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
Is energy required for targeting of lysosomal proteins? Are the proteins folded or unfolded during transfer?
Yes
Folded in a vesicle
What is the signal found in proteins that are targeted to the mitochondria? What is a feature of this sequence? Where on the protein is it found?
MTS
N terminus
Amphipathic
Are proteins for the mitochondria folded or unfolded during transfer?
Held (partially) unfolded by chaperone proteins
How is a mitochondrial protein moved to the mitochondria from the cytoplasm?
MTS binds to an import protein on the outer mitochondrial membrane
Protein is fed through the TOM protein on the outer mt membrane
Protein is fed through the TIM protein on the inner mt membrane
MTS cleaved by matrix peptidase
Protein folds in matrix
Is energy required in the targeting of proteins to the mitochondria?
Yes - hydrolysis of ATP for association of chaperone proteins
What is the signal found on proteins targeted for the nucleus? What composition does it have? Where is it found?
NLS
Basic amino acids
Can be found anywhere in protein
What are two examples of basic amino acids found in the NLS?
Lysine, arginine
What happens in the targeting of a protein to the nucleus?
Importin binds to the NLS of cargo protein, moves protein into the nucleus
Ran-GTP binds to importin in the nucleus
Displacement of the protein in the nucleus
Ran-GTP/Importin complex recycled to the cytoplasm
Does the targeting of proteins to the nucleus require energy?
Yes - hydrolysis of GTP
Is the NLS cleaved or retained? Why?
Retained, to allow protein to re-enter the nucleus after it breaks down during cell division
On which ribosomes are membrane proteins synthesised?
Ribosomes attached to the ER
What is the difference between regulated and constitutive secretion? Give an example of each.
Constitutive = constant secretion (e.g collagen from fibroblasts)
Regulated = at specific times, controlled (e.g. Endocrine, exocrine, neurocrine cells)
What do the ‘pre’ and ‘pro’ parts of a protein mean?
E.g. Preproalbumin
Pre = refers to part of the protein (signal sequence) that is cleaved during processing
Pro = referees to partially mature protein
Which receptor recognises the signal sequence? What is the composition of this receptor? What organelle does the receptor recognise?
SRP
6 proteins and a short piece of RNA
Ribosome
The signal sequence is found on cells destined for the ______
ER
For secretory proteins that need to enter the ER, how does translation begin? How is the protein produced?
Translation begins at a ‘free’ ribosome
Signal sequence is is produced and is recognised by SRP, binds
Translation stops
SRP binds to ribosome
This complex binds to the SRP receptor on the ER membrane
Translation begins
GTP hydrolysis
SRP falls off
Protein is made through the ER membrane into the lumen + folds
What do membrane proteins contain in their sequence that affects translation at ER ribosomes?
Stop transfer sequences
What is the function of stop transfer sequences in membrane proteins?
They halt the transfer of the peptide across the ER membrane
Acts as an anchor to hold the protein in position
What are some functions of the ER?
Specific proteolytic cleavage
Glycosylation
Formation of disulphides bonds
Hydroxylation of selected lysine and proline residues etc.
Where does n linked glycosylation take place? What is it?
In the ER
Sugars are added onto an asparagine side chain (NH group)
What is glycosylation important for in proteins?
Proper folding of proteins
Stability of proteins
Allows interactions with other molecules
Which enzyme in the ER ensures the correct disulphide bonds are formed?
Protein Disulphide Isomerase (PDI)
Misfolding of proteins in the ER can be corrected by which proteins?
ER chaperone proteins
If protein misfolding cannot be resolved by chaperone proteins in the ER, what happens to the protein? (2)
Retained in the ER
Returned to the cytosol for degradation
What modifications take place in the golgi?
Glycosylation
Sulfation
Removal of mannose
Where does O linked glycosylation occur?
Golgi
What is o linked glycosylation? In which molecule does o linked glycosylation play an important role?
Attachment of sugar to hydroxyl groups
Proteoglycans
