protein sorting in ER Flashcards
co translation
dont finish making the portein in the cytosol and translation continues into the ER- protein synthesis starts before synth is complete
SRP
single recognition particle
what does sorting rely on
a signal in the protein to know it goes in the ER and a receptor for that signal
what proteins are destined fo routside the cell
secreted protiens and plasma membrane proteins
targeting to the ER
proteins sorted before synthesis is complete and translated directly into the endoplasmic reticulum
-proteins destined for outside of the cell
where is the ER sorting signal
usually at the N terminus
how long is the ER sorting signal
10-30 aa long
structure of the ER sorting signal
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
why is size of the protein importantn
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
when does the entire signal appear in the cytosol to be accessed by a receptor
when the peptide is 82-100 aa long
targeting to the ER steps 1-3 (translocon closed)
- translation begins and signal sequence enters the cytoplasm
- signal recognition particle binds the signal sequence and also binds ribosome
- translation is halted and SRP facilitates moevement of complex to the ER - 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
targetting to the ER steps 4-5
- ribosome shunted onto translocon
-positioned with the exit point directly over the translocons opening - SRP released allowing translation to continue
binding to the ribosome results in conformational change opening the channel
where does post translational translocation occur
in yeast
what does sorting of membrane proteins require
insertion into the ER membrane and topogenic sequences
what are topogenic sequences
N temrinal signal sequences
internal stop transfer anchor sequences
internal signal anchor sequences
some cell surface proteins are initially synthesized as….
transmembrane protiens and transferred to a GPI anchor
what is the membrane spanning region of the anchoringprotiens in the ER membrane created by
a stop trnasfer anchor
what is a stop trnasfer anchor
hydrophobic region much like the original signal
what determines the orientation of ER membrane proteins
topogenic sequences
type I
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
type II
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
type III
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)
tail anchored protiens
lack N terminal signal sequence
has hydrophobic c terminus tail
tail not available until synth is complete and protein is released from ribosome
type IV
multiple signal anchors resluting in multiple membrane spanning regions
+ charged region determines orientation of N terminus
