Translation and Translocation

Question 1

name the places newly synthesised proteins can be targeted to

Answer 1

ER - if it has an ER signal sequence (co translation)
-> can become luminal or TM

cytosolic - no ER signal sequence and no protein targeting sequence

peroxisome/mitochondria/nucleus - no ER signal sequence, but has a targeting sequence

Question 2

amino acid types for nuclear and ER targeting

Answer 2

Nuclear = positive charge aa
ER = hydrophobic aa

Question 3

what are SRPs?

Answer 3

Signal recognition particles

they recognise peptides’ N-terminal “signal sequence” of 6-12 hydrophobic amino acids and direct the peptide to the membrane

Question 4

how to peptides enter the ER?

Answer 4

co-translational import

1. signal sequence bound by SRP - this causes translational arrest
2. SRP binds to SRP receptor
3. SRP receptor binds GTP and the polypeptide enters Sec-61 (going into the lumen
4. when the polypeptide is complete, the signal peptidase cleaves the signal to release the protein into the ER lumen

Question 5

purpose of translational arrest

Answer 5

If you allowed the protein to fold up then
i) you probably could not go through the Sec-61,

ii) the hydrophobic sequences would not like the ‘aqueous’ cytoplasm
iii) a folded protein may then not have the sequences ‘visible’ for correct targeting.

Question 6

post translational insertion into the ER

Answer 6

requires the complex sec 61 and sec 63

SRP and its receptor are not involved

deposits BiP molecules onto the translocating chain as it emerges into the ER lumen.

ATP-driven cycles of BiP binding and release pull the protein into the lumen,

Question 7

start/stop transfer sequences

Answer 7

Regions of high hydrophobicity can act as a stop transfer sequence – they stop the protein from being continuously extruded into the ER

Proteins can have ‘internal’ signal peptides that can cause the protein to stop transfer through the Sec61 - result in a TM protein

the signal peptide is a start-transfer sequence

Question 8

positive inside rule

Answer 8

the cytosolic end of TM domains is more positively charged than the lumenal end

the phosphate of phospholipids have a negative charge, and quite often as a protein goes back into the cytoplasm there is a positive charged amino acid to match with the negative charge of the phosphate