09_Synthesis of Membrane Proteins

Question 1

What are trans-membrane segments?

Answer 1

hydrophobic segments in integral proteins that interfere with their transfer into the RER lumen

Question 2

What assists the proper orientation of transmembrane sequences?

Answer 2

the translocon

Question 3

What is the charge on the majority of extramembrane domans facing the cytosol?

Answer 3

+

Question 4

What allows the TM segment to remain in the bilayer?

Answer 4

the hydrophobic nature of the TM segment

Question 5

The TM segment is inserted (laterally/horizontally) into the phospholipid bilayer.

Answer 5

laterally

Question 6

Where does protein glycosylation occur?

Answer 6

ER lumen

Question 7

Where is the core sugar (later to be added to protein) assembled? What lipid is it attached to?

Answer 7

built on both sides of the ER membrane

the sugar is attached to dolichol-pyrophosphate

Question 8

What amino acid sequence does N-linked glycosylation target?

Answer 8

Asn-X-Ser/Thr

Question 9

Which enzyme transfers the oligosaccharyl group to the Asn sidechain of a newly synthesized protein?

Answer 9

oligosaccharyl transferase

Question 10

How many terminal glucose residues are on a gycosylated protein?

Answer 10

three

Question 11

What three types of sugars are in the protein-linked oligosaccharide?

Answer 11

N-acetylglucosamine
Mannose
GLucose

Question 12

What is overall the role of chaperones in the ER?

Answer 12

ensure proteins are properly folded

Question 13

Misfolded proteins are destroyed by ________ in the (cytoplasm/ER lumen).

Answer 13

proteosomes

cytoplasm

Question 14

How does Calnexin recognize misfolded proteins?

Answer 14

Calnexin uses glucose as a signal to recognize misfolded glycoproteins.

Question 15

What are BiP and membrane sensor proteins?

Answer 15

chaperones that recognize misfolded proteins

Question 16

How are misfolded glycoproteins tagged?

Answer 16

by a terminal glucose

Question 17

Which enzymes remove glucose residues in quality control of glycoproteins?

Answer 17

Glucosidases (I and II)

Question 18

After removal of 2 of the 3 glucose residues on a glycoprotein, where does the protein bind?

Answer 18

Calnexin

Question 19

Which enzyme removes the terminal glucose residue after binding of the glycoprotein to Calnexin?

Answer 19

Glucosidase II

Question 20

If, after binding to Calnexin ad removal of the terminal glucose, the protein is still misfolded, what occurs?

Answer 20

Glucosyl transferase (GT or UGGT) adds a terminal glucose again, and the protein goes through a second round of quality control.

Question 21

How is a misfolded protein marked for degradation?

Answer 21

A polyubiquitin tail is added, marking the protein for degradation by the proteosome.

Question 22

What is the role of N-glycanase?

Answer 22

Plays a role in proteosome-mediated degradation of misfolded glycoproteins; removes oligosaccharyl group prior to ubiquitination.