Protein Targeting Flashcards
Proteins synthesised by ribosomes need to move to their correct cell compartment.
What x3 ways of transport do they need to be able to achieve?
1) through nuclear pores
2) across membranes
3) by vesicles
What is the difference between proteins destined for the mitochondria compared with those which are destined for the cytoplasm or nucleus of a cell?
Those for the mitochondria are released unfolded from the ribosome into the cytoplasm, the rest are folded
What is encoded within the newly formed polypeptides which aids them to get to the ER?
What does this bind to?
How does this help transport the protein and ribosome to the ER?
The amine end contains a series of hydrophobic AA’s called a signal sequence
These bind to a signal recognition particle (SRP) which can then dock with a receptor on the ER membrane
Once the ribosome and polypeptide it is synthesising has been bought to the ER by SRP’s, what happens next?
The SRP’s are displaced for re-use and the ribosome transfers its attachment to a translocation channel where it can synthesise the rest of its polypeptide into the ER lumen.
Once this is complete the signal sequence it cleaved off by a peptides enzyme and sits in the ER membrane ready for removal/degradation.
How do membrane bound proteins end up embedded in the membrane?
They have the same initial process as ER bound proteins (signal sequence, SRP’s, translocation channels etc..) but at the relevant point within their sequence they have a stop transfer sequence which is the width of the membrane the wish to be embedded in.
This sequence remains within the ER and the rest of the protein is continued to be synthesised outside of the membrane.
Which terminus of the protein is located inside the membrane and which outside in a membrane spanning protein?
N-terminus = inside
C-terminus = outside
How do vesicles fuse with their target membranes?
Their V-snare (vesicle) merged with the T-snare (target) to lock together as the vesicles fuses with the membrane
Which molecules allow refolding of improperly folded proteins?
Chaperone molecules
What happens to proteins in the Golgi (and ER)?
They re glycosylated, which acts as a postcode for the next stages of the proteins journey
With regards to import of proteins into the mitochondria, how does this occur?
1) proteins released unfolded into cytoplasm
2) chaperone molecules transport these proteins to the outer mitochondrial membrane
3) a receptor on the outer mitochondrial membrane recognises the proteins signal sequence and aligns it with 1 or 2 translocation channels (depending on the proteins destination)
4) the signal sequence is cleaved off
Where can proteins destined for the mitochondria end up?
In the outer membrane
Between the 2 membranes
In the inner membrane
In the matrix
What is the name of the signal sequence found on proteins destined for the cells nucleus?
What is special about this sequence of AA’s?
The nuclear localisation sequence
They are basic AA’s (lysine/argenine/histidine) therefore will stick out
Where is the nuclear localisation sequence located within the proteins structure?
Anywhere - not necessarily the N-terminus amine end
How are nucleus destined proteins transported across the nuclear envelope?
They are bound to a protein called importin
What are lysosomal bound proteins tagged with?
Mannose-6-phosphate
