3.3 Protein targetting to endoplasmic reticulum

Question 1

what are the two types of proteins made at the ER

Answer 1

Secretory proteins – water soluble
Membrane proteins – embedded in lipid bilayer

Question 2

common feature of ER signal sequence

Answer 2

stretch of 8+ hydrophobic amino acid residues usually near N-terminus
e.g.. leucine, isoleucine, valine

Question 3

what happens to ribosomes synthesising proteins without a signal sequence

Answer 3

remain free in cytosol

Question 4

what happens to ribosomes synthesising proteins with an ER signal sequence

Answer 4

directed to ER WHILST PROTEIN IS BEING MADE (co-tranlocation)

Question 5

what binds to the ER signal sequence as it emerges from ribosome?

Answer 5

cytosolic SRP

Question 6

what is SRP

Answer 6

signal recognition particle

Question 7

what does SRP allow when it binds to ER signal sequence

Answer 7

directs it to SRP receptors in ER membrane
then hands over the protein to the protein tranlocatior

Question 8

how is polypetide transferred in ER lumen

Answer 8

threaded through protein translocator channel as a loop whilst it’s being translated

Question 9

does ER signal sequence get carried thorugh into ER lumen with mature protein?

Answer 9

no, cleaved off by signal peptidase
different to nucleus remember

Question 10

name of protein translocator in ER membrane

Answer 10

Sec61

Question 11

how is GTP involved in ER protein targetting

Answer 11

SRP are GTPases