9/2/14 - Intracellular Compartments

Question 1

What is the function of the endosome?

Answer 1

Acts as a sorting center or “checkpoint” between the cell membrane and the lysosome. Some molecules are processed and sent to the lysosome for destruction, and others are recycled.

Question 2

Which organelle functions in the oxidation of fatty acids and some metabolic intermediates?

Answer 2

Peroxisome

Question 3

Which organelle is responsible for the breakdown of most biomolecules?

Answer 3

Lysosome

Question 4

What is the purpose of the Golgi apparatus?

Answer 4

To process and sort various molecules, especially those destined to be secreted by the cell.

Question 5

What are some functions of the endoplasmic reticulum?

Answer 5

It has several functions, including protein synthesis and protein transport. It is also the site of lipid biogenesis and calcium storage.

Question 6

What is the difference between a signal peptide and a signal patch?

Answer 6

A signal peptide is normally a sequence at the N-terminal, a signal patch is usually a combination of internal amino acid sequences that contribute to the folding of a protein.

Question 7

What is the purpose of signal peptides and signal patches?

Answer 7

These “tag” proteins and allow cellular machinery to direct them to the appropriate compartment.

Question 8

What features of signal peptides and signal patches direct proteins to specific compartments?

Answer 8

Amino acid sequence and folded structure.

Question 9

Signal peptides with a hydrophobic center are typically directed to which organelle?

Answer 9

Endoplasmic reticulum

Question 10

Signal peptides that are amphipathic alpha helices are typically directed to which organelle?

Answer 10

Mitochondria

Question 11

What protein helps regulate transport of macromolecules through the nuclear pore?

Answer 11

Ran

Question 12

What effect does protein size have on nuclear uptake?

Answer 12

Larger proteins require nuclear import signals or they will not pass through the pore.

Question 13

Describe how Ran facilitates the transport of proteins across nuclear pores.

Answer 13

RanGEF (guanine nucleotide exchange factor) phosphorylates Ran-GDP to form Ran-GTP inside the nucleus. The Ran-GTP gradient across the nuclear membrane powers transport of proteins across the nuclear pore. In the cytosol, RanGAP converts Ran-GTP to Ran-GDP before being transported back into the nucleus in GDP form.

Question 14

How does the function of Ran-GTP differ between nuclear export and nuclear import?

Answer 14

1. Nuclear import: Ran-GTP stimulates dissociation of cargo from the import receptor.
2. Nuclear export: Ran-GTP stimulates the association of cargo with the export receptor.

Question 15

How can the regulation of nuclear import lead to changes in gene expression?

Answer 15

Transcription factors have to enter the nucleus before they can activate gene expression. A disruption in nuclear import could alter gene expression depending on which TFs were affected.

Question 16

What are the four main structural elements of the mitochondrion?

Answer 16

1. Outer membrane
2. Intermembrane space
3. Inner membrane
4. Matrix space

Question 17

Which membrane complex facilitates transport of proteins through the outer mitochondrial membrane?

Answer 17

TOM (translocase of outer membrane)

Question 18

Which membrane complex facilitates transport of proteins into the mitochondrial matrix?

Answer 18

TIM23 (translocase of inner membrane)

Question 19

What role do cytosolic chaperone proteins play in mitochondrial protein import?

Answer 19

Cytosolic Hsp70 bind to unfolded cargo proteins and “push” them through the mitochondrial membranes when powered by ATP hydrolysis.

Question 20

How are transported proteins re-folded once inside the mitochondria? (2 related proteins)

Answer 20

1. Chaperone Hsp70 stabilizes unfolded regions of a protein while the rest of the protein folds up.
2. Chaperone Hsp60 forms a cage around the unfolded protein which protects it while it re-folds.

Question 21

True/False: Peroxisomes perform most oxidation reactions within the cell.

Answer 21

False. Mitochondia perform the most oxidation reactions.

Question 22

How are peroxisomes differentiated from mitochondria in EM imaging?

Answer 22

Peroxisomes lack inner cristae (inner folds of the mitochondrion) and have a dense core.

Question 23

What is the main function of the rough ER?

Answer 23

Contains many ribosomes, therefore involved in protein synthesis.

Question 24

Proteins translated on the ER typically end up in which two locations?

Answer 24

Membrane or secreted from the cell.

Question 25

What feature of proteins directs them through the ER?

Answer 25

Signal peptides.

Question 26

What are the main steps in co-translational transport?

Answer 26

1. SRP identifies a signal peptide and transports transcript to an ER membrane-bound ribosome.
2. ER ribosome continues translation, and now the protein product is transported into the ER lumen instead of the cytosol.

Question 27

How are multipass membrane proteins able to be inserted into the ER membrane?

Answer 27

They contain multiple stop/start transfer sequences.

Question 28

What is an important 7-transmembrane protein in cell signalling?

Answer 28

G-protein coupled receptor

Question 29

Describe, in general terms, how proteins are glycosylated in the lumen of the ER.

Answer 29

Sugars are added to asparagine residues (mostly) of ER proteins by enzymes.

Question 30

What are some functions of protein glycosylation?

Answer 30

1. To control ER protein folding
2. To protect ER proteins from degradation
3. To operate as a part of a sorting signal

Question 31

How does a cell respond to overwhelming protein levels within the ER?

Answer 31

Sensor molecules recognize over-abundance of proteins and activate to form a ribonuclease. Ribonuclease splices some mRNA to create a TF which promotes more chaperone expression. Higher chaperone levels allow ER to keep up with high levels of protein folding.