Session 6 Flashcards
What is required for protein sorting to occur?
A signal intrinsic to the protein, a receptor that recognises the signal and directs the protein to the correct membrane, a translocation machinery, energy to transfer the protein to its new location.
Describe the signal of a protein targeted to the ER lumen.
Signal is located at the N-terminus and is removed. Signal is a small chain of hydrophobic amino acids preceded by a basic amino acid.
Describe the signal of a protein targeted to the mitochondrial matrix.
Signal located at the N-terminus and is removed after entry into the mitochondrial matrix. Signal consists of an amphipathic helix with hydrophobic sides.
Describe the signal of a protein targeted to a peroxisome.
Signal located at the C-terminus and isn’t removed. Signal usually S-K-L.
Describe the signal of a protein targeted to the nucleus.
Signal is internal and isn’t removed. Signal is usually a cluster of 5 basic amino acids or 2 smaller clusters separated by 10 amino acids.
How are proteins kept unfolded?
By using chaperones.
What causes a pyruvate dehydrogenase deficiency?
Mutation in the helical signal causes an Arg to Pro substitution which breaks the helix. Causes reduced uptake into the mitochondria.
How does nuclear import of proteins occur?
Importin binds to the signal on the protein and the complex enters the nucleus. Ran-GTP binds to importin and displaces the protein. Importin bound with Ran-GTP is recycled into the cytoplasm. Energy for the process is provided by the hydrolysis of GTP.
How are proteins imported into the peroxisomeal matrix?
Import receptors bind to the signal sequence on the protein. The protein remains folded and the receptor integrates into the translocon to open it. The signal sequence dissociates from the receptor after the protein has entered the peroxisome and the receptor returns to the cytosol. Detaching of the receptor requires energy from ATP hydrolysis.
How are substances secreted from polar secretory cells?
Secreted only at one end of the cell.
How are membrane proteins delivered?
N-terminal signal enters a translocon while attached to a ribosome. N- terminal signal sequence is cleaved by a signal peptidase while the rest of the protein moves through the translocon. A hydrophobic stop-transfer anchor sequence anchors the protein to the membrane to prevent further transfer to the ER lumen.
What processes occur in the ER?
Insertion of proteins into membranes. Specific proteolytic cleavage. Glycosylation. Formation of Disulphide bonds. Folding of proteins. Assembly of multisubunit proteins. Hydroxylation of selected Lys and Pro residues.
What is the function of glycosylation?
Ensure correct protein folding and protein stability. Facilitates interactions with other molecules.
What causes congenital disorders of glycosylation?
Deficiencies in N-linked glycosylation.
What is N-linked glycosylation?
Addition of sugars into an asparagine side chain.
What do peptidyl-prolyl isomerases do?
Accelerate interconversion of cis and trans isomers of proline residues. This must occur during folding in several proteins.
What is the role of protein Disulphide isomerase?
Assist in the formation of Disulphide bonds.
How is protein Disulphide isomerase retained in the ER and not secreted?
KDEL receptor forms a transmembrane link between the protein and COPI-coat to anchor it within the ER.
What results from protein folding problems?
Protein may be trapped in misfolded conformation.
Protein may be incorrectly associated with other subunits.
How can unfolded proteins be corrected?
BiP, calnexin and calreticulin (chaperones) can recognise and bind to misfolded proteins. This causes them to be retained in the ER.
What results from a protein misfolding that isn’t corrected?
Protein may be returned to the cytosol for degradation or may accumulate to toxic levels in the ER and cause disease.
What signal is required for delivery of enzymes to a lysosomes from the Golgi?
Mannose-6-phosphate.
What enzymes are required to form a mannose-6-phosphate signal?
N-acetyl glucosamine phosphotransferase and a phosphodiesterase.
What is O-linked glycosylation?
Attachment if sugar to a hydroxyl group on serine or threonine.
