Protein Synthesis and Sorting Quiz

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Questions and Answers

What is the primary driving force behind the lateral movement of lipids in continuous membrane structures?

  • ATP dependent processes (correct)
  • Intermembrane channels
  • Passive diffusion across lipid bilayers
  • Hydrolysis of proteins

Which compartment is mentioned as a destination for lipids transported via vesicles?

  • Ribosomes
  • Golgi apparatus (correct)
  • Nuclear envelope
  • Lysosomes

Which structure is specifically identified for synthesizing lipids prior to their transport?

  • Plasma membrane
  • Mitochondria
  • Smooth Endoplasmic Reticulum (SER) (correct)
  • Rough Endoplasmic Reticulum (RER)

What process facilitates the transportation of lipids to organelles beyond the endoplasmic reticulum?

<p>Vesicular transport (A)</p> Signup and view all the answers

Which characteristic of the membranes allows for lateral lipid mobility?

<p>Physical connection between membranes (D)</p> Signup and view all the answers

What is the primary role of phospholipid exchange proteins in cellular transport?

<p>They facilitate lipid transport to organelles not connected to vesicular systems. (B)</p> Signup and view all the answers

Which organelles are specifically mentioned as recipients of lipids transported by phospholipid exchange proteins?

<p>Mitochondria and chloroplasts. (A)</p> Signup and view all the answers

What secondary role does the mentioned calcium ATPase play in muscle cells?

<p>It stores calcium ions (Ca²⁺). (A)</p> Signup and view all the answers

Which of the following processes is NOT directly associated with phospholipid exchange proteins?

<p>Protein synthesis. (A)</p> Signup and view all the answers

What type of modification occurs to toxins for their excretion?

<p>Hydroxylation. (C)</p> Signup and view all the answers

What type of peptide do proteins destined for the mitochondria use for targeting?

<p>N-terminal signal peptide (D)</p> Signup and view all the answers

Which transmembrane protein type is characterized by having positively charged residues following the hydrophobic segment?

<p>Type III (B)</p> Signup and view all the answers

How do proteins targeting the nucleus enter the nucleus?

<p>Transport through nuclear pores (A)</p> Signup and view all the answers

In which type of transmembrane protein is the N-terminal signal peptide followed by a stop-transfer sequence?

<p>Type I (B)</p> Signup and view all the answers

What primarily determines the orientation of Type II transmembrane proteins in the membrane?

<p>Distribution of positively charged residues (B)</p> Signup and view all the answers

What is true regarding the amino acid composition of Type I transmembrane proteins?

<p>They have a stop-transfer peptide and may contain charged residues. (D)</p> Signup and view all the answers

Which of the following is NOT typical for Type III transmembrane proteins?

<p>They have negatively charged residues. (C)</p> Signup and view all the answers

Which type of peptide primarily directs proteins to the Rough Endoplasmic Reticulum?

<p>N-terminal signal peptide (C)</p> Signup and view all the answers

What is the main purpose of utilizing a pulse-chase experiment in studying vesicular transport?

<p>To track the movement of radioactively labeled proteins (B)</p> Signup and view all the answers

In the context of vesicular transport, how does the polarity of vesicles relate to the endoplasmic reticulum (ER)?

<p>Vesicles maintain a similar polarity to the ER (D)</p> Signup and view all the answers

What key aspect of cellular function can be studied through vesicular transport research?

<p>Pathways of secretion from the ER (B)</p> Signup and view all the answers

Why is the study of lipid transport relevant when discussing the ER?

<p>Lipid transport is essential for maintaining ER structure and function (A)</p> Signup and view all the answers

What experimental technique is highlighted for revealing the pathway of secretion in cells?

<p>Pulse-chase experiment (A)</p> Signup and view all the answers

What can be inferred about the functionality of vesicles in relation to the ER?

<p>Vesicles serve a similar functional purpose as the ER (B)</p> Signup and view all the answers

What is a primary focus of studying the transport of labeled proteins using pulse-chase experiments?

<p>Mapping the intracellular pathways of protein secretion (A)</p> Signup and view all the answers

Which of the following best describes the relationship between vesicular transport and cellular function?

<p>Vesicular transport is essential for maintaining homeostasis (D)</p> Signup and view all the answers

What role do pulse-chase experiments play in understanding the synthesis of proteins within the ER?

<p>They map the pathways of protein secretion over time (C)</p> Signup and view all the answers

What occurs immediately after the SRP is released during protein translocation?

<p>The ribosome attaches to the translocon. (A)</p> Signup and view all the answers

Which process involves the movement of proteins between organelles?

<p>Vesicular transport (B)</p> Signup and view all the answers

What is the primary destination for proteins moving in an anterograde manner?

<p>From the ER to the Golgi (A)</p> Signup and view all the answers

What is the result of translation resuming after the ribosome attaches to the translocon?

<p>The protein is threaded into the ER lumen. (B)</p> Signup and view all the answers

Which of the following is incorrect about post-translational translocation?

<p>It happens in the cytoplasm before the protein reaches the ER. (C)</p> Signup and view all the answers

Which component plays a crucial role in guiding the ribosome to the translocon during protein synthesis?

<p>Signal Recognition Particle (SRP) (A)</p> Signup and view all the answers

How do start-transfer and stop-transfer signals function in protein translocation?

<p>They indicate where the polypeptide chain should enter or exit the translocon. (D)</p> Signup and view all the answers

What primarily occurs during the co-translational translocation mechanism?

<p>Protein synthesis and translocation occur simultaneously. (C)</p> Signup and view all the answers

Which statement best describes vesicular transport?

<p>It facilitates transport using vesicles between various organelles. (C)</p> Signup and view all the answers

What is the primary role of intracellular transfer sequences for proteins?

<p>To guide proteins to specific compartments within the cell. (A)</p> Signup and view all the answers

What characteristic do signal peptides possess in relation to soluble proteins?

<p>They are often cleaved by signal peptidase during translocation. (A)</p> Signup and view all the answers

What type of amino acids are found in intracellular transfer sequences?

<p>Hydrophobic amino acids. (A)</p> Signup and view all the answers

Which of the following statements about peroxisomes is accurate?

<p>Peroxisomes are involved in lipid metabolism and detoxification. (C)</p> Signup and view all the answers

What is expected to occur to proteins after the signal peptide is cleaved?

<p>Proteins are completely translocated into the endoplasmic reticulum. (B)</p> Signup and view all the answers

In the context of protein insertion into the membrane, what primarily dictates the final localization of a protein?

<p>The signal peptides and transfer sequences. (A)</p> Signup and view all the answers

What role does signal peptidase play in protein translocation?

<p>It cleaves signal peptides from proteins during transport. (C)</p> Signup and view all the answers

How do the components of the endoplasmic reticulum influence protein sorting?

<p>By dictating which proteins remain inside or are sent to the membrane. (A)</p> Signup and view all the answers

What is the significance of hydrophobic amino acids in the context of protein transport?

<p>They contribute to the anchoring of proteins within membranes. (B)</p> Signup and view all the answers

What would likely happen if the signal peptide were not cleaved from a soluble protein?

<p>The protein would remain in the cytoplasm. (B)</p> Signup and view all the answers

Flashcards

Protein Sorting

The movement of proteins from one compartment to another within a cell.

Protein Translocation

The process by which proteins are transported across biological membranes, such as the endoplasmic reticulum (ER) membrane.

Signal Recognition Particle (SRP)

A protein complex that recognizes and binds to the signal sequence of a protein, initiating its translocation into the ER.

Translocon

A protein channel in the ER membrane that allows the passage of proteins into the ER lumen.

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Signal Sequence

A short stretch of amino acids at the N-terminus of a protein that directs it to the ER.

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Anterograde Transport

The movement of proteins from the ER to the Golgi apparatus and beyond.

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Retrograde Transport

The movement of proteins from the Golgi apparatus back to the ER.

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Vesicles

Small, membrane-bound vesicles that transport proteins between organelles.

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Co-translational Translocation

The process of protein translocation that occurs during protein synthesis.

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Post-translational Translocation

The process of protein translocation that occurs after protein synthesis is complete.

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Pulse-Chase Experiment

A technique used to track the movement of proteins through the cell, particularly from the ER to the Golgi apparatus.

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Secretory Pathway

A series of compartments within eukaryotic cells, including the ER, Golgi apparatus, and vesicles, that work together to synthesize, modify, package, and deliver proteins and lipids.

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Vesicular Transport

The process of moving molecules, like proteins and lipids, between different compartments within a cell, using membrane-bound vesicles.

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Endoplasmic Reticulum (ER)

A network of interconnected membranes that form flattened sacs and tubes in eukaryotic cells.

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Golgi Apparatus

An organelle involved in processing and packaging proteins and lipids for secretion or delivery to other organelles.

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Secretion

The process of producing and releasing molecules, like proteins, from cells.

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Lipid Transport

The distribution and movement of lipids within the cell.

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Polarity

The relative arrangement of molecules within a cell, including the orientation of proteins in a membrane.

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Radioactively Labeled Proteins

Radioactively labeled proteins are used to track their movement through the cell, revealing the pathway of protein secretion.

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Lateral Mobility of Lipids

The ability of lipids to move freely within a continuous membrane structure, such as from the ER to the nuclear envelope.

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Lipid Transport from SER

The process by which lipids synthesized in the Smooth Endoplasmic Reticulum (SER) are transported to other parts of the cell.

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SER and Lipid Transport

The smooth endoplasmic reticulum (SER) is involved in the synthesis and transport of lipids.

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Type I transmembrane protein

A type of transmembrane protein anchored in the membrane with its N-terminus facing the ER lumen and its C-terminus facing the cytosol.

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Type II transmembrane protein

A type of transmembrane protein anchored in the membrane with its C-terminus facing the ER lumen and its N-terminus facing the cytosol.

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Type III transmembrane protein

A type of transmembrane protein similar to Type II, but with positively charged amino acids following the hydrophobic segment.

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Stop-transfer sequence

This sequence stops the protein from being fully translocated through the ER membrane, anchoring it in the membrane.

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Intracellular Transfer Sequences

Short stretches of hydrophobic amino acids within a protein sequence that guide the protein to a specific cellular compartment, such as the ER.

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Signal Peptide

Short stretch of amino acids at the beginning of a protein, acting as a signal for its translocation into the ER.

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Signal Peptidase

An enzyme that removes the signal peptide from a protein after it is translocated into the ER.

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ER Lumen

The space within the ER, separated from the cytosol by the ER membrane.

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Peroxisomes

Organelles responsible for various metabolic functions, including detoxification and lipid metabolism.

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Phospholipid Exchange Proteins

Specialized proteins that move lipids between cellular compartments, like the ER and mitochondria, which are not directly connected by vesicles.

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Molecular Detoxification

The process by which cells modify and detoxify harmful molecules, such as drugs and toxins, often involving enzymes that add hydroxyl groups.

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Steroid Hormone Synthesis (from Cholesterol)

Steroid hormones, like testosterone and estrogen, are synthesized from cholesterol using enzymes located in the smooth endoplasmic reticulum (SER).

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Calcium Storage (Ca²⁺ ATPase in muscle)

The SER plays a crucial role in storing calcium ions, which are essential for muscle contraction and other cellular processes.

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Study Notes

Protein Synthesis and Sorting

  • Proteins are primarily synthesized by ribosomes, either free in the cytosol or bound to the rough endoplasmic reticulum (RER).
  • Co-translational translocation: Proteins destined for the ER have a signal peptide (20-amino acid hydrophobic sequence) at their N-terminus. This signal directs the ribosome-nascent chain to the ER.
  • The signal peptide is recognized by the signal recognition particle (SRP), which pauses translation.
  • The SRP-ribosome complex binds to the SRP receptor on the ER membrane.
  • The SRP is released, and the ribosome attaches to the translocon, allowing the peptide to enter the ER lumen.
  • Translation resumes, and the protein is threaded into the ER lumen.
  • The signal peptide is cleaved by signal peptidase for soluble proteins, fully translocating into the ER.
  • Transmembrane proteins have internal signal peptides, determining their orientation based on positively charged residues.
  • Type I, Type II, Type IV are different types of transmembrane proteins.

Protein Folding and Modifications

  • Inside the ER, proteins undergo several post-translational modifications.
  • N-glycosylation: Addition of oligosaccharides (sugars) to asparagine residues (Asn-X-Ser/Thr).
  • Other modifications include addition of glycolipids (e.g., GPI anchors)
  • Quality control: The ER monitors protein folding. Misfolded or improperly glycosylated proteins are recognized and degraded by the proteasome.
  • Chaperones like BiP are key in this process.

Lipid Synthesis

  • Fatty acids are delivered to the ER by cytosolic fatty acid-binding proteins.
  • These fatty acids are used to synthesize phospholipids like phosphatidylcholine, crucial for membrane function.
  • Enzymes facilitate lipid transfer between membrane layers.
  • Flip-flops actively move phospholipids across the bilayer (ATP-dependent).
  • Lipids move laterally within the membrane.
  • Lipids can be transported via vesicles to other organelles (like the Golgi).
  • Transfer to mitochondria or chloroplasts via phospholipid exchange proteins.

Smooth Endoplasmic Reticulum (SER)

  • SER lacks ribosomes.
  • Functions include steroid hormone synthesis from cholesterol, calcium storage, and detoxification (e.g., hydroxylation of toxins like phenobarbital).

Protein Sorting

  • Proteins have signals (e.g., peptide signals) directing them to their proper compartments (secretory pathway).
  • Translocation (across membranes) and vesicular transport move proteins between compartments.
  • Vesicular transport: Proteins are transported in vesicles (e.g., ER to Golgi).
  • Anterograde transport: ER → Golgi → Plasma membrane.
  • Retrograde transport: Golgi → ER.

Golgi Apparatus

  • The Golgi apparatus further modifies proteins and sorts them for secretion or transport to other organelles.

Pulse-Chase Experiment

  • Demonstrates the secretory pathway in pancreatic cells.
  • Radioactively labeled proteins are tracked through the cell, revealing the pathway (RER → Golgi → secretion).
  • Microsomes (small vesicles from the ER) were used to study ER functions in a lab setting.

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Intracellular Compartments and Protein Sorting PDF

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