Protein Trafficking Flashcards
Where are cytosolic proteins made
Where are membrane and secretory proteins made.
Where are nuclear proteins made
The cytosol on free ribosomes
RER using the SRP cycle on the membrane bound ribosomes on the RER.
outer nuclear membrane
How is the ribosome bound to the ER membrane
By multiple nascent polypeptide chains
Where do proteins go after being made in the RER
The SER where lipids are made and vesicles form.
The lipids and proteins in the vesicles then travel to the Golgi and then to the membrane.
What can form because the SER has no ribosomes
Vesicles from the membrane.
Centrifugation can separate heavy and light vesicles
The SER will be lighter as it has no ribosomes.
The RER will be at the bottom of the tube.
How are the proteins sent to the right organelles
What happens if many of the amino acids are positively charged
It is based on their signal sequences. The first 10-12 amino acids that the nascent polypeptide chain will make.
They will go to the nucleus. If not they could go to the ER.
What is the signal recognition particle and what does it do
How does the peptide enter the RER
It binds to the signal sequence on the growing peptide
It will guide the ribosome to the RER.
The binding of the SRP to the ribosome causes a pause in translation.
There is a SRP receptor on the RER membrane which will bind to the SRP and ribosome complex.
The peptide will continue to be translated and will grow through the protein translocater on the membrane and into the RER lumen.
The SRP receptor is displaced and recycled.
How does the ribosome attach to the protein translocator on the RER
The large subunit is attached to the protein translocater
The small subunit faces the cytosol
What does ER based signal peptidase do
Removes signal peptides from secretory proteins because signal peptides are only needed early on.
What will happen to new transmembrane proteins that are entering the cell through the protein translocater
They will stay anchored to the ER membrane and won’t fully enter the ER lumen.
They have a stop transfer sequence which allows the protein to embed in the ER membrane.
How are peripheral proteins made
Swapping the signal peptide for a lipid anchor in the ER can take place to continue membrane association.
How does protein maturation occur in the ER
Disulphides bridges are formed between cystine residues in the chain to solidify the shape in the acidic environment.
The protein is glycosylated by a standard carbohydrate chain for quality control.
What are the vesicles from the ER coated in and where do they go
What if the protein made needs to return to the ER
What makes up the KDEL sequence
COPII
They go to the vesicular tubular cluster and then to the Golgi.
They carry a KDEL sequence which helps them be captured by the KDEL receptor so they can return to the ER lumen.
The KDEL sequence is lysine, aspartic acid, glutamic acid and leucine.
Why is addition of carbohydrates to proteins important.
For protein stability in the harsh extracellular environment
For cell cell recognition and cross species separation.
The two types of glycosylation
Where does initial addition of carbohydrates start.
N linked-
Where an aspergine on the protein uses its N to connect to a carbohydrate.
O linked-
A threonine on a protein uses its O to connect to a carbohydrate
ER
What does the quality control signal contain
Glucose, manose and N acetylglucosamine
Where does final addition of sugars and sorting take place
What happens
In the Golgi.
Trimming and growth of carbohydrates proceeds step by step in individual Golgi cisterns.
First the glucose residue is removed and the manose is trimmed.
N acetylglucosamine is added on two ends of the chain. Then N acetylneuraminic acid is added onto that.
Why does each glycosylation step require separate Golgi cisterns
To keep the different enzymes away from each other.
Why are organ transplants difficult
How do we overcome it
Sugar modifications.
Human cells make beta glucose and other animals make alpha glucose.
Humans will make antibodies against alpha galactose and reject transplants.
In beta galactose an OH group points up instead of down.
To overcome this GM pigs were made that lack galactose so their organs won’t have anything that makes humans reject it.
How is insulin matured
Where does it happen
What helps to keep insulin together
A sequence in the centre of pro insulin has to be removed to form insulin.
This happens after leaving the Golgi when it is in a vesicle on the way to the membrane.
The mature insulin has a C peptide cleaved off.
The disulphide bridges help to keep insulin together after its had its middle removed.
Genetic cause of type 1 diabetes
And what the body does to make it worse
Miss folding of pro insulin in the ER due to a mutation.
Meaning the protease cannot cleave the c peptide.
This causes secretion of faulty insulin that won’t be able to bind to its receptor.
The body makes antibodies against the pancreatic cells and destroys them so they stop making the bad insulin.
Cause high blood glucose.
What is pre pro insulin
It still has its signal sequence attached.
When it is removed it becomes pro insulin
Different cells can process the same polypeptide into different hormones.
Pro opiomelanocortin
This can be cleaved in different ways to make different hormones.
It can make beta endorphin and beta MSH in neurons.
It can make ACTH and beta lipotropin in the pituitary gland.
