Perroteau - L9 part 1

Questions and Answers

What distinguishes ribosomes that are associated with the endoplasmic reticulum from free ribosomes?

Associated ribosomes are responsible for the synthesis of secretory and membrane proteins. (correct)

Free ribosomes synthesize proteins that are only utilized in the nucleus.

Free ribosomes synthesize proteins for lysosomes only.

Associated ribosomes synthesize proteins exclusively for mitochondria.

Which of the following organelles is primarily recognized by its extensive membrane system and the presence of ribosomes on its surface?

Mitochondria

Golgi apparatus

Lysosomes

Rough endoplasmic reticulum (correct)

What recent understanding has changed the perception of peroxisomes in regards to their communication with other organelles?

Peroxisomes were always believed to be completely isolated.

Peroxisomes only communicate using direct contact without vesicles.

Peroxisomes can communicate through vesicles, contrary to previous beliefs. (correct)

Peroxisomes were previously thought to have no evolutionary connection to mitochondria.

Which of these organelles is known for being smaller and more difficult to visualize in cellular structure?

Secretory vesicles Signup and view all the answers

Which of the following accurately describes the primary role of the Golgi apparatus?

It plays a critical role in modifying, sorting, and packaging proteins. Signup and view all the answers

What is a key characteristic of the endomembrane system in relation to intracellular communication?

It provides a framework for organized communication among organelles. Signup and view all the answers

Which of these organelles is primarily involved in the degradation and recycling of cellular waste?

Lysosomes Signup and view all the answers

How does the structure of the rough endoplasmic reticulum aid in its function?

The ribosomes on its surface enable protein synthesis for cellular compartments. Signup and view all the answers

What is the primary role of kinases in protein modification?

Transfer phosphate groups from ATP to proteins. Signup and view all the answers

Which amino acid is specifically linked to acylation in lipid modification?

Cysteine Signup and view all the answers

What distinguishes covalent bonding, such as phosphorylation, from hydrogen bonding?

Hydrogen bonds can be easily disrupted by temperature changes. Signup and view all the answers

What effect does phosphorylation typically have on proteins?

It alters protein activity and stability. Signup and view all the answers

What role does the enzyme palmitoyl acyltransferase play in protein modification?

It transfers fatty acids to soluble proteins, making them integral. Signup and view all the answers

What is the primary role of peroxisomes in the cell?

Detoxification processes and consuming oxygen Signup and view all the answers

How are proteins destined for the endoplasmic reticulum synthesized?

By membrane-bound ribosomes on the rough endoplasmic reticulum Signup and view all the answers

Which proteins are modified through lipidation to become integral membrane proteins?

Proteins synthesized by free ribosomes in the cytosol Signup and view all the answers

What characterizes the rough endoplasmic reticulum compared to the smooth endoplasmic reticulum?

Rough ER is involved in protein synthesis and has ribosomes attached Signup and view all the answers

What determines the number of mitochondria in a cell?

The cell's energy requirements Signup and view all the answers

What is the primary function of the endoplasmic reticulum?

Synthesis of lipids and proteins Signup and view all the answers

Where do proteins destined for the mitochondria typically end up after synthesis?

In the matrix or intermembrane space of the mitochondria Signup and view all the answers

What initiates the targeting of proteins to their final destinations in the cell?

Signal peptides recognized by particles Signup and view all the answers

Which statement best describes the relationship between free ribosomes and the endomembrane system?

Proteins from free ribosomes can become integral membrane proteins within the endomembrane system. Signup and view all the answers

What results from endocytosis in the context of lysosomes and endosomes?

Internalization of extracellular materials Signup and view all the answers

What initiates the binding of the signal recognition particle (SRP) to a protein being synthesized?

The presence of the signal peptide Signup and view all the answers

What is the primary role of the SRP receptors located on the endoplasmic reticulum (ER)?

To interact with SRP and facilitate ribosome release Signup and view all the answers

What happens to the signal peptide during the protein synthesis process?

It is cleavage from the protein after entering the ER Signup and view all the answers

How do vesicles from the Golgi apparatus target specific proteins for secretion or transport back to the ER?

Through protein-protein interactions and specific domains Signup and view all the answers

What role do motor proteins play in the transportation of vesicles?

They enable vesicles to move along cytoskeletal filaments Signup and view all the answers

What distinguishes a protein destined for the lysosome from one destined for secretion?

The presence of a specific signal for lysosome targeting Signup and view all the answers

What happens to proteins after they have been fully processed in the Golgi apparatus?

They may be sent to various destinations including the extracellular environment Signup and view all the answers

Which structure is critical for the translocation of proteins across the ER membrane?

The translocon Signup and view all the answers

What post-translational modifications are primarily conducted in the Golgi apparatus?

N-glycosylation and phosphorylation Signup and view all the answers

Which motor proteins would typically be responsible for transporting vesicles towards the plasma membrane?

Kinesins Signup and view all the answers

What is the primary role of NLS (nuclear localisation sequence) in protein sorting?

It imports proteins into the nucleus. Signup and view all the answers

Which modification is specifically related to the process of increasing protein diversity through cleavage?

Proteolytic activation Signup and view all the answers

What distinguishes chaperonins from chaperons in protein folding?

Chaperonins facilitate proper folding and covalent binding. Signup and view all the answers

What are proteins synthesized by free ribosomes typically targeted to?

Endoplasmic reticulum and various organelles Signup and view all the answers

How is the signal peptide for import into the endoplasmic reticulum identified?

By the presence of a methionine in the N-terminal region. Signup and view all the answers

What is the primary function of the SRP (signal recognition particle) in protein synthesis?

To recognize and direct nascent polypeptides to the ER. Signup and view all the answers

Which post-translational modification can signal protein degradation?

Ubiquitination Signup and view all the answers

In gel electrophoresis, which factor primarily affects the movement of proteins within the gel?

The molecular weight of the proteins Signup and view all the answers

Which component's organization ultimately determines the localization of NES (nuclear export sequence)?

The three-dimensional structure of the protein Signup and view all the answers

Which of the following is a primary function of chaperons during protein synthesis?

Preventing premature folding of nascent peptides Signup and view all the answers

Study Notes

Intracellular Compartments and Protein Sorting

Cells contain various intracellular compartments, including the nucleus, mitochondria, peroxisomes, endoplasmic reticulum (ER), Golgi apparatus, and lysosomes.
Ribosomes can be free in the cytosol or bound to the ER; this affects the destination of synthesized proteins.
The ER is a single organelle comprising rough (with ribosomes) and smooth regions, responsible for lipid and protein synthesis.
Mitochondria generate energy and their quantity is regulated based on cellular energy needs; they also consume oxygen, similar to peroxisomes, which detoxify harmful substances.
Lysosomes form from endosomes and contain enzymes for degradation of cellular material.

Protein Synthesis and Post-Translational Modifications

Proteins synthesized by free ribosomes may either remain cytosolic, interact with organelle membranes, or become integral proteins through lipidation.
Free ribosome-synthesized proteins complexly target their final destinations post-translation, being directed to mitochondria, peroxisomes, or remain cytosolic.
Proteins intended for the secretory pathway first enter the ER, potentially passing through the Golgi apparatus before reaching destinations such as the plasma membrane or lysosomes.

SRP and Co-Translation Mechanism

Signal Recognition Particle (SRP) recognizes the N-terminal signal peptide of proteins destined for the ER, temporarily halting translation until the ribosome associates with the ER.
The SRP receptor and translocon on the ER facilitate the continuation of translation and insertion of the growing polypeptide into the ER lumen.
After translation, proteins may undergo modification in the Golgi apparatus, determining whether they are secreted, returned to the ER, or directed to lysosomes.

Protein Sorting

Protein localization relies on specific sequences encoded within their primary structure, such as Nuclear Localization Sequences (NLS) and Nuclear Export Sequences (NES).
Mitochondrial, plastid (chloroplast), peroxisome, and ER targeting sequences direct proteins to their respective organelles primarily located at the N-terminus.
Gunter Blobel's research highlighted the importance of these sequences in protein targeting, ensuring accurate cellular function.

Post-Translational Modifications

Proteins undergo various modifications after translation, including folding, proteolytic activation (cleavage), phosphorylation, lipid modifications, acetylation, and ubiquitination.
Proper protein folding is critical for functionality; chaperones and chaperonins assist in this folding process within the cytosol or ER lumen.
Misfolded proteins are targeted for degradation to maintain cellular integrity.

Electrophoresis and Protein Analysis

Proteins can be separated by molecular weight using electrophoresis; an electric field causes proteins to migrate through a gel matrix.
Larger proteins experience hindrance in the gel, resulting in slower migration compared to smaller proteins, enabling analysis of protein size and abundance.

Communication Between Organelles

Organelles such as peroxisomes communicate with the ER and mitochondrial systems using vesicular transport, contributing to cellular homeostasis.
Sorting mechanisms ensure that proteins are dispatched correctly to maintain specialized functions in different cellular compartments.### Protein Cleavage and Molecular Weight
Observations show a precursor protein at approximately 60 kDa and a shorter protein at about 40 kDa.
The smaller protein is generated by cleaving a fragment of about 15 kDa from the precursor.
Treatment leading to decreased levels of the longer protein form is indicated by a lighter dark spot on the visual representation.

Role of Phosphorylation

Phosphorylation alters protein stability and activity; it is a reversible process.
Key amino acids involved in phosphorylation are serine, threonine, and tyrosine; important for understanding disease mechanisms and regulatory pathways.
Kinases are enzymes that phosphorylate proteins using ATP, transferring a phosphate group and releasing ADP.
Phosphatases reverse phosphorylation by removing phosphate groups, leading to inactivation in some proteins.
While phosphorylation generally activates proteins, it is not exclusively so—some proteins can only be activated via dephosphorylation.

Cytoplasmic Lipid Modification

Lipid modification can be significant in disease contexts, acting on proteins after translation.
Proteins can become integral membrane proteins through lipid tail attachment via esterification with fatty acids.
Acylation involves cysteine, while prenylation involves glycine for protein modification.
The modification process is reversible, facilitated by specific enzymes that can attach or remove these hydrophobic tails.

Cleavage and Reversibility

Unlike reversible modifications, cleavage of proteins is an irreversible process.
Covalent binding, such as phosphorylation, requires enzymes and is energy-dependent, contrasting with hydrogen bonds that can easily destabilize via temperature or pH changes.

Palmitoylation and Depalmitoylation Cycle

Palmitoylation involves a transferase enzyme (DHHC family) that covalently binds palmitic acid (a 16-carbon fatty acid) to cysteine in proteins.
Depalmitoylation is the reverse process, re-separating the protein and fatty acid, facilitating membrane association or disassociation.

Localization Changes of Proteins

Soluble proteins can transition to integral membrane proteins through covalent lipid binding, showcasing dynamic cellular localization.

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Description

This quiz covers Unit 2 of cell biology, focusing on intracellular compartments and protein sorting mechanisms. We will explore the differences between proteins synthesized by free ribosomes and those directed to various organelles like the nucleus and mitochondria. Test your understanding of these essential biological processes.