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Define cytosol

unstructured aqueous phase of the cytoplasm, excluding organelles, membranes and insoluble structures (cytoskeleton)


Define targeting

process by which a newly synthesised protein is directed to a particular membrane of an organelle/location within a cell


Define sorting

processing a protein from one membrane bound compartment to another


What is the signal sequence hypothesis?

proteins contain a.a. codes within them that direct them to particular membranes e.g. PTS - peroxisome targeting sequence @ C-termiuns


Give the 7 step general mode for protein targeting

- signal sequence
- receptor
- docking @ target membrane
- translocation
- recycling of receptor (SRP)
- energy source
- protein folding


What was the role of Cesar Milstein with regards to protein signalling?

completed an experiment involving cell-free protein synthesis. Microsomes were present allowing the co-translationa import of translated proteins and removal of the signal sequence.


Give evidence for co-translational import (3 points)

- ribosomes associate with ER membrane
- newly synthesised proteins associate with the ER at time of translation
- in vitro translation in absence of microsomes does not allow protein import


Where is the signal peptide located on the polypeptide and what is it made up of? (for secretory proteins into the ER)

N - terminus
6-12 hydrophobic residues, preceded by a few +vely charge a.a.


Describe the SRP

signal sequence recognition particle
SRP binds signal sequence
SRP binds specific receptor on membrane
SRP blocks translation temporarily
SRP delivers nascent protein ribosome complex to ER


Where is the energy provided for the co-translational translocation of secretory proteins across the ER membrane?



Describe the Type 1 transmembrane protein

stop-transfer anchor sequence
lateral movement of sequence into bilayer creates a loop at C-terminus into the cytosol
ribosome dissociates when whole polypeptide is synthesised


Describe the Type 2 transmembrane protein

signal-anchor sequence
recognised by SRP and brought to translocon
the positve a.a. residues before the sequence direct the N-terminus side to the cytosol
sequence moved into bilayer and synthesis continues as normal


Describe post-translational import

The completed secretory protein is targeted to ER membrane (more specifically the translocon) by a signal sequence and not an SRP
unfolded chain pulled in by Hsp70 protein, with aid of ATP hydrolysis


What are chaperones?

Help to fold unfolded proteins in the lumen
Involve ATP -> ADP conversions and the chaperone surrounding the protein and folding it