ER, trafficking, Co-translational import (signal sequence)

Question 1

Signal sequence (for ribosomes targeted to the ER membrane)

Answer 1

-Located in the amino-terminus (N-terminus)

-Contains several consecutive hydrophobic amino acids

-Directs synthesis to the ER compartment

-Protein moves through channel into ER for co-translational import

Question 2

Step 1 for signal sequence (for ribosomes targeted to the ER membrane)

Answer 2

1. After translation signal sequence a signal recognition particle (SRP) binds to signal sequence and stops the translation process

Question 3

Step 2 for signal sequence (for ribosomes targeted to the ER membrane)

Answer 3

2. SRP binds SRP receptor to target the whole trannslation complex to ER (SRP + ribosome +mRNA + new polypeptide)

Question 4

Step 3 for signal sequence (for ribosomes targeted to the ER membrane)

Answer 4

3. SRP is released and ribosome binds to translocon

Once this is done, protein synthesis resumes

Question 5

Step 4 for signal sequence (for ribosomes targeted to the ER membrane)

Answer 5

• Polypeptide enters the ER (through the translocon) as it is translated

-In the end the signal peptide is cleaved off and chaperone folds the protein

Question 6

Proteins targeted to mitochondria

Answer 6

• Note that a different strategy exists for protein synthesized on free ribosomes that are targeted to the mitochondria or chloroplasts

-For mitochondria the TOM is the equivalent of SRP complex translocon

Question 7

Endocytic pathways of protein sorting (option 1)

Answer 7

-It is retained in the ER lumen, where it functions

Question 8

Endocytic pathways of protein sorting (option 2)

Answer 8

-It is transported from the ER to the Golgi complex for further modification and delivered to distal parts of the biosynthetic/secretory pathway

-In some cases the final destination could be outside the cell (secreted molecules like steroid hormones or mucin)