Flashcards in Protein Targetting Deck (34):
When ribosomes are attached to the ER, what is the destination of their secretion?
Either secretion or for the membrane.
What are the four components necessary for protein sorting?
A signal, a receptor, translocation machinery and energy to transfer the protein.
What is the signal found on a protein destined for the ER?
6-12 hydrophobic amino acids at the N terminus of the protein. This signal is cleaved after utilisation.
A protein with an amphipathic helix, and hydrophobic side chains found at the N terminus of the protein is destined for....
What is the signal for a protein targeted to the nucleus?
5basic amino acids or 2 small clusters 10 base pairs apart, internal on the protein. This is not cleaved.
How do proteins destined for the mitochondrial matrix enter this organelle?
They are kept unfolded by chaperones. They pass through a channel in the outer membrane and then an adjacent channel in the inner membrane. Here a protease cleaves off the signal.
In terms of protein targeting, explain Pyruvate dehydrogenase deficiency.
A point mutation on the gene means that there is a proline which breaks the helix meaning there is no signal. This means it is not taken up into the mitochondria, and so there is therefore deficiency. This leads to a build up of lactic acid which can lead to neurological problems.
What is the name of the carrier for proteins into the nucleus?
In transport of proteins into the nucleus, what displaces the NLS cargo from importin?
Ran-GTP. This is then hydrolysed in the cytoplasm so that importin can pick up more NLS containing cargo.
What is SRP?
SRP is a multidimensional riboprotein which mediates association between receptor, ribosome and signal peptide. This is present in the ER.
What are the two methods of secretion from the ER?
Constitutive pathway, regulated secretion.
Describe the process of proteins being targeted to the ER.
Protein synthesis begins in the cytoplasm, and then the SRP receptor recognises signal and stops synthesis. The unit is activated and GTP is hydrolysed which opens translocon. The signal is cleaved and the protein is delivered through the channel, before it folds in the ER.
Name the enzyme which cleaves the signal in protein targeting to the ER.
What happens in membrane synthesis?
A stop transfer anchor sequence anchors the protein in the membrane before the rest of the protein is synthesised in the cytoplasm.
What is the importance of glycosylation of proteins? Where does this take place?
This leads to correct protein folding, protein stability, and facilitates interaction with other molecules. This takes place in the ER
What is peptidyl-prolyl isomers see and why are they necessary?
They accelerate conversion from trans to cis isomer of proline and therefore are necessary for the correct folding of proteins.
What is KDEL?
It is a molecule which moves proteins to be retained in the ER back to the ER after they have moved to the Golgi.
Explain how KDEL accepts and released proteins.
At the Golgi, pH is low and so the protein is taken up with high affinity. At the ER, pH is high and so affinity is lower and protein is released.
What are the three causes of misfolding?
Protein trapped in mis folded shape, protein contains a mutation which leads to misfolding, protein is incorrectly associated with other subunits.
What is the purpose of chaperones in the ER?
These try to rectify misfolding, and also ensure that the protein is retained in the ER and a mis folded protein is not secreted.
When lysosomal enzymes pass through the Golgi, what is added as the signal?
Phosphate group is added to the hydroxyl group of the mannose sugar.
What happens if there is deficiency in the enzyme which gives lysosomal enzymes their signal?
This is fatal because undegraded material causes bloated lysosomes.
Where does O linked glycosylation occur?
In the Golgi. Sugar attaches to the hydroxyl group of Serine/Threonine and this is important in proteoglycans.
By what secretory pathway is collagen secreted?
What is the basic unit of collagen?
Describe the structure of collagen.
It is made up of 3 alpha chains which each have XYGly as their amino acid sequence. This is stabilised by hydrogen bonds.
Where does assembly of collagen fibres occur?
Outside of the cell. Otherwise this would be a destructive process.
What modification occurs to collagen in the ER?
Signal peptide is cleaved, some proline and lysine are hydroxylated, addition of N linked oligosaccharides and addition of galactose to hydroxylysine residues.
What is the function of prolyl hydroxylase in collagen synthesis?
It allows hydrogen bonds to form which stabilise the triple helix.
How are collagen molecules structured?
They connect to form fibrils and these aggregate to form fibres.
In collagen synthesis, what is the action of Lysyl oxidase?
It oxidises NH3 groups so that covalent cross linkages can form.
Explain how insulin is synthesised.
Insulin is synthesised in the ER and it folds with two disulphide bonds forming. Post Golgi this connecting peptide is cleaved (peptideC). This is used as a biochemical marker as to how much insulin our bodies are producing.
Where is a ribosome located if the protein it synthesised is destined for the cytosol?
In the cytoplasm.