Lodish Ch 12 TB MC

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Questions and Answers

What is necessary for ER import according to signal sequences?

Signal recognition particles are not required for ER import.

N-terminal signal sequences can direct non-ER proteins to the ER.

N-terminal signal sequences can initiate ER import independently.

Ribosomal translation of an N-terminal sequence is crucial for ER import. (correct)

Which type of signal is not part of Type I membrane proteins?

Transmembrane domain sequence (correct)

Internal signal-anchor sequence

Internal stop-transfer sequence

Cleavable signal sequence

GPI-anchoring in polarized epithelial cells is primarily targeting proteins to the:

Lysosomes

Plasma membrane (correct)

Nuclear envelope

Rough endoplasmic reticulum

What type of segments do computer programs often identify to predict membrane protein topology?

Hydrophobic segments Signup and view all the answers

What is the function of signal-anchor and stop-transfer sequences in multipass membrane proteins?

They direct insertion of the protein segments into the membrane. Signup and view all the answers

What phenomenon occurs due to misfolded proteins in the rough ER?

Traffic jam affecting normal protein transport Signup and view all the answers

Which of the following is not a mechanism involved in protein targeting to the ER?

Glycosylation modification Signup and view all the answers

What primarily initiates the process of protein targeting to the ER?

Binding of SRPs to N-terminal sequences Signup and view all the answers

Where are misfolded proteins exported to from the RER for degradation?

To the cytosol Signup and view all the answers

What enzyme trims the N-linked carbohydrate chains of misfolded proteins in the RER?

α-mannosidase Signup and view all the answers

What sequence allows the transmembrane receptor to be inserted into the membrane?

Signal-anchor sequence Signup and view all the answers

Which protein involved in cotranslational translocation into the ER membrane is NOT a GTP-hydrolyzing protein?

Sec61 translocon Signup and view all the answers

Which process refers to the degradation of unassembled or misfolded proteins in the cytosol?

Dislocation Signup and view all the answers

Protein insertion into the mammalian ER membrane is typically what?

cotranslational Signup and view all the answers

In which cellular location does glycosylation predominantly occur?

Golgi apparatus Signup and view all the answers

What powers post-translational translocation of some secretory proteins in yeast?

ATP hydrolysis by BiP Signup and view all the answers

Which protein does NOT interact with exposed amino acids during protein folding in the ER?

Calnexin Signup and view all the answers

What is the default location for proteins synthesized on cytosolic ribosomes without targeting information?

cytosol Signup and view all the answers

What is the purpose of quality control in the ER?

To ensure proteins are properly modified and folded Signup and view all the answers

Which sequence is found in a transmembrane receptor protein?

Stop-transfer-anchor sequence Signup and view all the answers

What is a key feature of an N-terminal signal sequence that targets secretory proteins to the ER?

preceded by positively charged amino acids Signup and view all the answers

Under which condition can you determine if a new protein was imported into microsomes in a cell-free protein synthesis system?

Ribosomes and mRNA are incubated with microsomes, and a protease is added Signup and view all the answers

In protein synthesis, what does the term 'cotranslational' imply?

Synthesis and insertion occur simultaneously Signup and view all the answers

Which of the following best describes the process of protein targeting to the ER?

Involves signal recognition particles Signup and view all the answers

Why are bacteria often unsuitable for producing therapeutic proteins?

They do not facilitate disulfide bond formation. Signup and view all the answers

Sorting of proteins to mitochondria and chloroplasts occurs:

After translation is complete. Signup and view all the answers

Tom/Tim and Toc/Tic protein complexes are primarily involved in:

Import of proteins into mitochondria and chloroplasts. Signup and view all the answers

Protein sequences targeting mitochondria or chloroplasts are typically found at:

The N-terminus of the protein. Signup and view all the answers

Energy input for protein import into the mitochondrial matrix comes from:

ATP hydrolysis and the proton-motive force. Signup and view all the answers

When must mitochondria be added for the import of proteins synthesized on cytosolic ribosomes?

After the translation is complete. Signup and view all the answers

Which type of protein bonds play a significant role in stabilizing therapeutic proteins?

Disulfide bonds. Signup and view all the answers

Which process does NOT contribute to mitochondrial protein import?

Simple diffusion of proteins. Signup and view all the answers

What is required for cytosolically synthesized proteins to be imported into chloroplast thylakoids?

Multiple N-terminal uptake-targeting sequences Signup and view all the answers

Which of the following proteins are imported through a pathway specifically for proteins made in the chloroplast stroma?

Proteins containing matrix-targeting sequences Signup and view all the answers

A polypeptide chain with an amphipathic helix and basic residues will likely enter which cellular structure?

The mitochondria Signup and view all the answers

If a cell lacks cytosolic Hsc70, what is the expected outcome regarding protein import into the mitochondrial matrix?

Import would be diminished Signup and view all the answers

Which component is NOT required for a sequence containing both a matrix-targeting sequence and an intermembrane-space-targeting sequence?

Oxa1 Signup and view all the answers

How are many peroxisomal matrix proteins imported?

As folded proteins Signup and view all the answers

PTS1- and PTS2-bearing matrix proteins are targeted to which of the following?

A common import receptor and translocation machinery on the peroxisomal membrane Signup and view all the answers

Which option represents a common misconception about protein import mechanisms?

All proteins must be unfolded to enter organelles. Signup and view all the answers

What is the origin of peroxisomes aside from division of preexisting organelles?

Through de novo formation from nonperoxisomal membranes Signup and view all the answers

Which of the following best describes the import mechanism for peroxisomal matrix proteins?

They arrive as folded proteins. Signup and view all the answers

What characterizes the nuclear pore complex regarding molecular transport?

Facilitates both passive diffusion and active transport Signup and view all the answers

What is the significance of mutated cells in Zellweger syndrome concerning peroxisomal membrane proteins?

They form peroxisomal ghosts with normal membrane protein composition. Signup and view all the answers

In the protein import process to the nucleus, which component does the α importin subunit bind to directly?

Basic nuclear localization signals in cargo proteins Signup and view all the answers

What type of proteins can peroxisomes import from precursor membranes?

Matrix proteins through different mechanisms Signup and view all the answers

What do Pex19, Pex3, and Pex16 proteins have in common regarding peroxisomes?

They are involved in de novo formation of peroxisomes. Signup and view all the answers

What defines the method of import for proteins into mitochondria compared to peroxisomes?

Peroxisomes have a distinct import machinery for their proteins. Signup and view all the answers

Study Notes

Moving Proteins into Membranes and Organelles

Co-translational translocation of proteins into the ER membrane: Proteins involved, including SRP receptor subunit, elongation factors in ribosome-mediated mRNA translation, and P54 subunit of SRP, are not GTP-hydrolyzing proteins. The Sec61 translocon is.
Protein insertion into the ER membrane: Typically a co-translational process in mammals.
Post-translational translocation in yeast: Powered by ATP hydrolysis by BiP.
Default location of proteins synthesized on cytosolic ribosomes: Diffuse throughout the cytosol.

General Features of an N-Terminal Signal Sequence

Targets secretory proteins to the ER.
16-30 amino acids long, with a hydrophobic core (6-12 amino acids) preceded by positively charged amino acids.

Determining Protein Import into Microsomes

Incubate ribosomes and mRNA with microsomes.
Add a protease.
Analyze the results.

ER Import

N-terminal signal sequences are necessary for ER import.
Signal recognition particles (SRPs) are needed during co-translational import.
Ribosomal translation initiates through interactions with SRP.

Type I Membrane Proteins

Characterized by a cleavable signal sequence and either an internal signal anchor sequence or internal stop transfer sequence.
Have an N-out, C-in topology.

GPI-Anchoring

Targets proteins to the plasma membrane, particularly in polarized epithelial cells.

Membrane Protein Topology

Computer programs predict protein topology based on identifying hydrophobic segments.

Multipass Membrane Proteins

Employ signal-anchor sequences and stop-transfer sequences.
Proteins loop in and out of the membrane.

Protein Traffic Jam (Misfolded Proteins)

Misfolded proteins block normal protein transport.
Cells export misfolded proteins to be degraded by the proteasome.

Glycosylation

Occurs in the Golgi.
A post-translational modification of proteins.

ER Protein Folding

Protein folding in the ER involves interactions with ER chaperones (e.g., BiP, calnexin, PDI, prolyl isomerase).

ER Quality Control

Ensures proper protein folding and modification before transport.
Incorrectly folded proteins are targeted for degradation.

Protein import

Proteins are targeted to mitochondria and chloroplasts post translationally.
Different transport pathways exist.

Peroxisome

May arise from precursor membranes, and possibly de novo.
Many peroxisomal matrix proteins are imported as folded proteins.

Nuclear Import

Importin binds to nuclear localization signals (NLS) in cargo proteins.
Proteins move through nuclear pores into the nucleus.

Nuclear Export

Cargo protein complex binds to Ran-GTP.
Ran-GTP induces dissociation and releases the protein in the cytoplasm.
Requires specific sequences .

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Description

Test your knowledge on the mechanisms of protein import into membranes and organelles, focusing on co-translational translocation into the ER, signal sequences, and post-translational processes. This quiz covers key proteins involved and experimental methods for analyzing protein import.