Perroteau - L9 part 2

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Questions and Answers

Which protein undergoes lipidation and is not a transmembrane protein?

APT (correct)

Cysteine

DHHC

Transcription factor

How many isoforms of DHHC proteins have been identified in mammals?

23 (correct)

What is the primary site of synthesis for the DHHC protein?

Nucleus

Endoplasmic Reticulum (ER) (correct)

Golgi apparatus

Ribosomes

What is the main enzymatic activity associated with DHHC proteins?

Lipidation Signup and view all the answers

Where is APT localized after lipidation to perform its function?

Membrane Signup and view all the answers

What distinguishes APT from DHHC regarding their structure?

APT is synthesized by free ribosomes. Signup and view all the answers

Which cellular compartments are DHHC proteins primarily associated with?

ER, Golgi, and plasma membranes Signup and view all the answers

What is required for a substrate to interact with DHHC proteins for lipidation?

Presence of cysteine with specific amino acids Signup and view all the answers

What function does the reversible transmembrane protein serve compared to DHHC?

It cleaves the covalent binding to release the substrate. Signup and view all the answers

Why is the association with cysteine critical for the lipidation process?

It facilitates the binding of the enzyme. Signup and view all the answers

What occurs to the signal sequence after the interaction with the translocon is initiated?

It is cleaved off after the protein synthesis is completed. Signup and view all the answers

Which factor regulates the interaction of the SRP with the signal peptide?

The exchange of GTP for GDP. Signup and view all the answers

What happens if there is no signal sequence present during protein synthesis?

Translation continues without any interruption. Signup and view all the answers

What role does the SRP play in protein translation to the endoplasmic reticulum?

It blocks translation until associated with its receptor. Signup and view all the answers

What characterizes the nature of hydrophobic amino acids during translation through the translocon?

They are more likely to exit the translocon and integrate into the bilayer. Signup and view all the answers

When the ribosome reaches a stop codon, what is the first action taken?

The ribosome dissociates from the mRNA and translocons. Signup and view all the answers

What is the primary function of the structure maintained among isoforms of the DHHC protein family?

To ensure localization specificity for different cell types Signup and view all the answers

Which part of the translation process is specifically termed 'cotranslational'?

The simultaneous translation and translocation of proteins. Signup and view all the answers

What does the interaction between SRP and its receptor lead to in protein synthesis?

The hydrolysis of GTP to GDP preventing further SRP binding. Signup and view all the answers

What could potentially happen if there is a point mutation in the nuclear localization sequence of a protein?

The protein will remain in the cytoplasmic compartment Signup and view all the answers

Which aspect is crucial for the transport of proteins to mitochondria?

The need for a mitochondrial localization sequence and chaperones Signup and view all the answers

What characteristic of translocons is important for facilitating the transport of transmembrane proteins?

They can open to the extracellular environment. Signup and view all the answers

What common sequence variation is maintained across the DHHC protein family?

Consistent presence of Asp-Pro and Gly amino acids Signup and view all the answers

Which consequence can arise from mutations or inhibition of palmitoylation in DHHC proteins?

Development of neurodegenerative diseases Signup and view all the answers

How does histon acetylation primarily affect gene expression regulation?

By loosening chromatin structure for enhanced accessibility Signup and view all the answers

What characteristic differentiates proteins with six transmembrane domains from those with four in the DHHC family?

Variations in the N-terminal sequence and potential alternative splicing Signup and view all the answers

What role do Ran-GDP proteins play in the cell?

Assisting with transport between the cytoplasm and nucleus Signup and view all the answers

Why is yeast often utilized as an experimental model for studying protein functions related to DHHC proteins?

The structural similarities provide a basis for translational research Signup and view all the answers

What is the significance of the conservation of specific regions between yeast and mammals in biological research?

It establishes a framework for understanding functional mechanisms applicable to humans Signup and view all the answers

What role do chaperons play in the transport of proteins to the mitochondria?

They denature proteins to allow them to pass through the transporters. Signup and view all the answers

What are TOM and TIM in relation to mitochondrial protein import?

Transport proteins in the outer and inner membranes, respectively. Signup and view all the answers

Which statement best describes protein import into peroxisomes?

Proteins enter from both the cytosol and the endoplasmic reticulum. Signup and view all the answers

How do peroxisomes differ from lysosomes and endosomes?

Peroxisomes contain a nucleus within their structure. Signup and view all the answers

Which statement is false regarding the synthesis of mitochondrial proteins?

All mitochondrial proteins are imported post-synthesis. Signup and view all the answers

What initiates the targeting of proteins to the rough endoplasmic reticulum?

Interaction between a signal peptide and the signal-recognition particle (SRP). Signup and view all the answers

What process is described when peroxisomes increase in number?

Fission of existing peroxisomes. Signup and view all the answers

In what way do peroxisome proteins differ from mitochondrial proteins in their synthesis?

Some peroxisome proteins can enter without being denatured. Signup and view all the answers

What distinguishes the method of protein import into mitochondria from that into peroxisomes?

Mitochondrial proteins require denaturation, while peroxisome proteins do not. Signup and view all the answers

Study Notes

DHHC PAT and APT as Transmembrane Proteins

DHHC PAT and APT are transmembrane proteins involved in lipidation processes.
DHHC acts as a transmembrane enzyme, promoting the transfer of palmitate to substrates, requiring a cysteine for specificity.
APT, while not a transmembrane protein, interacts with substrates to catalyze hydrolysis and release soluble proteins.
APT is synthesized by free ribosomes, while DHHC proteins are synthesized in the endoplasmic reticulum (ER).

Diversity of DHHC Proteins

At least 23 mammalian DHHC isoforms exhibit specific localizations in intracellular compartments, including ER, Golgi, and plasma membranes.
All isoforms are characterized by a common catalytic activity, despite differences in their sequences.
Primary sequences of DHHC proteins typically contain conserved regions and structural domains (4 or 6 transmembrane domains).
Yeast proteins similar to mammalian DHHC proteins reveal evolutionary conservation in enzymatic function.

DHHC Enzymes and Neurodegenerative Diseases

DHHC enzymes show specificity for various substrates, including G proteins.
Mutations or inhibition of palmitoylation in these enzymes are linked to neurodegenerative diseases.

Histone Acetylation and Protein Recognition

Acetylation influences transcription regulation while ubiquitination targets proteins for degradation.
Mutation of nuclear localization sequences affects protein transport, demonstrating crucial roles in cellular localization.

Protein Import into Mitochondria

Proteins destined for mitochondria are synthesized by free ribosomes, requiring a mitochondrial localization sequence for transport.
Chaperone proteins denature the synthesized proteins so they can pass through mitochondrial transporters (TOM for outer and TIM for inner membranes).
The import process is energy-consuming and ensures proper folding occurs once inside the mitochondria.

Peroxisomes: Unique Features

Peroxisomes are distinguished by high concentrations of catalase and are central to detoxification processes.
Proteins can enter peroxisomes from the cytosol or ER, with some being synthesized by free ribosomes and others delivered via vesicular transport from the ER.

Secretion Route and SRP Function

The majority of mammalian proteins synthesized by ribosomes associated with the ER are coded by about one-third of the genome.
Signal recognition particles (SRP) guide the ribosomes to the ER, blocking translation until the SRP interacts with the receptor at the rough ER.
Translation resumes once the SRP releases the signal peptide and allows the polypeptide chain to enter the translocon within the ER membrane.

Translocon Mechanism

The polypeptide chain passes through an aqueous channel formed by the translocons located in the ER membrane.
Specific interactions with hydrophobic amino acids determine whether regions of the polypeptide will integrate into the membrane or remain within the ER lumen.
Upon reaching a stop codon, ribosomes dissociate, allowing recycling of the ribosomal components and translocon machinery.

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Description

This quiz explores the roles of DHHC PAT and APT transmembrane proteins in the lipidation process. It focuses on the enzymatic functions of these proteins, their specificity related to cysteine interactions, and their significance in protein modification. Test your knowledge on these critical cellular mechanisms.