protein sorting in ER

Question 1

co translation

Answer 1

dont finish making the portein in the cytosol and translation continues into the ER- protein synthesis starts before synth is complete

Question 2

SRP

Answer 2

single recognition particle

Question 3

what does sorting rely on

Answer 3

a signal in the protein to know it goes in the ER and a receptor for that signal

Question 4

what proteins are destined fo routside the cell

Answer 4

secreted protiens and plasma membrane proteins

Question 5

targeting to the ER

Answer 5

proteins sorted before synthesis is complete and translated directly into the endoplasmic reticulum

-proteins destined for outside of the cell

Question 6

where is the ER sorting signal

Answer 6

usually at the N terminus

Question 7

how long is the ER sorting signal

Answer 7

10-30 aa long

Question 8

structure of the ER sorting signal

Answer 8

terminal portion usually includes charged amino acids (hydrophilic)
middle region 6-12 consists of hydorphobic amino acids

at the signal cleavage site, it is sometimes charged

Question 9

why is size of the protein importantn

Answer 9

bc when the protein is being translated, the start of the chain is covered by the ribosome

translation continues for about 100 aa and about 70 of them are hidden inside the LARGE SUBUNIT first aa appearing in cytosol when the peptide is 70 aa long

Question 10

when does the entire signal appear in the cytosol to be accessed by a receptor

Answer 10

when the peptide is 82-100 aa long

Question 11

targeting to the ER steps 1-3 (translocon closed)

Answer 11

1. translation begins and signal sequence enters the cytoplasm
2. signal recognition particle binds the signal sequence and also binds ribosome
   - translation is halted and SRP facilitates moevement of complex to the ER
3. SRP associates with SRP receptor present in the ER membrane
   -SRP receptor is associated with a closed or possibly not assembled translocon
   -SRP and SRP receptor bind GTP
   -translocon remains closed until interaction with ribosome

Question 12

targetting to the ER steps 4-5

Answer 12

4. ribosome shunted onto translocon
   -positioned with the exit point directly over the translocons opening
5. SRP released allowing translation to continue
   binding to the ribosome results in conformational change opening the channel

Question 13

where does post translational translocation occur

Answer 13

in yeast

Question 14

what does sorting of membrane proteins require

Answer 14

insertion into the ER membrane and topogenic sequences

Question 15

what are topogenic sequences

Answer 15

N temrinal signal sequences
internal stop transfer anchor sequences
internal signal anchor sequences

Question 16

some cell surface proteins are initially synthesized as….

Answer 16

transmembrane protiens and transferred to a GPI anchor

Question 17

what is the membrane spanning region of the anchoringprotiens in the ER membrane created by

Answer 17

a stop trnasfer anchor

Question 18

what is a stop trnasfer anchor

Answer 18

hydrophobic region much like the original signal

Question 19

what determines the orientation of ER membrane proteins

Answer 19

topogenic sequences

Question 20

type I

Answer 20

stop transfer anchor within the growing peptide

c terminus on the cytosolic side
n terminus on the ER lumen side

has N terminal signal sequence

Question 21

type II

Answer 21

lack N terminal signal sequence

+ charged region oreints N terminal portion in cytosol

internal hydrophobic signal anchor acts as both an ER signal sequence and membrane anchor

Question 22

type III

Answer 22

similar to type II
lack signal sequence
+ charged region
signal anchor

+charged region is after the signal anchor resulting in c terminus in the cytoplasm (reverse of type II)

Question 23

tail anchored protiens

Answer 23

lack N terminal signal sequence

has hydrophobic c terminus tail

tail not available until synth is complete and protein is released from ribosome

Question 24

type IV

Answer 24

multiple signal anchors resluting in multiple membrane spanning regions

+ charged region determines orientation of N terminus

Question 25

how are GPI anchored proteins interted

Answer 25

like a type I proteins followed by cleavage oft the precursor protein by a transamidase and covalently links the new c terminus to a termnial amino group of preformed GPI anchor

Question 26

fact about GPI anchor

Answer 26

its amphipathic with a hydrophobic end and a polar end near the N terminus

Question 27

what does GPI stand for

Answer 27

amphipathic glycosyl phosphatidyl inisitol molecule