Lecture 10 Review

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Questions and Answers

What is the role of the SRP in protein translocation to the endoplasmic reticulum?

It synthesizes the entire protein before translocation.

It cleaves the signal sequence before translocation.

It facilitates the folding of the newly synthesized protein.

It binds to the ribosome and pauses translation. (correct)

What event occurs immediately after the SRP binds to the SRP receptor on the ER?

Translocation of the protein across the membrane begins.

Signal sequences are cleaved from the protein.

The SRP leaves the ribosome and translation resumes. (correct)

The ribosome detaches from the mRNA.

Which process describes the movement of the protein across the ER membrane during translation?

Non-cotranslational translocation

Post-translational translocation

Co-translational translocation (correct)

Transcriptional translocation

What happens to the N-terminal signal sequences after the SRP-mediated translocation process?

They are cleaved on the lumenal side of the ER membrane. Signup and view all the answers

What is the function of the translocon during protein translocation?

It acts as a channel for the newly synthesized protein to enter the ER. Signup and view all the answers

What happens to Ran when it is bound to the nuclear transport receptor?

It accumulates in the cytosol. Signup and view all the answers

Where are most mitochondrial proteins encoded?

In the nuclear genome. Signup and view all the answers

What occurs to proteins as they are translocated into mitochondria?

They are unfolded during translocation. Signup and view all the answers

What function does the endoplasmic reticulum (ER) serve?

Synthesis of secreted proteins. Signup and view all the answers

Which of the following is true about the nuclear transport receptor?

It accumulates in the cytosol. Signup and view all the answers

Which process involves sorting proteins into chloroplasts?

Translocation similar to mitochondrial import. Signup and view all the answers

What percentage of genes are involved in synthesis related to the ER?

Approximately 30%. Signup and view all the answers

Which of the following is NOT a mechanism of protein sorting mentioned?

Transport into lysosomes. Signup and view all the answers

What kind of proteins mainly require active transport to enter the nucleus?

Large proteins like transcription factors Signup and view all the answers

Which protein family competes with cargo for binding to the import receptor?

Ran GTPase Signup and view all the answers

What must occur for import to require high Ran-GTP levels in the nucleus?

GTP hydrolysis must happen in the nucleus Signup and view all the answers

What role do GTP-binding proteins play in the context of nuclear transport?

They act as molecular switches controlling transport. Signup and view all the answers

Which statement accurately reflects the process of cargo protein release from the import receptor?

It requires binding of Ran GTPase to the receptor. Signup and view all the answers

What happens when Ran-GAP is depleted in a cell?

Ran GTPase is unable to hydrolyze GTP effectively. Signup and view all the answers

What is the function of the GEF in relation to GTP-binding proteins?

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

Which of the following accurately describes the nuclear pore complex?

It has unstructured protein loops creating a diffusion barrier. Signup and view all the answers

What characterizes small GTP-binding proteins in their role as molecular switches?

They activate pathways upon binding GTP. Signup and view all the answers

How do larger proteins typically enter the nucleus?

Using nuclear transport receptors for active transport. Signup and view all the answers

What designation is given to the first signal sequence during protein translocation?

Start-transfer sequence Signup and view all the answers

What is cleaved off by signal peptidase in the ER lumen?

Signal sequences Signup and view all the answers

Which orientation results when the start-transfer sequence is positioned correctly?

N-terminus in the lumen Signup and view all the answers

What is true regarding multipass membrane proteins?

They comprise multiple stop-transfer and start-transfer sequences Signup and view all the answers

What type of glycosylation occurs on asparagine side-chains in the ER?

N-linked glycosylation Signup and view all the answers

What is the primary role of signal sequences in protein sorting?

They direct proteins to their specific cellular destinations. Signup and view all the answers

What is one function of protein glycosylation?

Enhances protein folding and solubility Signup and view all the answers

What happens during the Ran-GTPase cycle regarding nuclear import of proteins?

It regulates the transport of proteins into and out of the nucleus. Signup and view all the answers

How do proteins get targeted to mitochondria and chloroplasts post-translationally?

By being tagged with a post-translational signal sequence. Signup and view all the answers

Where does glycosylation of proteins primarily occur?

Only in the ER lumen Signup and view all the answers

What is the mechanism by which the signal recognition particle (SRP) functions?

It couples protein synthesis with their import into the endoplasmic reticulum. Signup and view all the answers

Which amino acid is specifically mentioned in the glycosylation sequence?

Serine Signup and view all the answers

What role does protein glycosylation play in the endoplasmic reticulum?

It facilitates the folding and stability of proteins. Signup and view all the answers

What role can protein glycosylation play in protein interaction?

Facilitates protein sorting Signup and view all the answers

Which of the following correctly describes transmembrane proteins during their insertion into the endoplasmic reticulum?

They are integrated into the membrane through a series of hydrophobic domains. Signup and view all the answers

Which statement about the sugar chain in glycosylation is incorrect?

Occurs solely in the cytosol Signup and view all the answers

Which organelle is associated with post-translational import of proteins primarily via signal sequences?

Mitochondria Signup and view all the answers

What determines whether a protein will remain in the cytosol or be directed elsewhere?

The presence of signal sequences. Signup and view all the answers

What is the first step in the transport of a protein into the nucleus?

Recognition of the nuclear localization signal by importins. Signup and view all the answers

Study Notes

Protein Sorting Overview

Eukaryotic cells have membrane-bound organelles
Proteins are sorted by different mechanisms:
- Transport through nuclear pores
- Transport across membranes
- Transport by vesicles

Learning Objectives

Understand mechanisms for protein targeting to various cellular compartments
Understand signal sequences
Understand how proteins are targeted post-translationally to mitochondria and chloroplasts
Understand how SRP couples translation and translocation of proteins into the endoplasmic reticulum
Understand how proteins with multiple transmembrane regions are inserted into the endoplasmic reticulum
Understand protein glycosylation and folding in the ER

Nuclear Envelope Architecture

The nuclear envelope comprises an outer and inner membrane
The space between the two nuclear membranes is called the perinuclear space
Contains nuclear pores
The inner membrane and outer membrane are linked by nuclear pores
Nuclear pores allow the transport of molecules between the nucleus and the cytoplasm
Small proteins can passively diffuse
Larger proteins need an active process for transport
Nuclear transport receptors move molecules through the nuclear pores

Nuclear Pore Complexes

Unstructured protein loops create a diffusion barrier
Very small proteins (<5 kD) can passively diffuse through nuclear pores. Larger proteins require an active transport process.
Many proteins enter the nucleus through these pores requiring an active shuttling process

Nuclear Transport Receptors

Prospective nuclear protein (cargo) binds to the nuclear import receptor
The nuclear import receptor "lets go" of cargo protein in the nucleus
GTP-binding proteins (GTPases or monomeric GTPases) are molecular switches
- The "GEF" (guanine nucleotide exchange factor) triggers GTP binding.
- The 'GAP' (GTPase-activating protein) triggers GTP hydrolysis.
GTP hydrolysis drives nuclear transport
Ran GTPase competes with cargo for binding to the receptor

Mitochondrial Protein Import

Most mitochondrial proteins are encoded in the nuclear genome
Proteins are unfolded as they are translocated across both membranes simultaneously
- Binding to import receptors in the outer membrane
- Protein translocator in the inner membrane
Proteins are then translocated into the matrix
Proteins are refolded within the mitochondrion

ER Protein Import - SRP and Co-translational translocation

SRP (signal recognition particle) binds signal sequence and slows/pauses translation
SRP binds to the SRP receptor on the ER
SRP leaves and ribosome engages translocation channel (aka "translocon")
Translation resumes and protein is translocated across the bilayer
Signal sequence is cleaved off protein by signal peptidase
Protein folds in the ER lumen

Transmembrane Proteins

Proteins with transmembrane segments are directed into the ER membrane in a "co-translational translocation"
First segment (start-transfer sequence)
Second segments (stop-transfer sequence)
The mechanism dictates the direction of the protein in the membrane (N-terminus in the lumen)

Glycosylation

Many proteins undergo glycosylation (N-linked) in the ER
Sugar chains attach to asparagine in the ER
Never occurs in the cytosol

Multipass Membrane Proteins

Multipass membrane proteins contain multiple stop-transfer and start-transfer sequences
These sequences relate to translocation and not translocation.

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Description

Test your knowledge on the mechanisms of protein translocation and sorting within cellular organelles such as the endoplasmic reticulum, mitochondria, and chloroplasts. This quiz covers key concepts including the role of the signal recognition particle (SRP), translocon function, and the transport receptors involved in protein import. Dive into the complexities of cellular transport processes!