GTPases and Microsomes in Translation

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Questions and Answers

What is the role of microsomes in the translation process?

They assist in the synthesis of free ribosomes.

They enhance protein aggregation.

They degrade misfolded proteins.

They mimic the plasma membrane environment. (correct)

What must occur for a secreted protein to mature and fold correctly?

The signal peptide must be removed. (correct)

The protein must be synthesized in a free ribosome.

The protein must aggregate first.

The signal peptide must remain attached.

What is the primary role of the EMC complex in transmembrane protein formation?

To aid Sec61 as an accessory protein for translocase (correct)

To facilitate the synthesis of lipid membranes

To cleave the N-terminus from type II proteins

To initiate the glycosylation process in the cytoplasm

What characterizes the SReceptor in the translation process?

It contains a submerged beta subunit as an anchor. Signup and view all the answers

Which type of transmembrane protein has its N-terminus found in the ER lumen?

Type II Signup and view all the answers

What is the function of the Signal Recognition Particle (SRP)?

It halts translation by binding to the ribosome. Signup and view all the answers

How does N-glycosylation occur for type II transmembrane proteins?

When the N-terminus is in the ER lumen Signup and view all the answers

What distinguishes type III transmembrane proteins from type II regarding their N-terminus?

Type III is cleaved off later from calmodulin/ca2+ Signup and view all the answers

How does NAC prevent polyQ aggregates?

By binding to nascent proteins. Signup and view all the answers

What happens to the N-terminal signal sequence during protein translocation?

It is cleaved by SPase after membrane insertion. Signup and view all the answers

What is the composition of the Sec61p trimer?

A heterotrimer that forms aqueous pores Signup and view all the answers

What role do ribosomes have in protein synthesis depending on their location?

Bound ribosomes produce secreted proteins with a larger yield. Signup and view all the answers

Which of the following statements accurately describes the effectiveness of paraformaldehyde?

It can compromise cellular integrity by killing the cell Signup and view all the answers

In the context of the positive inside rule, which best describes the sequence of the mRNA chain?

Positive-hydrophobic core-polar residues indicates a correct N-terminus signal. Signup and view all the answers

What is the function of chaperones such as BiP, calnexin, and calreticulin in the ER?

They facilitate proper protein folding and quality control Signup and view all the answers

Which type of glycosylation involves the attachment of sugar to hydroxyl groups on side chains?

O-linked glycosylation Signup and view all the answers

Study Notes

Translation and Microsomes

Microsomes must be added before translation for effective protein synthesis; they mimic the plasma membrane and facilitate processing.
Addition of ribosomes (RM) before translation is crucial; without it, protein processing does not occur.

Experimental Findings

Higher yield of synthesized secreted proteins compared to authentic proteins observed in experiments by Dobberman & Bobblehead.
Free and membrane-bound ribosomes produce different amounts of light chains, particularly secreted proteins.

Protein Targeting and Signal Peptides

Proteins possess specific domains (SSH) that direct them to their destinations.
Signal peptides must be removed for proper protein maturation and folding, as they are typically hydrophobic.

Role of Microsomes

Microsomes consist of membrane-bound ribosomes that aid newly synthesized protein translocation.
Signal Recognition Particle (SRP) recognizes the signal directing proteins to the endoplasmic reticulum (ER).

NAC and Aggregation Prevention

NAC prevents nascent proteins from aggregating; in its presence, polyQ cannot aggregate.

Structure and Function of SRP

SRP binds nascent polypeptides and halts translation.
Comprises two parts: one binds to ribosomes; the other binds mRNA and recognizes protein signals.

SRP Receptor Dynamics

SRP receptor is a membrane-bound heterodimer; the beta subunit anchors in the membrane, while the alpha subunit remains in the cytosol.
SR is adjacent to the translocon, facilitating the delivery of nascent polypeptides to the ER lumen.

GTP and Polypeptide Processing

GTP binding to SR and SRP releases the signal sequence.
The N-terminal signal sequence is cleaved during entry into the cell by enzymes (SPase and SPPase).

Membrane Protein Types and Cleavage

Transmembrane proteins undergo specific cleavage: N-terminal signal for type I and internal signal for types II and III.
Type I proteins have their N-terminus in the cytosol; must cleave at the N-terminus first.

Type II & III Proteins

Type II proteins have N-terminus in the cytosol, while type III proteins have it in the ER lumen.
N-glycosylation occurs exclusively in the ER lumen; Type II proteins need to flip to allow for this process.

Channel Components and Functionality

Sec61p forms the aqueous channel for translocation.
The SR recruits ribosomes during protein synthesis.

Experimental Observations

Paraformaldehyde treatment is lethal; immunofluorescence staining reveals SRP receptor colocalization with BiP.
Protein trafficking begins at specific regions of the ER (ERES), with distinct domains for various functions.

Ion Current and Experimentation

Ion currents through translocons can be measured experimentally; they can be blocked or unblocked by NAC.
Puromycin acts as an SRP-like element and aids in experimentation regarding protein translocation.

Folding and Quality Control in the ER

Oxidoreductive enzymes, PDI and PPI, play critical roles in protein folding.
Chaperones such as BiP, calnexin, and calreticulin assist in maintaining protein structure.

Glycosylation Types

O-linked glycosylation involves sugar attachment to the hydroxyl groups of side chain residues.

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Description

This quiz delves into the role of microsomes and GTPases in the translation process of protein synthesis. Understanding the necessity of adding microsomes for optimal protein yield is crucial, as it mimics the cellular environment. Explore the differences in yield between synthesized secreted proteins and authentic proteins in this interactive assessment.