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Flashcards in 09_Synthesis of Membrane Proteins Deck (22)
1

What are trans-membrane segments?

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

2

What assists the proper orientation of transmembrane sequences?

the translocon

3

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

+

4

What allows the TM segment to remain in the bilayer?

the hydrophobic nature of the TM segment

5

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

laterally

6

Where does protein glycosylation occur?

ER lumen

7

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

built on both sides of the ER membrane
the sugar is attached to dolichol-pyrophosphate

8

What amino acid sequence does N-linked glycosylation target?

Asn-X-Ser/Thr

9

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

oligosaccharyl transferase

10

How many terminal glucose residues are on a gycosylated protein?

three

11

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

N-acetylglucosamine
Mannose
GLucose

12

What is overall the role of chaperones in the ER?

ensure proteins are properly folded

13

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

proteosomes
cytoplasm

14

How does Calnexin recognize misfolded proteins?

Calnexin uses glucose as a signal to recognize misfolded glycoproteins.

15

What are BiP and membrane sensor proteins?

chaperones that recognize misfolded proteins

16

How are misfolded glycoproteins tagged?

by a terminal glucose

17

Which enzymes remove glucose residues in quality control of glycoproteins?

Glucosidases (I and II)

18

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

Calnexin

19

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

Glucosidase II

20

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

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

21

How is a misfolded protein marked for degradation?

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

22

What is the role of N-glycanase?

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