L20- Post translational modifications Flashcards
what happens after translation of the polypeptide chain by the ribosome during translation
it folds to form a 3D activated structure
some proteins may need
additional processing after translation
what sorts of additional processing may proteins require after translation
- Proteolytic cleavage - Chemical modification
proteolytic cleavage
breaking peptide bonds to remove part of protein o e.g. N and C terminals
chemical modification
addition of functional groups to amino acid residues o E.g. glycosylation
what is protein targeting
how proteins know where to go into their cell e.g. proteins have intrinsic signals that governs heir transport and localisation in the cell e..g SRP
what is required for protein sorting
- A signal intrinsic to the protein - e.g. Signal sequence in polypeptide of secretory proteins 2. A receptor that recognises the signal and which directs it to he correct membrane 3. A translocation machinery 4. Energy for translocation (GDP)
two locations where translation (protein synthesis) can occur
- on free ribosomes - by ribosomes on the RER
proteins translated by free ribosomes are destined
for the cytosol or post translational import into organelles e.g. - nucleus - mitochondria - chloroplast - peroxisome
proteins translated by ribosomes found on the RER
proteins destined for the membrane or secretory (Golgi) pathway via translational. insertion
what sort of proteins are targeted for secretion
- extracellular proteins (collagen) - membrane protein (channels etc) - vesicular proteins (lysosomes)
an example of a cell that has a high secretory rate
pancreatic acing cells
pancreatic acinar cells are the
make up the exocrine tissue of the pancreas - stuffed full of secretory granules - e.g. pancreatic amylase
how are pancreatic acinar cells designed to carry out their function?
- lots of RER —> high rate of secretory protein synthesis
secretory pathway of a cell
1) the endoplasmic reticulum,
2) Golgi apparatus and the
3) vesicles that travel in between them as well as the cell membrane and lysosomes.
It’s named ‘secretory’ for being the pathway by which the cell secretes proteins into the extracellular environment.
Types of secretion from cells
- Constitutive
- Regulated
constitutive secretion
happens all the time to maintains basal levels e.g. albumin
regulated secretion
Controlled
- can be upregulated or downregulated by calcium
e. g. endocrine cells- secreting hormones
e. g. exocrine cells- secreting digestive
e. g. Neurocrine cells- secreting NT
what part of a protein targets it for the ER
signal sequence at the N-terminus of secretory proteins e.g. Preproalbumin has a signal sequence which targets it to certain areas of the cell
N terminus
at the beginning of the protein (will be on outside of the cell)
when secretory proteins, destined for the ER are being translated by free ribosomes, what binds to the signal sequence (first part of the mRNA translated) to target it to the ER
the signal recognition particle
the SRP is able to
bind to the SRP receptor on the ER - causing protein synthesis of mRNA to occur within the RER and not in the cytosol
preproalbumin as an example- the N terminal
N -terminal aa sequence has a central region rich in hydrophobic residues (red part)
- 5-30 aa in length - able to form alpha helix
what is the ‘pre’ part of the preproalbumin
defines the signal sequence which is removed during processing
when we lose signal sequence it becomes
pro albumin
Synthesis of secretory proteins and their translocation across the ER membrane: Process
- Free ribosome starts to translate the mRNA
- first part of the mRNA sequence (N-terminus) is translated and signal sequence produced
- as soon as signal sequence appears it can be bound by the SRP
- when the SRP bidns ot the ribosome and the signal seqeunce, it halts protein synthesis- translational arrest
- SRP protein binds to SRP receptor- binding the ribosome to the translocon (GDP dependent-energy)
- causes translocon to open to allow the translated protein to enter the ER
- Signal seqence get cleaved by signal peptidase befor ethe res tof the protein is translated
- mRNA translated into a protein in the ER lumen
- polypeptide chain folds to form a protein
- once trasnlation is finished, the ribosome will detach fromt he ER and go off to repeat the process
Function of the ER
- Glycosylation (N-linked)
- Formation of S-S bonds (disulphide bonds)
- Proper folding of proteins
There are more but these are the ones to concentrate on
N- linked glycosylation involves
sugars being added onto an asparagine side chain
what does N-linked eman
sugars added to amino group (asparagine)
why is glycosylation of proteins important
- Correct protein folding
- Protein stability
- Facilitates interactions with other molecules- e.g. recognition of self proteins
deficincies in N-linked glycosylation leads to
a severe inherited human disease e.g. congenital disorders of glcosylation (CDG)
Proper folding of proteins is facilaited by
chaperone proteins
chaperone proteins
ER chaperone proteins attempt to correct problem
- Retain unfolded proteins in ER
- Act as sensors to monitor extent of misfolding
- Mediate increased transcription of protein mis-folding
- Mediate reduction in translation
which enzyme catalyses the formation of disulphide bonds during protein folding
protein disulphide isomerise (PDI)
where does formation of disulphide bonds occur
the ER lumen
formation of disulphide bonds includes
- disulphide bond between 2 cystein residues (SH)
- correctly folded protein will have the correct pairsings of S-S bonds
- PDI ensure correct disulphide bond forms and therefore correct protein folding
what happens if there are fodling prolems - linekd to formation of disulphide bonds
- Trapped in wrong compartments
- Trapped in mis-folded conformation
- Protein contains mutation resulting in mis-folding
- Protein may be incorrectly associated with other sub-unit
What happened if mis-folding cannot be corrected?
- Protein may be returned to the cytosol for degradation
- Protein may accumulate to toxic levels in the ER resulting in disease- may arise due to single mutation
what modifies proteins are they leave the ER
golgi- a continuation of the RER
