Protein Trafficking

Question 0

Is there a difference between free and RER ribosomes?

Answer 0

Nope. None.

Question 1

Where is the signal on a protein for destination located on the protein?

Answer 1

Signal may be a run of 3 to <30 amino acids at N- or C-terminal or internal
Some may be post translational modifications
Some signals may be 3D domains on protein

Question 2

What kinds of proteins are synthesized in free ribosomes?

Answer 2

Proteins destined for peroxisomes, mitochondria (except ones made in mitochondria) and the nucleus

Question 3

What sort of signals are used for proteins to enter the nucleus?

Answer 3

Internal nuclear localization sequences

Question 4

What sort of signal sequences are used for proteins to enter peroxisomes?

Answer 4

C-terminal sequence SKF

Question 5

What sort of signal sequences are used for proteins to be imported into the mitochondrial matrix?

Answer 5

N-terminal sequence rich in positive charged amino acids and serine, threonine

Question 6

What are the 3 major classes of proteins produced in the RER?

Answer 6

Secretory proteins
Lysosomal proteins
Integral membrane proteins

Question 7

How is a signal sequence removed from a secreted protein?

Answer 7

By signal peptidase

Question 8

What are the common signal sequences of N-terminal secretory peptides?

Answer 8

Range in length from 13-26
Amino terminal contains at least one positively charged residue
A hydrophobic 10-15 residue stretch forms the center of the signal
Residue of the signal peptidase cleavage site has small, neutral side chain (alanine common)

Question 9

Where does synthesis of all proteins begin?

Answer 9

By free ribosomes binding to mRNA and commencing synthesis of N-terminal region of polypeptide

Question 10

What is signal recognition particle (SRP)?

Answer 10

An RNA protein compex that recognizes and binds to signal sequences

Question 11

What doe SRP when bound to a signal sequence?

Answer 11

It temporarily arrests translation by the ribosome

Question 12

Can a protein return to the cytosol once it enters the endoplasmic reticulum?

Answer 12

No, entry into ER is irreversible

Question 13

What chaperones assist correct peptide chain folding?

Answer 13

ATP driven heat shock proteins

Question 14

What do N-linked glycoproteins use as a phosphate carrier and how?

Answer 14

They use dolichol embedded in the ER membrane

Question 15

What is dolichol phosphate?

Answer 15

An isoprenoid derivative
A lipid
Localized to membranes

Question 16

What happens to dolichol phosphate as oligosaccharides are synthesized?

Answer 16

On the cytosolic side sugar precursors are added to the phosphate group forming dolichol pyrophosphate intermediate. Once the oligosaccharide is translocated across the ER membrane into the lumen the incomplete core-dolichol pyrophosphate is released as the oligosaccharide is transferred to an Asn residue

Question 17

Where are oligosaccharides synthesized?

Answer 17

Half on cytosol side of ER, half in the ER lumen

Question 18

How is dolichol phosphate regenerated?

Answer 18

Using a phosphatase

Question 19

What are some drugs that affect the addition of sugars?

Answer 19

Tunicamycin - blocks first step in oligosaccharide surface

Bacitracin - blocks phophatase that recycles dolichol phosphate

Question 20

How is bacitracin a useful antibiotic?

Answer 20

The bacterial enzyme used to recycle an isprenoid pyrophosphate in cell wall synthesis is very sensitive to it.

Question 21

How are proteins transferred to the Golgi apparatus from the ER?

Answer 21

Via transport vesicles (transfer vesicles) to the cis side of the Golgi complex

Question 22

What happens in the Golgi apparatus to proteins and carbohydrates?

Answer 22

They are additionally modified. Proteins are then sorted and directed to their destination by transfer vesicles leaving the trans face of the Golgi

Question 23

How is lysosomes targeting conducted?

Answer 23

Phospho-N-acetyl-glucosamine added to mannose by phosphotransferase > phosphotransferase recognizes 3D motif > phosphodiesterase removes N-AC-Gln leaving mannose-6-phosphate > mannose-6-phosphate binds to receptor in Golgi membrane > vesicles targeted to lysosome

Question 24

What is the autosomal recessive disease where individuals are deficient in phosphotransferase?

Answer 24

I-cell disease (mucolipidosis II)

Question 25

What happens in I-cell disease?

Answer 25

Eight acid hydrolases are not targeted to lysosomes but secreted instead which results in glycosaminoglycans and glycolipids build up as inclusions in lysosomes

Question 26

What are the symptoms of I-cell disease?

Answer 26

Severe psychomotor retardation and skeletal deformities

Question 27

How are heat shock proteins returned to the ER after traveling to the Golgi?

Answer 27

They have KDEL (lys-asp-glu-leu) as their return signal which has them be repackaged in vesicles in the Golgi and returned to the ER.

Question 28

What disease results from disorders in receptor mediated uptake?

Answer 28

Some forms of familial hypercholesterolemia

Question 29

What kind of regions of the plasma membrane are many receptors located in?

Answer 29

Coated pits with a thick coat of the protein clathrin at the cytosolic side of the indentation

Question 30

How are endosomes different than other vesicles?

Answer 30

Endosomes are often larger, often irregular vesicles with acidic lumens

Question 31

How does endocytosis of receptors begin?

Answer 31

Invagination of the coated pit triggered by receptor-ligand binding > clathrin then forms a lattice around the coated pit forming a coated vesicle > vesicle loses clathrin shell and fuses with endosome

Question 32

What is the function of transferrin?

Answer 32

Transports iron from sites of absorption and storage to sites of utilization Two Fe3+ ions bind/apotransferrin molecule to make transferrin, the two together bind to the coated pits

Question 33

How is transferrin processed?

Answer 33

Transferrin internalized into endosome then the endosome is acidified which allows Fe3+ to dissociate from apotransferrin which remains bound to the receptor

Question 34

When the receptor is recycled how is Fe3+ recaptured?

Answer 34

By ferritin in the cytosol

Question 35

How does the semliki virus (related to yellow fever) enter cells?

Answer 35

It enter susceptible cells by binding to receptors in coated pits and is endocytosed

Question 36

How does diphtheria toxin enter cells?

Answer 36

By binding to growth factor receptors and being internalized by the endocytosis

Question 37

How are proteins turned over?

Answer 37

Ubiquity in tags proteins for destruction in eukaryotic cells.