Protein sorting Flashcards
What is protein sorting?
The transport of proteins to their specific destination
What are the possible destinations of proteins synthesised by free ribosomes?
Cytosol
Nucleus
Mitochondria
Peroxisome
What are the possible destinations of proteins synthesised by ribosomes on the ER?
ER
Golgi apparatus
Lysosome
Plasma membrane
Secretory vesicle
What are the requirements of protein sorting?
Signal in the protein
Receptor for signal
Translocation machinary
Energy
What is the signal for peroxisome proteins?
Peroxisome targeting sequence (PTS)
What is the receptor for the signal in peroxisome proteins?
Pex5
Where does Pex5 bind to PTS in the cell?
In the cytoplasm
What is the translocation machinery for peroxisome proteins?
Pex proteins form channel
across peroxisomal membrane
How are peroxisome proteins transported into the peroxisomes?
Pex channel binds to Pex5-protein complex
peroxisome protein enters peroxisome
How is Pex5 recycled?
ATP hydrolysis releases energy for Pex5 to be released back into cytoplasm
What is the signal for mitochondrial proteins?
Mitochondrial targeting signal (MTS)
What happens to mitochondrial proteins in the cytoplasm?
Kept unfolded by chaperone proteins
using energy from ATP hydrolysis
What is the translocation machinery for mitochondrial proteins?
Transporter on outer membrane (TOM)
Transporter on inner membrane (TIM)
Chaperone proteins
How are mitochondrial proteins transported into the mitochondrial matrix?
Pulled through TOM and TIM by chaperone proteins in the mitochondrial matrix
using energy from ATP hydrolysis
What happens to mitochondrial proteins in the mitochondrial matrix?
MTS is cleaved off
protein folds up
What is the signal for nuclear proteins?
Nuclear targeting sequence (NTS)
What is the receptor for the signal in nuclear proteins?
Importin
What is the translocation machinery for nuclear proteins?
Importin
Ran-GTP
How are nuclear proteins transported into the nucleus?
Importin binds to NTS
complex moves through nulear pore into nucleus
Ran-GTP binds to importin
causes release of nuclear protein into nucleus
How is importin recycled?
Ran GTP-importin complex leaves nucleus through nuclear pore
Ran-GTPase hydrolyses Ran-GTP into Ran-GDP and Pi
causes release of importin
What is the molecular cause of Swyer syndrome?
Defect in NTS in SRY protein
What is the genotype and phenotype of Swyer syndrome?
Genotype - XY
Phenotype - female
Why do individuals with Swyer syndrome appear female despite an XY sex chromosome combination?
SRY protein not transported to nucleus
doesn’t bind to promoter regions of genes to initiate male sex development
What is the secretory pathway?
Proteins passing through ER, Golgi
then on to secretory vesicles, lysosomes, membrane proteins etc.
What is the signal for proteins in the secretory pathway?
Signal sequence
What is the signal sequence called in proteins?
Given prefix pre-
What is the receptor for the signal in secretory proteins?
Signal recognition particle (SRP)
What is the signal recognition particle made up of?
Proteins
RNA
What does the signal recognition particle bind to?
Signal sequence
Ribosome
What happens when SRP binds to the signal sequence and a ribosome for secretory proteins?
SRP-signal sequence-ribosome complex binds to SRP receptor in ER membrane
What happens when the SRP-signal sequence-ribosome complex binds to SRP receptor in ER membrane?
Ribosome synthesises the rest of the secretory protein into the ER lumen
What happens to secretory proteins inside the ER lumen?
Signal sequence is cleaved off by signal peptidase
secretory protein is entirely produced
secretory protein folds up
What happens to the membrane protein in the ER lumen and membrane?
Signal sequence is cleaved off by signal peptidase
stop-transfer sequence of amino acids remains in ER membrane
rest of protein synthesised remains outside of the ER membrane
What shape does the stop-transfer sequence in membrane proteins form?
Alpha helix
What part of the membrane protein does the stop-transfer sequence form?
The transmembrane spanning region
What happens to the ribosome when it has finished producing the secretory protein?
Dissociated from ER membrane
Where are secretory proteins transported to from the ER?
Golgi
How is SRP receptor recycled?
Dissociates using energy released from hydrolysis
What is the signal of ER proteins?
KDEL sequence
What is the receptor for the signal in ER proteins?
KDEL receptor
Where is the KDEL receptor located?
Golgi membrane
How are ER proteins transported from the Golgi back to the ER?
KDEL sequence binds to KDEL receptor in Golgi membrane
KDEL receptor is bound to COP1 coat outside of Golgi membrane
Where do vesicles with a COP1 coat from the Golgi
travel to?
Go back to the ER
How is the KDEL receptor recycled?
Change in pH in ER
KDEL receptor loses affinity for KDEL sequence
so it dissociates
returns to the Golgi
What is the signal for lysosomal proteins?
Mannose-6-phosphate
When is the signal for lysosomal proteins formed?
Formed in the Golgi
How is the signal for lysosomal proteins formed?
Enzymes add phosphate to mannose residues in lysosomal protein
using energy from hydrolysis
How are lysosomal proteins targeted for the formation of mannose-6-phosphate signal?
Have a signal patch - a group of amino acids forming a recognisable site
How are lysosomal proteins transported from the Golgi to lysosomes?
In Golgi, M6P binds to M6P receptors in membrane
vesicles which bud off here travel to lysosomes
How are M6P receptors recycled?
Acidic pH in lysosome causes dissociation of M6P and M6P receptors
M6P receptors bud off in vesicles that travel back to the Golgi
What happens to lysosomal proteins in lysosomes? Why?
Phosphatase enzymes remove phosphate from M6P
To prevent it from binding to M6P receptors again
What is the molecular cause of I-cell disease?
Defect in enzyme that phosphorylates mannose residues in lysosomal proteins to form M6P signal
How are lysosomes affected in I-cell disease?
Lysosomes lack lysosomal proteins e.g. enzymes
become bloated with undigested material
