Protein Trafficking and Degradation Flashcards
How are proteins whose final destination is beyond the ER transported?
In vesicles
Where does protein synthesis begin?
At ribosomes in the cytosol
What are the steps in the default secretory pathway?
Protein synthesis begins at ribosome in ER
Protein is taken to the ER and then the Golgi
Protein is transported in a vesicle from the Golgi to the plasma membrane and released outside if the cell
Proteins destined for the plasma membrane or lysosome, following a variation if which pathway?
The secretory pathway
Proteins destined for the nucleus, mitochondria or peroxisome follow a variation of which pathway?
The cytosolic pathway
How do proteins following the secretory pathway get to the ER?
These proteins have a signal peptide which corresponds to a signal recognition particle (SRP) which will pick up the protein and take it to the ER. The SRP will then bind to a SRP receptor on the ER membrane. The SRP receptor is close to a translocon - a pore through which the protein can now be threaded. Once the protein has been threaded through into the lumen of the ER a signal peptidase will cleave off the signal peptide from the protein.
Describe the movement of proteins through the Golgi stack.
Vesicles carrying proteins from the ER fuse at the Golgi to form the new cis cisternae. The proteins then move through the Golgi stack and are acted upon by enzymes as they do so. Proteins are sorted within the trans Golgi network so that proteins with the same target sequence gather together. The trans Golgi network then buds off inti vesicles which migrate to their target location.
What kind of proteins will have a ‘stop translocation’ sequence and why is this sequence important?
Proteins bound for the plasma membrane will have this sequence. It is important because it stops the protein from being threaded all the way through into the lumen of the ER. Instead the protein remains embedded in the ER membrane, then is embedded in the vesicle and Golgi membrane. When the vesicle from the Golgi reaches the plasma membrane the membrane and the vesicle will fuse so that the embedded protein can become a part of the plasma membrane.
What sugar will proteins destined for the lysosome be labelled with?
Mannose-6-phosphate
Farnesylation is a post-translational modification that occurs in the Golgi. What does it do?
Farnesylation is the addition of a farnesyl group which targets proteins to the cytoplasmic face of the plasma membrane
What is ubiquitination?
Ubiquitination is a post-translation modification which adds a ubiquitin group to a protein that labels the protein for degradation.
What are the two main methods of getting rid of proteins that are no longer required by the cell?
Lysosomal degradation
Proteosomal degradation
Under what circumstances will the lysosomal degradation pathway be used?
When the protein that needs to be degraded has a long half life, is a membrane protein or is an extracellular protein.
Under what circumstances will the proteosomal degradation pathway be used?
When the protein which needs to be degraded has a short half life, is a key metabolic enzyme or is a defective protein. In short: when a protein needs to be gotten rid of quickly.
Which degradation pathway will proteins tagged with ubiquitin undertake?
Proteosomal
