Protein Targeting Flashcards
What is proteolytic cleavage?
Breaking of peptide bonds to remove part of a protein
What is chemical modification of a protein?
Addition of functional groups to the amino acid residues of a protein
On which ribosomes are proteins for lysosomes synthesised?
Ribosomes associated with the rough ER
What is required for protein sorting? (4)
A signal intrinsic to the protein
A receptor that recognises to the signal and directs it to the membrane
Translocation machinery (e.g. Proteins/cell structures to move and allow sorting)
Energy to transfer the protein
What is a peroxisome and its function?
Small spherical membrane enclosed organelle
Contains enzymes for oxidation reactions, e.g. Catalase to break down hydrogen peroxide and the breakdown of very long fatty acids
What signal do proteins targeted to peroxisomes possess? Where on the protein is this located?
Serine-Lysine-Leucine
(PTS)
On c terminus of protein
Are proteins targeted to peroxisomes usually folded or unfolded before import?
Folded
What happens to a protein destined for peroxisome import in the cytoplasm?
Pex5 receptor recognises PTS and binds to cargo protein
13 Pex proteins form a transport channel across the peroxisomal membrane
The transport channel binds to the Pex5-cargo complex - allowing entry into peroxisome
Is energy required during targeting of proteins to peroxisomes?
Yes, hydrolysis of ATP for recycling of Pex5
What signal is found in proteins targeted to the ER? Where on the protein is this found? When and by which protein is the signal recognised?
Signal sequence
N terminus
During translation - SRP
After binding of the SRP how do proteins get imported into the ER?
On the ER membrane there is a translocon (channel) associated with a docking protein and SRP receptor.
SRP receptor binds to SRP/Protein complex
Hydrolysis of GTP - translocon opens
Protein synthesised through ER membrane into lumen
Signal sequence cleaved by signal peptidase
Folded in ER lumen
Is energy required when targeting proteins to the ER?
Yes - hydrolysis of GTP by SRP - opens channel
Is the signal sequence cleaved or retained in protein targeting to the ER? Is it folded or unfolded during transfer?
Cleaved - signal peptidase
During transfer = unfolded
What is the signal for proteins that are retained in the ER? Where on the protein is it found?
KDEL Sequence
Lys-Asp-Glu-Leu
C terminus
How is a protein destined to be retained in the ER, moved out and returned to the ER?
COP2 transports the KDEL protein towards the golgi
KDEL receptors bind to the KDEL sequence in cis golgi
COP1 transports the KDEL sequence back towards the ER
Is a protein destined to be retained in the ER folded or unfolded during transfer? Does it use energy?
Folded
No energy required - pH dependent
What is the signal for proteins that are targeted to the lysosome?
Addition of mannose-6-phosphate in the cis golgi through N linked glycosylation
What happens to the lysosomal protein after Mannose-6-phosphate has been added to it?
Travels through the golgi, binds with M6P receptors in trans golgi
Receptor dependent transport takes place, vesicle moves towards lysosome membrane
There is dissociation of the receptor from the molecule at the acidic pH of a lysosome
Phosphate is cleaved of by phosphatase