L25 - Protein Trafficking Flashcards
Where are cytosolic proteins made
In the cytosol
Where are membrane and secreted proteins made
RER
The RER is studded by
Ribosomes
Where are nuclear proteins made
The outer nuclear membrane
Describe what happens to the common pool of ribosomes
Form either a polyribosome in the cytosol
Form a polyribosome bound to the RER membrane
Describe how the polyribosome on the RER membrane forms
Signal sequence of a protein destined for the ER is translated
This is recognised by a signal recognition particle (SRP)
SRP bins to an SRP receptor particle on the ER membrane
WHat occurs at the RER
Protein synthesis
What occurs at the SER
Lipid synthesis and formation of vesicles
What is a heavy microsome
One made from the RER - studded with ribosomes
What is a smooth microsome
From the smooth ER - no ribosomes
Describe how you may separated rough and smooth microsome
Differnetial centrifugation
Establish concentration gradient of sucrose solution (high at the bottom)
Rough microosmes are heavier so wil lfloat at higher concnetrations of sucrose - at the bottome of the tube
What is the role of the golgi
Final addition of sugars and sorting
Describe the organisation of the golgi
Cis to trans golgi network
Describe what happens during the maturation of vesicles
Proteases act to trim and activate hormones and enzymes
A sequence of positive AA targets the protein to
the nucleus
What does an SRP have
What does this allow
Hinge region
Means that it is able to travel with the ribosome
Describe the process of a signal sequence targetting a protein to the ER
Signal sequence is produced through translation
SRP binds to sig seq and the ribosome
Translation slows
SRP then captured by the SRP receptor protein and protein translocated
Polypeptide is made through the trnalsocator into the ER
Signal peptidase catalyes cleavage of the signal sequence
What enzyme catakyses cleavage of the signal sequence
Signal peptidase
What is the topology of the ribosome once it has bound to the translocator
Large SU faces the translocator
Small SU faces the cytosol
Describe how a protein is inserted into the ER membrane
At the start there is a start-transfer sequence - hydrophobic
This is cleaved by a signal peptidase
Stop-transfer sequence marks the transmembrane domains
What is a GPI anchor
Glycosylaphosphatidyllinositol anchor
What does a GPI anchor allw
Attachement of a protein to the membrane without the need for a transmembrane doamin
Once the signal sequence has been cleaved what happens
Conformational maturation
Conformation maturation involves the formation of ____________
Where do these form
What is the function
Disulphide bridges
From between cystenines
Solidify
Where do DS bridges NOT form
In the cytosol since it is a reducing environment
What is the role of standard glycosylation
Quaity control
Describe how glycosylation acts as quality control
If the protein is in the incorrect shape then the sugar cant be attached at the correct place - if this happens the protein is degraded
What (3) is glycosylation important for
Protein stability in a harsh EC environment
Cell-cell recognition
Cross species separation
What sugar do humans use
What sugar do other animals use
What is the different
B-galactose
A-galactose
At C1 OH and H are in differnent positions
What are the two linkages of glycosylation - from which AA does each occur
N-linked - from asparagine
O linked - from thereonine
What three molecules are contained in the quality control tag
Glucose
Mannose
N-acetylglucosamine
Trimming and growth of carbohydrate chain proceeds in a
Step-by step in each individual cisternae
Each glycosylation step takes place in …
What is the importance of this
A sepaate golgi compartment
Keeps the specific glycoylaion enzymes away from one another
Describe why animal organs are rejected when transplanted into humans
Humans use B-galactose whilst animals will make a-galacose
Humans make antibdodies agaisnt a-galactose - so the animal derived organ will be rejected by the humn
Describe how the problems of organ transplant can be overcome
Using an organsism which doesnt have the enzymes required for the synthesis of a-galactose
So there is no a-galactose produced s no antibodies produced
Describe the processing of insulin
Preproinsulin translation occurs in the ER - cleaveage of insulin leads to the production of proinsulin
Proinsulin is transported to the golgi where it is packaged into sec vesicles
Proinsulin cleaved to mature insulin and a C peptide
What enzymes are required for the cleavage of prepro-insulin and pro-insulin
Specific proteases
Even though two chains of insulin are separate how are they held together
Disulphide bonds between the two chains
Describe Type 1 diabetes
Misfolding of proinsulin in the ER due to mutation
Protease in secretory vesicles unable to cleave the C peptide
Secretion of dyfunctional proinsulin
Antibodies make agaisnt pancreatic cells
Destruction of pancreatic cells
A patient with type 1 diabetes will have
Increased levels of blood glucose
Different cells cotnain ____________ which cleave the protein into ____________________
Different enzymes
Different hormones
What is an example of a large protein which can be cleaved into many different proteins
Pro-opiomeanocortin
ERresident enzymes have what
KDEL sequence
What is the role of the KDEL sequence
KDEL sequence of ER resident enzymes binds to the KDEL receptor
Ensures that they return to the ER instead of entering the secretory pathway
What is the KDEL seq
Lys - Asp - Gly - Leu
What can Pro-opiomeanocortin be cleaved to in the pittuitry
B-lipotrophin
Corticotrophin
What can Pro-opiomeanocortin be cleaved in in neurones
B-endorphin
B-MSH
A-MSH
Gama-lipotrophin
When do neurones produce B-endorphin
In response to excersise and stress
Describe what happens upon binding of insulin to its receptor
Causes downstream signalling
Increased transport of glucose transports into the PM of muscle cells
