Protein Sorting II

Question 1

Common pathway for production and secretion of proteins.

Answer 1

1) Protein synthesis and translocation across ER membrane
2) Protein folding and modifications in the lumen of the ER
3) Protein transport to Golgi, lysosomes, or cell surface in vesicles

Question 2

Organelles a protein might pass through on its way to it’s final destination

Answer 2

Starts with synthesis of polypeptides in the cytoplasm.

Can then go to the ER.
Then it can go through the golgi to Lysosomes or the plasma membrane.

Or it can be a cytosolic protein. It can have a targeting sequence that sends it to mitochondrion (or chloroplasts) or peroxisomes or the nucleus.

Question 3

Pathway from ER to plasma membrane

Answer 3

Rough ER, Cis-golgi network, Cis-golgi, Medial-golgi, trans-golgi, trans-golgi network, secretory vesicle, exocytosed

Question 4

ER signal peptides

Answer 4

typically at the N-terminus, (16-30 aa long,) Contain 6-12 hydrophobic residues (at their core) and are typically flanked by positive amino acids on one side or the other; otherwise their amino acid sequence is not conserved.
Most signal peptides removed in ER.

Question 5

Translation is ____ in proteins targeting the secretory system.

Answer 5

Cotranslational

Question 6

SRP

Answer 6

Signal recognition particle; a ribonucleoprotein complex, comprising 6 proteins and a single 300-nt RNA molecule

SRP binds to the SP on nascent polypeptide in the cytoplasm and stops translation until docked on the ER.

SRP also binds the large ribosomal subunit and the SRP-receptor on the ER

Question 7

What does P-54 do?

Answer 7

Binds ER signal sequence

Question 8

What does P68/P72 do?

Answer 8

It is required for protein translocation

Question 9

What does P9/P14 do?

Answer 9

It interacts with ribosomes

Question 10

SRP receptor

Answer 10

integral membrane protein, heterodimeric complex with alpha and beta subunits. SRP-receptor docks the SRP, ribosome, and nascent polypeptide onto the ER. GTP hydrolysis releases the SRP from the ribosome and allows continued translation and translocation into the ER

Question 11

Both SRP-p54 subunit and the SRP receptor-alpha-subunit are _____.

Answer 11

GTP-binding proteins (but are not GTPases on their own)

Question 12

ER translocon

Answer 12

Sec61alpha (makes a protein-lined pore in the ER membrane)

Question 13

Sec61alpha

Answer 13

makes a protein-lined pore in the ER membrane, comprises 10 membrane spanning alpha-helices that form a pore. At the middle is a constriction or gasket lined with hydrophobic isoleucine residues.
The channel is gated by a peptide plug, so that it is only open during translocation.

Question 14

The ribosome associates directly with the translocon and the polypeptide is translocated across the ER membrane in _____ form.

Answer 14

unfolded

Question 15

Where does the energy for translocation across the ER membrane comes from ____.

Answer 15

polypeptide synthesis / translation

Question 16

A ____ removes the signal peptide soon after translocation into the lumen of the ER.

Answer 16

signal peptidase

Question 17

BiP

Answer 17

Binding protein;
molecular chaperone that facilitates protein import;
Hsc70 chaperone that uses ATP hydrolysis to draw in protein

Question 18

Examples of Type I Integral membrane proteins

Answer 18

Glycophorin, LDL receptor, Influenza HA protein, Insulin receptor, Growth hormone receptor

Question 19

Type I integral membrane protein

Answer 19

Contains a cleavable, N-terminal SP;
Contains 20-25 hydrophobic, transmembrane spanning alpha-helix;
Has second, STA (stop-transfer anchor) sequence;
So two total signals: Signal peptide and STA. Direct orientation

Question 20

Examples of Type II integral membrane proteins

Answer 20

Asiaglycoprotein receptor, transferrin receptor, Sucrase-isomaltase precursor, golgi galactosyltransferase, Golgi sialyltransferase, Influenza HN protein

Question 21

Type II integral membrane protein

Answer 21

Lacks N-terminal signaling peptide, but has an internal signal-anchor sequence.
The signal anchor functions as a signal peptide and a membrane anchor.
Positively charged amino acids adjacent to the signal peptide keep the N-terminus opposite to Type I proteins.

Question 22

Type III integral membrane protein.

Answer 22

Lacks N-terminal signaling peptide, but has an internal signal-anchor sequence.
The signal anchor functions as a signal peptide and a membrane anchor.
Type III integral membrane proteins have positive amino acids on opposite side of transmembrane sequence than type II (adjacent to signal peptide).

Question 23

Example of a Type III integral membrane protein

Answer 23

cytochrome p450

Question 24

Example of a Type IV integral membrane protein

Answer 24

Sec61, Connexin, G protein-coupled receptors, Glucose transporters, Voltage-gated Ca2+ channels, ABC small molecule pumps, CFTR channel

Question 25

Type IV integral membrane protein

Answer 25

Nearly all multipass proteins lack a cleavable signal sequence. Transmembrane sequences alternate as Start and Stop transfer sequences.

Question 26

The positive amino acids in transmembrane proteins assembled in the ER are located in the ____.

Answer 26

Cytoplasm (not in the ER lumen) (Have to check about cell membrane)

Question 27

True or false: Type IVA proteins have the positive aa residues on the N-terminus side.

Answer 27

True (I think, from how I interpreted the slide)

Question 28

Hydropathy plots show ___.

Answer 28

Stretches of hydrophobic regions are given positive values on the graph and can help predict the type and class of integral membrane proteins.

Question 29

Types of membrane-anchored proteins

Answer 29

Type 1, 2, 3, and 4 and GPI-anchored proteins (Glycophosphatidylinositol-anchored)

Question 30

GPI-anchored proteins

Answer 30

synthesized from type 1 proteins by the activity of luminal transamidase

Play roles in signaling, cell polarity, or interactions with extracellular matrix.

Question 31

Post-translational protein modifications that occur in the ER

Answer 31

Proteolytic cleavage, GPI-anchors, N-linked glycosylation (e.g. influenza HA protein), disulfide bond formation, hydroxylation of proline and lysine residues (e.g. procollagen - Ehlers Danos Syndrome, Connective tissue disorders)

A common 14-residue precursor of N-linked oligosaccharides is added to proteins in the RER

Question 32

Functions of the ER (as far as protein synthesis goes)

Answer 32

Some post-translational protein modifications occur there (see previous card), proper protein folding occurs (disulfide bond formation, petidyl prolyl isomerase, lectins (calnexin, calreticulin), unfolded protein response), assemly of multiprotein complexes permitted, misfolded proteins targeted for degredation

Question 33

All mature N-linked glycoproteins contain ___.

Answer 33

the core (GlcNAc)2(Man)3

Question 34

N-linked glycosylation occurs on a subset of ___ residues in appropriate context

Answer 34

Asn (asparagine)

Question 35

dolichol pyrophosphoryl oligosaccharide precursor

Answer 35

generated on ER membrane on a dolichol lipid anchor. This is attached to the polypeptide during translocation and subsequently trimmed of Glc & Man residues
N-linked glycosylation may promote folding and stability of glycoproteins

Dolichol phosphate is a strongly hydrophobic lipid

Question 36

Tunicamycin blocks ___.

Answer 36

synthesis of dolichol pyrophosphoryl oligosaccharide

Question 37

Oligosaccharides can play a role in ___.

Answer 37

Cell-cell adhesion and immune responses.

Question 38

Leukocytes contain ___ that are glycosylated and oligos can interact with a sugar-binding domain in certain ___(same blank)__s on endothelial cells lining blood vessels and this tethers the leukocytes to the endothelium and assists in movement into tissues during inflammatory response to infection.

Answer 38

cell-adhesion molecules

Question 39

The A, B, O blood group antigens are ____ on the surface of erythrocytes and can induce an immune response.

Answer 39

glycoproteins

Question 40

After transfer, glycosidases trim ___ from the oligosaccharide.

Answer 40

3 Glc and 1 specific Man

Question 41

Formation and rearrangement of disulfide bonds is carried out by ___.

Answer 41

protein disulfide isomerase in the RER (rough endoplasmic reticulum)

Question 42

Folding and assembly of hemagglutinin trimer in the ER

Answer 42

BiP, PDI, calnexin, and calreticulin (lectins) work together

Question 43

lectins

Answer 43

carbohydrate-binding proteins

Question 44

Unfolded protein response

Answer 44

Transcriptional activation

BiP binds to unfolded proteins. Ire1 dimer no longer bound to BiP and forms a dimer and acts as an endonuclease that activates transcription

Question 45

What happens to misfolded proteins in the ER?

Answer 45

They are transferred to the cytosol for degradation by the proteasome

Question 46

Emphysema

Answer 46

a hereditary form is caused by a point mutation in alpha1-antitrypsin, secreted by macrophages and hepatocytes. Wild-type alpha1-antitrypsin functions to inhibit trypsin and the blood protease elastase. When alpha1-antitrypsin is missing, elastase degrades the fine tissue in the lung that participates in the absorption of oxygen, eventually producing symptoms of emphysema.
Mutant alpha1-antitrypsin is synthesized in rER, but doesn’t fold properly and forms crystalline inclusions. This also impairs the secretion of other proteins in hepatocytes.

Question 47

Diseases related to ER stress

Answer 47

Alzheimer’s disease, Parkinson’s disease, Amyotrophic lateral sclerosis, Type 2 diabetes, Atherosclerosis, Nonalcoholic fatty liver disease, HCV and HBV infection (hepatitis), Alcholic liver disease, Cancer