MCGB - Protein Processing And Targeting - done to slide 21 Flashcards
If proteases were activated while they are still inside the cell they were synthesised in, they would destroy the cell. How is this issue overcome?
They are activated outside the cell in “post-translational modification.”
What is proteolytic cleavage?
Breaking peptide bonds to remove part of the protein.
What is chemical modification?
The addition of functional groups to amino acid residues.
If a protein is destined for the cytosol or for posttranslational import into organelles, where is it synthesised?
Free ribosomes
If a protein is destined for the membrane, lysosomes, secretory vesicles or other organelles, where is it synthesised?
Ribosomes on rough ER.
What is required for protein sorting?
A SIGNAL intrinsic to the protein, a RECEPTOR which can recognise the signal and direct it to the correct membrane, a TRANSLOCATION machinery, ENERGY to transfer protein to new place.
Which type of protein targeting is this? Three amino acids termed the “PTS” are present on the C-terminus of the protein. PTS is recognised by PTS receptor Pex5 which binds to “cargo protein”, 13 Pex proteins make up a transport channel across membrane of organelle, binds to Pex5-cargo complex. ATP hydrolysis required for recycling of receptor.
Protein targeting to peroxisomes
What is the difference between constitutive and regulated secretion?
Constitutive - going on all the time, constant flow of extracellular proteins out of the cell, eg. Fibroblasts
Regulated - occurs at specific times from cell, eg. Neurocrine cells
Give some characteristics of a signal sequence of a secreted protein.
N-terminal amino acid sequence, 5-30 amino acids in length, central region rich in hydrophobic residues, able to form alpha helix.
What is post translational modification?
The changes made to proteins after they are formed.
Give some examples of proteins that would be targeted for secretion.
Extracellular proteins, membrane proteins, vesicular proteins eg lysosomes.
Ina cell that secretes proteins there is a lot of one particular organelle. Which one?
RER
Why is it important that secreted proteins are able to form an alpha helix?
It allows them to quickly cross the lipid bilayer.
What is a signal recognition particle?
A molecule composed of six proteins and a short piece of RNA which recognises the signal peptide and the ribosome.
Give some functions of the endoplasmic reticulum.
Insertion of proteins into membranes, specific proteolytic cleavage, glycosylation, formation of disulphide bonds, proper folding/assembly of proteins, hydroxylation of residues.
What is N-linked glycosylation?
Adding a sugar group to an amino acid through a nitrogen group in the ER.
Why is glycosylation of proteins important?
Provides stability, facilitates interaction with other molecules, allows correct protein folding.
What is the role of protein disulphide isomerase?
It ensures the correct disulphide bond forms and replaces them if incorrect.
Why is it less of an issue if protein misfolding occurs in the ER?
ER chaperone proteins can help the protein to refold correctly.
What happens to a mis-folded protein which cannot be corrected?
It is returned to the cytosol for degradation.
What is O-linked glycosylation?
Attachment of a sugar to an OH group, which occurs in the Golgi complex.
Collagen is not very abundant in the body - true or false?
False, it’s the most abundant.
What is a collagen fibre made up of?
Molecules called TROPOCOLLAGEN (rod shaped protein, made of 3 polypeptides, repeated glycine structure, triple helix)
Give some characteristics of the triple helix structure of collagen.
Non-extensible, non-compressible, high tensile strength.
Why is glycine present in every third position of the alpha helix of collagen?
Glycine is the only amino acid small enough to fit into the middle of the helix.
What sort of bonds form between alpha chains to stabilise the structure?
Hydrogen bonds.
Which organelle do fibroblasts have an uncommonly large amount of?
RER
The signal peptide is cleaved off as the prepro alpha chains of collagen enter the ER. What are they then called?
Pro alpha chains
Why are collagen molecules weaker if sufficient vitamin C is not present?
The amino acids cannot be hydroxylated, so they cannot form hydrogen bonds and the molecule is weaker (scurvy).
Outline the stages of tropocollagen formation and modification.
1) synthesis and entry to lumen of RER
2) cleavage of signal peptide
3) hydroxylation of Pro and Lys
4) N-glycosylation and galactose added
5) chain alignment, S-S bonds
6) precollagen forms
7) glucose added, enters transport vesicle
8) exocytosis, removal of N and C terminal propeptides
What is removed when procollagen is transformed into tropocollagen?
N and C terminal peptides
How does tropocollagen form collagen?
N and C terminal propeptides removed, collagen molecules laterally associated and covalently cross-linked, aggregation of fibrils.
What does lysyl oxidise cause tropocollagens to do?
Form cross-links
Why must tropocollagen be secreted outside the cell?
It is extremely large when cross-linked and would destroy the cell if it formed inside.
Give some examples of disorders that can occur if targeting to peroxisomes goes wrong.
Zellweger syndrome, Rhizomelic Chondrodysplasia Punctata (both are peroxisome biogenesis disorders)
What is a signal sequence?
N-terminal sequence of 5-30 amino acids
