Lodish Ch 13 TB MC

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Questions and Answers

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

elongation factors in ribosome-mediated mRNA translation

a subunit of the SRP receptor

P54 subunit of SRP

Sec61 translocon (correct)

Protein insertion into the mammalian ER membrane is typically:

post-translational

quasitranslational

pretranslational

cotranslational (correct)

Post-translational translocation of some secretory proteins in yeast is powered by:

phospholipid hydrolysis by phospholipase C

cAMP hydrolysis by cAMP phosphodiesterase

ATP hydrolysis by BiP (correct)

GTP hydrolysis EF-Tu

In the absence of targeting information, what is the default location of proteins synthesized on cytosolic ribosomes?

Proteins synthesized on cytosolic ribosomes that contain no information for targeting to organelles diffuse throughout the cytosol. Signup and view all the answers

In a cell-free protein synthesis system utilizing microsomes from fragmented ER, under which condition could you determine if the new protein was imported into the microsome?

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

Which of the following is true about ER import?

Ribosomal translation of a 6-12 amino acid sequence on the N-terminus will initiate the process through interactions with SRP. Signup and view all the answers

Type I membrane proteins have all of the following properties, except:

internal signal-anchor sequence Signup and view all the answers

GPI-anchoring serves a special function, especially in polarized epithelial cells, because this modification serves to target proteins to the:

plasma membrane Signup and view all the answers

The topology of membrane proteins can often be predicted by computer programs that identify ______ topogenic segments.

hydrophobic Signup and view all the answers

Glycosylation, a post-translational modification of proteins, occurs in the:

Golgi Signup and view all the answers

All the following proteins interact with exposed amino acids during protein folding in the ER, except:

calnexin Signup and view all the answers

Unassembled or misfolded proteins in the RER can be damaging to the physiology of a cell and therefore are transported to the cytosol where they are degraded. This transport process is referred to as:

dislocation Signup and view all the answers

Sorting of proteins to mitochondria and chloroplasts is:

post-translational Signup and view all the answers

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

protein translocation into mitochondria and chloroplasts, respectively. Signup and view all the answers

Protein sequences for targeting to mitochondria or chloroplasts are located at:

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

Protein import into the mitochondrial matrix is supported by energy input from:

all of the above Signup and view all the answers

A polypeptide chain contains an amphipathic helix, with arginine and lysine residues on one side and hydrophobic residues on the other. It will likely enter:

the mitochondria Signup and view all the answers

In a cell that lacks cytosolic Hsc70:

import of proteins into the mitochondrial matrix would be diminished Signup and view all the answers

Which of the following components of mitochondrial import are NOT required for a sequence containing a matrix-targeting sequence and an intermembrane-space-targeting sequence?

Oxal Signup and view all the answers

Many peroxisomal matrix proteins are imported as:

folded proteins Signup and view all the answers

PTS1- and PTS2-bearing matrix proteins are targeted to:

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

Unlike mitochondria and chloroplasts, peroxisomes can arise ______ from precursor membranes, as well as by division of preexisting organelles.

de novo Signup and view all the answers

The nuclear pore complex allows for:

all of the above Signup and view all the answers

During the import of proteins into the nucleus, the importin subunit binds directly to:

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

Which type of RNA participates in nuclear export of mRNA?

hnRNA Signup and view all the answers

Which of the following is present in the nuclear export sequence of PKI (an inhibitor of protein kinase A)?

a leucine-rich sequence Signup and view all the answers

Transport of unspliced HIV mRNA from the nucleus to the cytoplasm of host cells is promoted by a virus-encoded protein named:

Rev Signup and view all the answers

How does Ran-GTP participate in the nuclear export of the HIV Rev protein?

In the nucleus, Ran·GTP binds to the nuclear export receptor exportin 1 and then to the leucine-rich nuclear export sequence (NES) in Rev. Exportin 1, in this trimolecular cargo complex, interacts transiently with FG repeats in FG-nucleoporins, allowing it to traverse the nuclear pore complex (NPC). In the NPC, the cargo complex encounters Ran·GAP, stimulating Ran to hydrolyze GTP, which reduces its affinity for exportin 1. Exportin 1 subsequently loses its affinity for the NES, releasing Rev to the cytoplasm. Signup and view all the answers

The nuclear pore complex (NPC) contains ______ structures that form a gel-like matrix that allow small molecules to diffuse through, but require larger proteins to enter via importin or other nuclear chaperones.

FG-nucleoporin Signup and view all the answers

During the process of nuclear import, a GEF works in the:

nucleus to exchange GTP for GDP bound to Ran Signup and view all the answers

In multipass membrane proteins synthesized in association with membrane-bounded ribosomes of the rough ER, signal-anchor and stop-transfer anchor sequences alternate. What do these sequences do?

Signal-anchor sequences direct insertion of internal segments of the protein into the ER membrane; stop transfer sequences stop the transfer of the protein across the membrane. The alternation of the two produces a protein that loops in and out of the membrane multiple times. Signup and view all the answers

Having misfolded soluble or secretory proteins in the RER contributes to what investigators call the “traffic jam,” a scenario associated with a number of human diseases where the normal transport of proteins is blocked by these abnormal proteins and the inability of protein complexes to arrive at their correct site and function properly. Briefly describe how the cell overcomes this particular traffic jam by exporting the misfolded proteins out of the RER into the cytosol, where they are degraded by the proteasome.

Essentially, the misfolded proteins have N-linked carbohydrate chains that are trimmed by the enzyme α-mannosidase. Once trimmed, these proteins are recognized by the lectin-like protein EDEM and/or OS-9, which targets the protein to an ER-associated degradation or ERAD complex, which serves as a type of channel needed to export the protein into the cytosol. Once in the cytosol, these proteins are subjected to enzymes that eventually target them to the proteasome for degradation. Signup and view all the answers

What are the general features of an N-terminal signal sequence that targets secretory proteins to the ER?

N-terminal signal sequences targeting proteins to the ER are 16 to 30 amino acids in length and have a hydrophobic core of 6 to 12 amino acids. Preceding the core is one or more positively charged amino acids. Otherwise, N-terminal signal sequences have little in common. Signup and view all the answers

In the absence of targeting information, what is the default location of proteins synthesized on cytosolic ribosomes?

Proteins synthesized on cytosolic ribosomes that contain no information for targeting to organelles diffuse throughout the cytosol. Signup and view all the answers

How does Ran·GTP participate in the nuclear export of the HIV Rev protein?

In the nucleus, Ran·GTP binds to the nuclear export receptor exportin 1 and then to the leucine-rich nuclear export sequ ence (NES) in Rev. Exportin 1, in this trimolecular cargo complex, interacts transiently with FG repeats in FG-nucleoporins, allowing it to traverse the nuclear pore complex (NPC). In the NPC, the cargo complex encounters Ran·GAP, stimulating Ran to hydrolyze GTP, which reduces its affinity for exportin 1. Exportin 1 subsequently loses its affinity for the NES, releasing Rev to the cytoplasm. Signup and view all the answers

To what extent do peroxisomal matrix protein import and peroxisomal membrane protein import share the same machinery?

The fact that mutated cells giving rise to Zellweger syndrome, a defect in peroxisomal matrix protein import, still form peroxisomal membranes (peroxisomal ghosts) with the normal composition of peroxisomal membrane proteins strongly indicates that the import machinery for membrane proteins is very different from that for matrix proteins. This situation is different from that for other organelles such as the ER, mitochondria, and chloroplasts. Signup and view all the answers

What is meant by de novo formation of peroxisomes?

This is the concept that peroxisomes can arise from nonperoxisomal membranes. The peroxisomal proteins Pex19, Pex3, and Pex16 are involved. The nature of the precursor membrane is unclear. Signup and view all the answers

What is the meaning of “quality control in the ER?”

“Quality control within the ER” refers to the need for proteins to be properly modified and folded before they can exit from the ER and travel to the Golgi apparatus. Improperly modified and folded proteins are typically translocated into the cytosol for degradation. Signup and view all the answers

Why are bacteria often a poor choice for the production of proteins for therapeutic purposes?

Typically, proteins used for therapeutic purposes are secreted proteins in animals; disulfide bonds stabilize their structures. Disulfide-bond formation occurs spontaneously in the lumen of the ER but not within bacteria. With this realization, animal cells became the preferred choice for the production of such proteins. Signup and view all the answers

During in vitro translation of mitochondrially targeted proteins, when must mitochondria be added for import of proteins synthesized on cytosolic ribosomes?

Proteins are imported into mitochondria post-translationally. Therefore, although mitochondria can be added during the translation process for import to occur, cotranslational presence is not a requirement as it is for import into the ER. The mitochondria can be added post-translationally. Signup and view all the answers

How are proteins imported into the thylakoids of chloroplasts?

For cytosolically synthesized proteins targeted to chloroplast thylakoids, multiple N-terminal uptake-targeting sequences are required. These act sequentially with the N-terminus, most targeting sequences being removed in the chloroplast stroma to expose the next targeting sequence. Four different pathways are known for the import of proteins from the chloroplast stroma into thylakoids. Three are for proteins imported from the cytosol and one is for proteins made in the chloroplast stroma. All pathways are variations of those used for export of proteins by bacteria. Examples of proteins homologous between bacteria and chloroplasts have been identified. ffff Signup and view all the answers

Study Notes

Moving Proteins into Membranes and Organelles

Cotranslational translocation into the ER membrane involves proteins that are not GTP-hydrolyzing, such as the Sec61 translocon.
Protein insertion into the mammalian ER membrane is typically cotranslational.
Post-translational translocation of some secretory proteins in yeast is powered by ATP hydrolysis by BiP.
In the absence of targeting information, proteins synthesized on cytosolic ribosomes will diffuse throughout the cytosol.

N-terminal Signal Sequences

N-terminal signal sequences targeting proteins to the ER are 16-30 amino acids long, with a hydrophobic core of 6-12 amino acids. Positively charged amino acids often precede the core.
Proteins targeted to the ER are typically 16 to 30 amino acids in length and have a hydrophobic core region of 6 to 12 amino acids in length, with one or more positively charged amino acids preceding the core.

ER Import Conditions

Determining if a protein was imported into microsomes involves incubating ribosomes and mRNA with microsomes, then adding a protease before analysis. Proteases break down proteins.

ER Import

N-terminal signal sequences are necessary for targeting to the ER, even if added to proteins not normally targeted to the ER.
Signal recognition particles (SRPs) are needed during co-translational protein import.
Ribosomal translation of a 6-12 amino acid signal sequence on the N-terminus initiates the import process via interaction with SRP.

Type I Membrane Proteins

Features of Type I membrane proteins except for an internal signal-anchor sequence include a cleavable signal sequence, an internal stop-transfer sequence, and an N-out, C-in topology.

GPI-Anchoring

GPI-anchoring targets proteins to the plasma membrane in polarized epithelial cells.

Membrane Protein Topology Prediction

Computer programs that predict protein topology identify hydrophilic and hydrophobic segments.

Multipass Membrane Proteins

Signal-anchor sequences insert internal protein segments into the ER membrane, while stop-transfer sequences prevent proteins from further passing through the membrane.
These sequences alternate in multipass membrane proteins, creating proteins that loop in and out of the membrane multiple times.

Misfolded Protein Traffic Jam

Misfolded proteins in the RER cause a "traffic jam," blocking normal protein transport.
Cells overcome this by exporting misfolded proteins to the cytosol for degradation via the proteasome system after trimming the carbohydrate chains of misfolded proteins.

ER Quality Control

Proteins must be properly folded and modified before exiting the ER to the Golgi apparatus.
Improperly folded proteins are degraded in the cytosol.

Bacterial Protein Production

Bacteria are often not suitable for therapeutic protein production because disulfide bond formation is harder in bacteria than in animal cells.

Mitochondrial and Chloroplast Protein Sorting

Proteins are sorted post-translationally to mitochondria and chloroplasts.

Chloroplast Protein Import

Multiple N-terminal targeting sequences are often necessary for import into chloroplast thylakoids.

Mitochondrial Protein Import

Protein import into the mitochondrial matrix requires energy input from ATP hydrolysis by chaperone proteins, especially in the cytosol.

Peroxisomal Protein Import

Peroxisomal matrix proteins are typically imported as folded proteins.

Nuclear Protein Import

Importin subunits bind directly to the basic nuclear localization signals in cargo proteins.

Nuclear mRNA Export

Certain types of RNA, like heterogenous nuclear RNA (hnRNA), are involved in mRNA export from the nucleus.

HIV Rev Protein Export

Ran-GTP, a GTP-binding protein, plays a role in nuclear export mechanisms of the HIV Rev protein. It binds to nuclear export receptor exportin 1 and facilitates its movement through the nuclear pore complex (NPC) before releasing Rev in the cytosol.

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Description

This quiz covers the processes involved in moving proteins into membranes and organelles, focusing on mechanisms like cotranslational translocation and post-translational import. Key concepts include N-terminal signal sequences and the conditions necessary for protein import into the ER. Test your understanding of these fundamental cellular processes.

Lodish Ch 13 TB MC

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Questions and Answers

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

Protein insertion into the mammalian ER membrane is typically:

Post-translational translocation of some secretory proteins in yeast is powered by:

In the absence of targeting information, what is the default location of proteins synthesized on cytosolic ribosomes?

In a cell-free protein synthesis system utilizing microsomes from fragmented ER, under which condition could you determine if the new protein was imported into the microsome?

Which of the following is true about ER import?

Type I membrane proteins have all of the following properties, except:

GPI-anchoring serves a special function, especially in polarized epithelial cells, because this modification serves to target proteins to the:

The topology of membrane proteins can often be predicted by computer programs that identify ______ topogenic segments.

Glycosylation, a post-translational modification of proteins, occurs in the:

All the following proteins interact with exposed amino acids during protein folding in the ER, except:

Unassembled or misfolded proteins in the RER can be damaging to the physiology of a cell and therefore are transported to the cytosol where they are degraded. This transport process is referred to as:

Sorting of proteins to mitochondria and chloroplasts is:

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

Protein sequences for targeting to mitochondria or chloroplasts are located at:

Protein import into the mitochondrial matrix is supported by energy input from:

A polypeptide chain contains an amphipathic helix, with arginine and lysine residues on one side and hydrophobic residues on the other. It will likely enter:

In a cell that lacks cytosolic Hsc70:

Which of the following components of mitochondrial import are NOT required for a sequence containing a matrix-targeting sequence and an intermembrane-space-targeting sequence?

Many peroxisomal matrix proteins are imported as:

PTS1- and PTS2-bearing matrix proteins are targeted to:

Unlike mitochondria and chloroplasts, peroxisomes can arise ______ from precursor membranes, as well as by division of preexisting organelles.

The nuclear pore complex allows for:

During the import of proteins into the nucleus, the importin subunit binds directly to:

Which type of RNA participates in nuclear export of mRNA?

Which of the following is present in the nuclear export sequence of PKI (an inhibitor of protein kinase A)?

Transport of unspliced HIV mRNA from the nucleus to the cytoplasm of host cells is promoted by a virus-encoded protein named:

How does Ran-GTP participate in the nuclear export of the HIV Rev protein?

The nuclear pore complex (NPC) contains ______ structures that form a gel-like matrix that allow small molecules to diffuse through, but require larger proteins to enter via importin or other nuclear chaperones.

During the process of nuclear import, a GEF works in the:

In multipass membrane proteins synthesized in association with membrane-bounded ribosomes of the rough ER, signal-anchor and stop-transfer anchor sequences alternate. What do these sequences do?

What are the general features of an N-terminal signal sequence that targets secretory proteins to the ER?

In the absence of targeting information, what is the default location of proteins synthesized on cytosolic ribosomes?

How does Ran·GTP participate in the nuclear export of the HIV Rev protein?

To what extent do peroxisomal matrix protein import and peroxisomal membrane protein import share the same machinery?

What is meant by de novo formation of peroxisomes?

What is the meaning of “quality control in the ER?”

Why are bacteria often a poor choice for the production of proteins for therapeutic purposes?

During in vitro translation of mitochondrially targeted proteins, when must mitochondria be added for import of proteins synthesized on cytosolic ribosomes?

How are proteins imported into the thylakoids of chloroplasts?

Study Notes

Moving Proteins into Membranes and Organelles

N-terminal Signal Sequences

ER Import Conditions

ER Import

Type I Membrane Proteins

GPI-Anchoring

Membrane Protein Topology Prediction

Multipass Membrane Proteins

Misfolded Protein Traffic Jam

ER Quality Control

Bacterial Protein Production

Mitochondrial and Chloroplast Protein Sorting

Chloroplast Protein Import

Mitochondrial Protein Import

Peroxisomal Protein Import

Nuclear Protein Import

Nuclear mRNA Export

HIV Rev Protein Export

Studying That Suits You

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