Chapter 15: Intracellular Compartments and Protein Transport

How are mitochondria thought to have originated?

when an aerobic bacterium was engulfed by a larger anaerobic eukaryotic cell

What are the three transport mechanisms for proteins into organelles? In which is the protein folded and unfolded?

transport through pores(folded), membranes (unfolded), or by vesicles (folded)

What type of sequence is present on proteins destined for the ER? How can this destination be changed?

N-terminal signal sequence

via recombinant DNA

What is the role of Ran-GTP in the nuclear transport process?

All of the above

3 multiple choice options

How do proteins enter the nucleus? How is this process conducted?

nuclear pores

The proteins contain a nuclear localization signal that is recognized by nuclear import receptors, which interact with the cytosolic fibrils that extend from the rim of the pore

- after cargo delivery, the receptors return to the cytosol via nuclear pores for reuse

- Similar types of transport receptors, operating in the reverse direction, export mRNA from the nucleus

Differentiate Ran-GTP and Ran-GEF

Ran-GTP converts Ran GTP to Ran-GDP - dissociated from receptor

Ran-GEF causes Ran-GDP to release its GDP and take up GTP - binds to receptor

What is the role of Ran-GDP (in the cytosol)?

Causes dissociation of Ran from the import receptor, freeing the receptor for reuse

3 multiple choice options

A researcher disrupts the function of Ran-GEF in a eukaryotic cell while leaving Ran-GAP unaffected.

Which of the following outcomes is most likely to occur?

Failure of cargo proteins to be released from import receptors in the nucleus

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How are proteins diffused into the chloroplasts and mitochondria?

Proteins must unfold

Proteins with mitochondrial signal sequence is recognized by a receptor in the outer mitochondrial membrane

The complex of receptor, precursor protein, and translocator then diffuses laterally in the outer membrane until the signal sequence is recognized by a second translator in the inner membrane

OM Receptor > OM translocator > IM translocator > Matrix

proteins are imported into chloroplasts by a similar mechanism

How does an ER signal sequence and an SRP direct a ribosome to the ER membrane?

SRP binds to signal sequence and ribosome temporarily slow translation

SRP-ribosome complex then bind to SRP-receptor in ER membrane

Protein synthesis resumes and injects the protein to the ER lumen

Which component is CRITICAL for directing a ribosome to the ER membrane during protein synthesis?

ER signal sequence

3 multiple choice options

Which sequence accurately represents the pathway of a protein entering the mitochondria?

OM Receptor > OM translocator > IM translocator > Matrix

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Which characteristic is typical of the N-terminal signal sequences that direct proteins to the endoplasmic reticulum (ER)?

Contains a hydrophilic n-region, a central hydrophobic region, and a cleavage site at the c-region.

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What sequence directs proteins to the ER?

N-terminal signal sequence

On average 16 to 30 amino acid residues in length comprising a characteristic tripartite structure:

1. a hydrophilic, usually positive charged n-region

2. A central hydrophobic region of 5-15 residues

3. a c-region with the cleavage site for signal peptidase

What signals determine the arrangement of a transmembrane protein in the lipid bilayer?

start and stop signals

Where is the ER signal sequence cleaved and what is left?

at its c-terminus, leaving the transmembrane protein anchored in the membrane

What is unique about a double-pass transmembrane protein

internal ER signal sequence

the internal sequence not only acts as a start-transfer signal, but it also helps to anchor the final protein in the membrane

What is the start and stop signals for the ER transmembrane protein and what does it do?

start: N-terminal signal sequence initiates transfer

stop: second hydrophobic sequence, which acts as a stop-transfer sequence

Differentiate endo- and exocytosis?

inward pathway: endocytotic

outward pathway: exocytotic

Describe the process of receptor mediated endocytosis?

cargo receptors, with their bound cargo molecules, are captured by adaptins

adaptins bind clathrin in cytosolic surface

Dynamin proteins pinch off the vesicles by the help of GTP hydrolysis

Which of the following statements accurately describes the function of clathrin-coated vesicles within cellular transport processes?

Adaptins play a role in selecting which molecules are transported in clathrin-coated vesicles.

3 multiple choice options

Which of the following components of receptor-mediated endocytosis of LDL is incorrectly matched with its function?

Forms the coated vesicle

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Describe the three proteins responsible for directing transport vesicles to their target membranes

Rab proteins, tethering proteins, and SNAREs

Interactions between tethering proteins and rab proteins determine the destination of the vesicle

Rab is a type of G (GTPase) protein

SNARE proteins can catalyze the fusion of the vesicle and target membranes

Where are proteins covalently modified?

ER

Which carbohydrate residue distinguishes blood type A from blood type O?

N-acetylgalactosamine

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Describe the contents of each blood group

O = H antigen

A = H antigen + N-acetylgalactosamine

B = H antigen + Galactose

AB = A + B

What role does the endoplasmic reticulum (ER) play in the processing of proteins required for membrane and secreted proteins?

It assists in the folding and glycosylation of newly synthesized proteins.

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What is the most common method to determine the destination of a protein bearing a particular signal sequence?

radioactive molecule tagging

In the context of studying protein destinations within a cell, why might a researcher choose to use GFP tagging instead of radioactive molecule tagging?

Both B and C

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What is the function of Unfolded Protein Response (UPR)?

keeps unfolded proteins within the ER

checks cargo proteins for errors or deformation in every compartment from ER all the way to cell membrane

responsible for proper folding of proteins in the ER

What are the three unfolded proteins responses?

IRE1, PERK, and ATF6

What are the two purposes of all of the unfolded proteins responses?

1. increase folding of proteins in the ER

2. limit translation

Which of the following best describes the role of the unfolded protein response (UPR) in the endoplasmic reticulum (ER)?

To assist in the folding of newly synthesized proteins and limit unnecessary protein translation

3 multiple choice options

Proteins travel through the Golgi in sequence in what two ways?

1. by means of transport vesicles that bud from one cisterna and fuse with the next

2. by a maturation process in which the Golgi cisternae themselves migrate through the Golgi stack

Proteins are secreted from the cell by what two secretory pathways?

constitutive secretory pathway: supplies the plasma membrane with lipids and proteins

- e.g., collagen and membrane proteins

Regulated exocytosis pathway: extracellular signal stimulates their secretion

- e.g., insulin, neutrotransmitters

How does LDL and viruses enter animal cells?

receptor-mediated endocytosis

How is endocytosis conducted?

endocytosed macromolecules are sorted in endosomes

after uptake through one of many endocytic pathways

- internalized proteins merge into a common early/sorting endosome

- from there they diverge along distinct sorting pathways

What are the three pathways of endocytosis?

recycling: returned to the same plasma membrane

degradation: to lysosomes

transcytosis: or to a different plasma membrane

During the process of apoptosis (programmed cell death), a cell must signal to the immune system that it needs to be cleared. Which of the following best describes the enzymatic activity that occurs at the plasma membrane to create this "eat me" signal?

Calcium-activated Scramblases randomize lipids, exposing phosphatidylserine (PS) on the outer leaflet.

3 multiple choice options