Post translation modification of proteins. Flashcards
What proteins are synthesised on ribosomes on the RER?
Those destined for secretory pathway
Membrane proteins
What proteins are synthesised on cytosolic ribosomes?
Proteins that remain in the cytosol
Proteins that are transported into organelles post translation
What is the targeting sequence of a protein destined for the lumen of the RER?
One or more basic amino acids followed by 6-12 hydrophobic amino acids, located at the N terminus.
Is the signal for a protein destined for the lumen of the ER cleaved?
Yes
What is the signal for a protein destined for the mitochondrial matrix?
Amphipathic helix of 20-50 residues - hydrophobic side and side containing arginine and lysine (charged) at the N terminus
May also have internal ‘stop transfer’ - hydrophobic.
Is the signal that directs a protein into the mitochondria cleaved?
Yes.
What signal directs a protein into the peroxisome?
S K L at C terminus (serine, lysine, leucine)
Is the signal that targets a protein to the peroxisomes cleaved?
No
What signal targets a protein to the nucleus?
An internal signal - either a cluster of five basic amino acids or two smaller clusters separated by roughly ten amino acids. Must be on the surface of the folded protein.
How do proteins arrive in mitochondrial matrix?
Maintained unfolded by chaperones (MSF, mitochondrial import stimulation factor)
Signal binds receptor
Fed through pore in outer membrane (translocase of the outer membrane, TOM), and through adjacent channel in inner membrane (translocase of the inner membrane, TIM)
Target signal cleaved by matrix processing protease
Is the signal that targets a protein to the nucleus cleaved?
No
Is energy required for targeting proteins to the mitochondrial matrix, and if so what for?
MSF (mitochondrial import stimulating factor) uses ATP to maintain the protein in an unfolded form.
mHsp70 (mitochondrial heat shock protein 70) uses ATP hydrolysis to drive translocation.
What causes pyruvate dehydrogenase deficiency?
Mutation at codon 10 of nuclear matrix targeting signal (N-MTS) of the E1alpha subunit
Results in arg to pro substitution
Removal of one hydrophobic residue from hydrophobic face of amphipathic helix
Proline is a helix breaker.
Reduced uptake of protein into mitochondrial matrix.
How are proteins transported to the nucleus?
Folded cargo protein with nuclear localising signal (NLS) binds importin (carrier protein)
Enters nucleus via nuclear pore
Ran-GTP binds importin, cargo protein displaced by conformational change, importin bound to Ran-GTP recycled to cytoplasm.
Hydrolysis of GTP releases Ran-GDP, Pi and importin.
What is the point of retaining the signal sequence in a protein targeted to the matrix?
Allows re-importing when nucleus is reformed following cell division.
What is Swyer syndrome?
Loss of mutation of nuclear localising signal (NLS) means sex determining region Y (SRY) protein not targeted to nucleus.
Required for testis differentiation.
XY but outwardly female.
What causes:
Leri-Weill dischondrosteosis
And
Langer mesomelic dysplasia?
Mutation in nuclear localisation signal of SHOX transcription factor.
SHOX required for skeletal development - leads to short stature.
How are proteins targeted to peroxisomes?
Peroxisomal import receptor binds folded proteins with peroxisome targeting sequence.
Peroxisomal import receptor integrates with translocon and opens it.
Peroxisome targeting sequence dissociates.
Receptor returns to cytoplasm - requires hydrolysis of ATP.
What is rhizomelic chrondrodysplasia punctata?
Mutation in Pex7 (receptor for a set of peroxisomal enzymes)
What cells undergo regulated secretion?
Neurocrine cells - neurotransmitters
Endocrine cells - hormones
Exocrine cells - digestive juices
How are organelles distributed in a polarised cell such as a pancreatic acinar?
Nucleus and RER at basal end of the cell.
Secretory granules closer to lumen of the duct.
What is SRP?
Multidomain riboprotein
Mediates a three way interaction between a receptor in the ER, the ribosome and the signal peptide.
Allows proteins to enter the lumen of the ER as they are being synthesised.
How does a protein targeted to the ER arrive in the lumen?
Ribosome starts to synthesise protein. Signal recognised by SRP (signal recognition particle)
Binds SRP receptor on ER membrane along with 2 GTP molecules.
Protein is synthesised through the translocon.
Signal sequence cleaved by signal peptidase.
Protein is folded in the lumen.
Is energy required for translocation of proteins into the lumen of the ER?
Yes - GTP is cleaved to release SRP
How are membrane proteins targeted?
Same as for proteins destined for lumen of ER.
Then a second hydrophobic sequence (stop transfer anchor sequence) prevents the protein moving any further into the lumen and hence anchors it in the plasma membrane.
List some (7) functions of the ER.
Hydroxylation of lysine and proline Glycosylation Insertion of proteins into membranes Formation of disulphide bridges Folding of proteins Proteolytic cleavage Assembly of multisubunit proteins