Post-translational Processing of Proteins Flashcards
What are some examples of how a protein can be processed after translation?
Proteolytic cleavage and chemical modification
What is proteolytic cleavage?
Breaking peptide bonds to remove part of a protein
Where are proteins destined for the cytosol or post-translational import into organelles synthesised?
Free ribosomes
Where are proteins destined for the membrane or secretory pathway synthesised?
Ribosomes on the rough ER
What is required for protein sorting?
An intrinsic signal, a receptor that recognises the signal, a translocation machinery and energy
What is the intrinsic signal in proteins destined for peroxisomes?
Serine-lysine-leucine (SKL)
Usually present on the C terminus of the protein
What is the receptor that recognises the signal in protein targeting to peroxisomes?
PTS receptor Pex5
Binds to cargo protein in the cytoplasm
What is the translocation machinery in protein targeting to peroxisomes?
13 Pex proteins make up a transport channel across the peroxisomal membrane which binds to the Pex5-cargo complex
What is required to allow recycling of the PTS receptor?
ATP hydrolysis
What is dissociation often due to?
Change in pH
What are some examples of peroxisome biogensis disorders?
Zellweger syndrome and Rhizomelic Chondrodysplasia Punctata
What is constitutive secretion?
Occurs all the time - constant flow of extracellular proteins out of the cell. Eg collagen from fibroblasts
What is regulated secretion?
Occurs at specific times - only secreted in response to signals. Eg endocrine cells secreting hormones
What do secretory cells have an abundance of?
Rough ER
What is a signal sequence?
N-terminal amino acid sequence, 5-30 amino acid’s in length, central region rich in hydrophobic resides, able to form alpha helix which allows it to cross membrane easily
What does the ‘pre’ part of preproalbumin define?
The signal sequence which is removed during processing
What is the signal recognition particle?
Receptor needed to bind the signal peptide on proteins destined for the ER, composed of 6 proteins and a short piece of RNA, recognises the signal peptide and the ribosome
What does the stop transfer sequence do?
Halts transfer of the peptide across the ER membrane, acts as an anchor to hold protein in position - type I
What are the functions of the Endoplasmic Reticulum?
Insertion of proteins into membranes, specific proteolytic cleavage, glycosylation, formation of S-S bonds, proper folding of proteins, assembly of multisubunit proteins, hydroxylation of selected Lys and Pro residues
What is N-linked glycosylation?
Sugars are added on an asparagine side chain - occurs in ER
Why is glycosylation of proteins important?
Correct protein folding, protein stability, facilitates interactions with other molecules
Between which residues are disulphide bonds formed?
Cystine
What is the role of protein disulphide isomerase (PDI)?
Ensure correct disulphide bonds form
Why may there be protein folding problems?
Proteins may be trapped in mis-folded conformation, protein contains mutation resulting in mis-folding, protein may be incorrectly associated with other subunits
What is the role of ER chaperone proteins in folding problems?
Retain unfolded proteins in the ER, act as sensors to monitor extent of protein mis-folding
What are examples of ER chaperone proteins?
BiP: ‘Binding Immunoglobulin Protein’
Calnexin and Calreticulin
What happens if mis-folding can not be corrected?
Protein may be returned to cytosol for degradation or accumulate to toxic levels in the ER resulting in disease
What is O-linked glycosylation?
Attachment of sugar to OH group
Where does O-linked glycosylation occur?
Golgi
What is O-linked glycosylation important in?
Proteoglycans - component of ECM and mucus secretions
What post-translational modifications does preproinsulin undergo?
The signal peptide at the N terminus of preproinsulin is cut, directing it to enter the endoplasmic reticulum. Proinsulin results and folds into the correct conformation with the formation of disulphide bonds. The B chain is removed and there is trimming of the caboxyl termini. It then enters the golgi as insulin
Where is the signal sequence located in proteins targeted for the ER?
N terminus
Where is the signal sequence located for proteins targeted for the nucleus?
Various positions, must be on surface of folded protein
Where is the signal sequence located for proteins targeted for mitochondria?
N terminus
Where is the signal sequence located for proteins targeted for the lysosomes?
Signal patch distinguishes lysosomal proteins from other mannose-labelled proteins
