Session 9-Post-translational Processing Of Proteins Flashcards
What is post-translational modification?
Some proteins need additional processing after translation as some proteins are first non-functional and then become active in the right location
What are the two methods of post-translational modification?
Proteolytic cleavage
Chemical/covalent modification
What is proteolytic cleavage?
Breaking peptide bonds to remove part of a protein
What is chemical/covalent modification?
Addition of functional groups to AA residues
Where are proteins destined for the cytosol synthesised?
On free ribosomes
Where are proteins destined for the membrane or secretory pathway synthesised?
Ribosomes on RER
What is required for protein sorting? (4)
- Signal, intrinsic to protein
- Receptor that recognises signal and which directs it to the correct membrane
- Translocation machinery-to transport protein across membrane
- Energy to transfer protein to new place
What is the signal in protein targeting to peroxisomes and where is it present on the protein?
-serine-lysine-leucine = SKL
This SKL sequence is present in proteins going to the peroxisome and is called the peroxisome targeting sequence (PTS).
It is present on the C terminus of the protein
What is the receptor that recognises the signal in protein targeting to peroxisomes?
PTS receptor Pex5 which recognises SKL and binds to the protein with this sequence.
What is the translocation machinery involved in protein targeting to peroxisomes?
13 Pex proteins make up the transport channel across the peroxisomal membrane - pore is produced which allows the protein to pass through the membrane. These proteins bind to Pex5 to form a cargo complex
Where does the energy to transfer the protein in protein targeting to peroxisomes come from?
ATP hydrolysis
Which disorder is a result of protein targeting to peroxisomes going wrong and give two examples of this disorder?
Peroxisome biogenesis disorder:
- Zellweger syndrome
- Rhizomelic Chondrodysplasia Punctata
What causes Zellweger syndrome?
Mutation in several different Pex proteins-cannot break down fatty acids so they accumulate and cause impairment in neuronal function
What happens in Rhizomelic Chondrodysplasia Punctata?
Mutation in Pex7 protein which leads to long bone shortening
What are the two types of secretion from cells?
Constitutive secretion
Regulated secretion
What is constitutive secretion?
Continuous secretion of extracellular proteins (eg collagen, immunoglobulins, albumin)
What is regulated secretion?
Controlled secretion-proteins secreted at specific times
What do endocrine cells secrete?
Hormones
What do exocrine cells secrete?
Digestive juices
What do neurocrine cells secrete?
Neurotransmitters
What does the pre- part of preproalbumin define?
Signal sequence which is removed during processing
What does the -pro- part of preproalbumin mean?
Partially mature protein
What is a signal sequence?
N-terminal AA sequence, 5-39 AA in length. There is a central region, rich in hydrophobic residues that are able to form an alpha helix, allowing the protein to cross the lipid bilayer
What is a signal recognition particle (SRP)?
Composed of 6 proteins and a short piece of RNA, recognises signal peptide and ribosome
Describe the synthesis of secretory proteins and their translocation across the ER membrane (5)
- Starts as free ribosomes
- Signal sequence recognised by SRP
- Binds to ribosome
- Pauses translation
- SRP falls off and translation continues
What are the functions of the ER? (7)
- Insertion of proteins into membranes
- Specific proteolytic cleavage
- Formation of S-S bonds
- Proper folding of proteins
- Assembly of multisubunit proteins
- Hydroxylation of selected Lys and Pro resides for collagen processing
- Glycosylation
What are the two types of glycosylation?
N-linked
O-linked
Why is glycosylation of proteins important? (4)
- Correct protein folding
- Protein stability
- Facilitates interactions with other molecules
- Deficiencies in N-linked glycosylation can lead to severe inherited diseases (congenital disorders of glycosylation or CDG)
What happens in N-linked glycosylation?
Sugars are added on the asparagine (Asn) side chain, the reaction involves an amino group hence N-linked. This occurs in the ER
What happens in O-linked glycosylation?
Attachment of sugar to -OH group and occurs in the golgi. The sugar is attached to the hydroxyl group of serine or threonine.
What happens if there are folding problems?
Protein may be trapped in misfolded conformation
Protein may be incorrectly associated with other sub-units
What attempts to correct any issues with protein folding?
ER chaperone proteins
Give examples of ER chaperone proteins
BiP (binding immunoglobulin protein)
Calnexin and calreticulin
What do ER chaperone proteins do?
Retain unfolded proteins in the ER and act as sensors to monitor the extent of protein mis-folding
What happens if mis-folding cannot be corrected?
Protein may be returned to cytosol for degradation
What happens if there is too much misfolding in proteins?
Protein may accumulate to toxic levels in the ER resulting in disease
