the first ~60 amino acids.
This signal is identified by a signal recognition particle (SRP), halting synthesis and directing the ribosomal complex to the rER membrane.
The ribosome aligns with a translocator, resuming synthesis and guiding the peptide into the ER lumen where the signal is cleaved.

Regulation of Secretory Protein Synthesis

The SRP and its receptor function as G proteins with GTPase activity, binding and hydrolyzing GTP.
Interactions between SRP and its receptor occur only when both are GTP-bound, leading to signal sequence release and translocator engagement.

Integral Membrane Protein Synthesis

Translocating integral membrane proteins partially embed in the ER membrane without complete passage.
These proteins possess hydrophobic segments that are funneled into the lipid bilayer through a lateral gate in the translocon.

Protein Processing in the rER

Signal peptides at the N-terminus are removed by signal peptidase; unfolded proteins are assisted by chaperones.
Protein disulfide isomerase forms disulfide bonds, while transferases add N-linked oligosaccharides to proteins.
Initial sugar units are added in the cytosol before being flipped into the lumen for further modification and transfer to specific asparagine residues.

Summary of Functions in Protein Sorting

Distinct proteins localize to specific organelles, governed by sorting signals recognized by sorting receptors.
Proteins lacking sorting signals remain in the cytosol, synthesized on free ribosomes.
The sER is involved in lipid synthesis, carbohydrate metabolism, and detoxification, while the rER produces various specialized proteins and conducts N-glycosylation.
Proteins for cytosolic, mitochondrial, nuclear, and peroxisomal use are synthesized on free ribosomes.

Protein Movement Between Compartments

Proteins are synthesized on free ribosomes in the cytosol or on ribosomes attached to the endoplasmic reticulum (ER).
Proteins without sorting signals remain in the cytosol; those with signals are directed elsewhere.
Transport mechanisms include:
- Gated transport between the cytosol and nucleus.
- Transmembrane transport through membrane-bound translocators into distinct compartments.
- Vesicular transport, which involves vesicles moving proteins between compartments.

Endoplasmic Reticulum (ER)

Comprises smooth ER (sER) and rough ER (rER), organized in a network of tubules and sacs, with the rough ER having ribosomes attached.
Proteins are either co-translationally translocated, embedded, or fully translocated into the ER lumen, fated for various destinations (ER, Golgi, lysosomes, plasma membrane, secretion).
Polyribosomes form when multiple ribosomes bind to a single mRNA, allowing simultaneous synthesis of proteins.
The ER lumen and extracellular environment are topologically distinct from the cytosol but equivalent to each other.

Functions of the Smooth ER (sER)

Lacks ribosomes and is primarily involved in lipid synthesis.
Synthesizes steroid hormones, including cortisol, testosterone, and estrogens, in steroid-secreting cells.
Oxidizes long-chain fatty acids and plays a role in detoxification processes, especially in liver cells.
Stores and releases calcium ions in muscle cells, triggering muscle contraction.

Carbohydrate Metabolism in sER

Glycogen serves as a multi-branched glucose polymer, stored primarily in liver and muscle cells.
Synthesis is stimulated by insulin; breakdown occurs in response to glucagon.
In hepatocytes, glucose-6-phosphatase in sER membranes converts glucose-6-phosphate into glucose, allowing it to enter the bloodstream through GLUT2 transporters.

Functions of the Rough ER (rER)

Responsible for synthesizing secretory proteins, lysosomal proteins, integral membrane proteins, and resident proteins for the ER and Golgi.
Involved in the folding of proteins and quality control, removing misfolded proteins through ER-associated degradation.
Assemblies of multimeric proteins and performs co-translational modifications such as the formation of disulfide bonds and glycosylation.

Protein Synthesis on Membrane-bound Versus Free Ribosomes

Membrane-bound ribosomes on the rER synthesize proteins targeted for secretion, integral membranes, and the endomembrane system.
Free ribosomes produce proteins destined for the cytosol, peripheral membrane proteins, and proteins for peroxisomes, mitochondria, or the nucleus.

Signal Sequences and Patches

A signal sequence is a continuous stretch of amino acids, often removed post-sorting, while a signal patch involves a three-dimensional arrangement on protein surfaces.
Sorting receptors recognize these signals, facilitating protein transport to appropriate organelles.
Proteins synthesized on rER typically have an N-terminal signal sequence.

Secretory Proteins

Initial polypeptide translation includes a destination signal within the first ~60 amino acids.
This signal is identified by a signal recognition particle (SRP), halting synthesis and directing the ribosomal complex to the rER membrane.
The ribosome aligns with a translocator, resuming synthesis and guiding the peptide into the ER lumen where the signal is cleaved.

Regulation of Secretory Protein Synthesis

The SRP and its receptor function as G proteins with GTPase activity, binding and hydrolyzing GTP.
Interactions between SRP and its receptor occur only when both are GTP-bound, leading to signal sequence release and translocator engagement.

Integral Membrane Protein Synthesis

Translocating integral membrane proteins partially embed in the ER membrane without complete passage.
These proteins possess hydrophobic segments that are funneled into the lipid bilayer through a lateral gate in the translocon.

Protein Processing in the rER

Signal peptides at the N-terminus are removed by signal peptidase; unfolded proteins are assisted by chaperones.
Protein disulfide isomerase forms disulfide bonds, while transferases add N-linked oligosaccharides to proteins.
Initial sugar units are added in the cytosol before being flipped into the lumen for further modification and transfer to specific asparagine residues.

Summary of Functions in Protein Sorting

Distinct proteins localize to specific organelles, governed by sorting signals recognized by sorting receptors.
Proteins lacking sorting signals remain in the cytosol, synthesized on free ribosomes.
The sER is involved in lipid synthesis, carbohydrate metabolism, and detoxification, while the rER produces various specialized proteins and conducts N-glycosylation.
Proteins for cytosolic, mitochondrial, nuclear, and peroxisomal use are synthesized on free ribosomes.

Protein Movement Between Compartments

Proteins are synthesized on free ribosomes in the cytosol or on ribosomes attached to the endoplasmic reticulum (ER).
Proteins without sorting signals remain in the cytosol; those with signals are directed elsewhere.
Transport mechanisms include:
- Gated transport between the cytosol and nucleus.
- Transmembrane transport through membrane-bound translocators into distinct compartments.
- Vesicular transport, which involves vesicles moving proteins between compartments.

Endoplasmic Reticulum (ER)

Comprises smooth ER (sER) and rough ER (rER), organized in a network of tubules and sacs, with the rough ER having ribosomes attached.
Proteins are either co-translationally translocated, embedded, or fully translocated into the ER lumen, fated for various destinations (ER, Golgi, lysosomes, plasma membrane, secretion).
Polyribosomes form when multiple ribosomes bind to a single mRNA, allowing simultaneous synthesis of proteins.
The ER lumen and extracellular environment are topologically distinct from the cytosol but equivalent to each other.

Functions of the Smooth ER (sER)

Lacks ribosomes and is primarily involved in lipid synthesis.
Synthesizes steroid hormones, including cortisol, testosterone, and estrogens, in steroid-secreting cells.
Oxidizes long-chain fatty acids and plays a role in detoxification processes, especially in liver cells.
Stores and releases calcium ions in muscle cells, triggering muscle contraction.

Carbohydrate Metabolism in sER

Glycogen serves as a multi-branched glucose polymer, stored primarily in liver and muscle cells.
Synthesis is stimulated by insulin; breakdown occurs in response to glucagon.
In hepatocytes, glucose-6-phosphatase in sER membranes converts glucose-6-phosphate into glucose, allowing it to enter the bloodstream through GLUT2 transporters.

Functions of the Rough ER (rER)

Responsible for synthesizing secretory proteins, lysosomal proteins, integral membrane proteins, and resident proteins for the ER and Golgi.
Involved in the folding of proteins and quality control, removing misfolded proteins through ER-associated degradation.
Assemblies of multimeric proteins and performs co-translational modifications such as the formation of disulfide bonds and glycosylation.

Protein Synthesis on Membrane-bound Versus Free Ribosomes

Membrane-bound ribosomes on the rER synthesize proteins targeted for secretion, integral membranes, and the endomembrane system.
Free ribosomes produce proteins destined for the cytosol, peripheral membrane proteins, and proteins for peroxisomes, mitochondria, or the nucleus.

Signal Sequences and Patches

A signal sequence is a continuous stretch of amino acids, often removed post-sorting, while a signal patch involves a three-dimensional arrangement on protein surfaces.
Sorting receptors recognize these signals, facilitating protein transport to appropriate organelles.
Proteins synthesized on rER typically have an N-terminal signal sequence.

Secretory Proteins

Initial polypeptide translation includes a destination signal within the first ~60 amino acids.
This signal is identified by a signal recognition particle (SRP), halting synthesis and directing the ribosomal complex to the rER membrane.
The ribosome aligns with a translocator, resuming synthesis and guiding the peptide into the ER lumen where the signal is cleaved.

Regulation of Secretory Protein Synthesis

The SRP and its receptor function as G proteins with GTPase activity, binding and hydrolyzing GTP.
Interactions between SRP and its receptor occur only when both are GTP-bound, leading to signal sequence release and translocator engagement.

Integral Membrane Protein Synthesis

Translocating integral membrane proteins partially embed in the ER membrane without complete passage.
These proteins possess hydrophobic segments that are funneled into the lipid bilayer through a lateral gate in the translocon.

Protein Processing in the rER

Signal peptides at the N-terminus are removed by signal peptidase; unfolded proteins are assisted by chaperones.
Protein disulfide isomerase forms disulfide bonds, while transferases add N-linked oligosaccharides to proteins.
Initial sugar units are added in the cytosol before being flipped into the lumen for further modification and transfer to specific asparagine residues.

Summary of Functions in Protein Sorting

Distinct proteins localize to specific organelles, governed by sorting signals recognized by sorting receptors.
Proteins lacking sorting signals remain in the cytosol, synthesized on free ribosomes.
The sER is involved in lipid synthesis, carbohydrate metabolism, and detoxification, while the rER produces various specialized proteins and conducts N-glycosylation.
Proteins for cytosolic, mitochondrial, nuclear, and peroxisomal use are synthesized on free ribosomes.

Protein Movement Between Compartments

Proteins are synthesized on free ribosomes in the cytosol or on ribosomes attached to the endoplasmic reticulum (ER).
Proteins without sorting signals remain in the cytosol; those with signals are directed elsewhere.
Transport mechanisms include:
- Gated transport between the cytosol and nucleus.
- Transmembrane transport through membrane-bound translocators into distinct compartments.
- Vesicular transport, which involves vesicles moving proteins between compartments.

Endoplasmic Reticulum (ER)

Comprises smooth ER (sER) and rough ER (rER), organized in a network of tubules and sacs, with the rough ER having ribosomes attached.
Proteins are either co-translationally translocated, embedded, or fully translocated into the ER lumen, fated for various destinations (ER, Golgi, lysosomes, plasma membrane, secretion).
Polyribosomes form when multiple ribosomes bind to a single mRNA, allowing simultaneous synthesis of proteins.
The ER lumen and extracellular environment are topologically distinct from the cytosol but equivalent to each other.

Functions of the Smooth ER (sER)

Lacks ribosomes and is primarily involved in lipid synthesis.
Synthesizes steroid hormones, including cortisol, testosterone, and estrogens, in steroid-secreting cells.
Oxidizes long-chain fatty acids and plays a role in detoxification processes, especially in liver cells.
Stores and releases calcium ions in muscle cells, triggering muscle contraction.

Carbohydrate Metabolism in sER

Glycogen serves as a multi-branched glucose polymer, stored primarily in liver and muscle cells.
Synthesis is stimulated by insulin; breakdown occurs in response to glucagon.
In hepatocytes, glucose-6-phosphatase in sER membranes converts glucose-6-phosphate into glucose, allowing it to enter the bloodstream through GLUT2 transporters.

Functions of the Rough ER (rER)

Responsible for synthesizing secretory proteins, lysosomal proteins, integral membrane proteins, and resident proteins for the ER and Golgi.
Involved in the folding of proteins and quality control, removing misfolded proteins through ER-associated degradation.
Assemblies of multimeric proteins and performs co-translational modifications such as the formation of disulfide bonds and glycosylation.

Protein Synthesis on Membrane-bound Versus Free Ribosomes

Membrane-bound ribosomes on the rER synthesize proteins targeted for secretion, integral membranes, and the endomembrane system.
Free ribosomes produce proteins destined for the cytosol, peripheral membrane proteins, and proteins for peroxisomes, mitochondria, or the nucleus.

Signal Sequences and Patches

A signal sequence is a continuous stretch of amino acids, often removed post-sorting, while a signal patch involves a three-dimensional arrangement on protein surfaces.
Sorting receptors recognize these signals, facilitating protein transport to appropriate organelles.
Proteins synthesized on rER typically have an N-terminal signal sequence.

Secretory Proteins

Initial polypeptide translation includes a destination signal within the first ~60 amino acids.
This signal is identified by a signal recognition particle (SRP), halting synthesis and directing the ribosomal complex to the rER membrane.
The ribosome aligns with a translocator, resuming synthesis and guiding the peptide into the ER lumen where the signal is cleaved.

Regulation of Secretory Protein Synthesis

The SRP and its receptor function as G proteins with GTPase activity, binding and hydrolyzing GTP.
Interactions between SRP and its receptor occur only when both are GTP-bound, leading to signal sequence release and translocator engagement.

Integral Membrane Protein Synthesis

Translocating integral membrane proteins partially embed in the ER membrane without complete passage.
These proteins possess hydrophobic segments that are funneled into the lipid bilayer through a lateral gate in the translocon.

Protein Processing in the rER

Signal peptides at the N-terminus are removed by signal peptidase; unfolded proteins are assisted by chaperones.
Protein disulfide isomerase forms disulfide bonds, while transferases add N-linked oligosaccharides to proteins.
Initial sugar units are added in the cytosol before being flipped into the lumen for further modification and transfer to specific asparagine residues.

Summary of Functions in Protein Sorting

Distinct proteins localize to specific organelles, governed by sorting signals recognized by sorting receptors.
Proteins lacking sorting signals remain in the cytosol, synthesized on free ribosomes.
The sER is involved in lipid synthesis, carbohydrate metabolism, and detoxification, while the rER produces various specialized proteins and conducts N-glycosylation.
Proteins for cytosolic, mitochondrial, nuclear, and peroxisomal use are synthesized on free ribosomes.

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MODULE 14