Protein Targeting and Sorting

Question 1

as proteins are synthesized, they must be…?

Answer 1

sorted and directed to their final locations

Question 2

ribosomes may reside… how many? what do they do there?

Answer 2

up to 10% of ribosomes may reside in the nucleus, translating new mRNAs in a mainly quality control capacity

Question 3

most polypeptide synthesis take place in the..?

Answer 3

cytosol with transcripts leaving the nucleus and associating with free ribosomes

Question 4

shortly after translation begins, two pathways for routing

the protein products diverge: (2)

Answer 4

1. cotranslational import

2. posttranslational import

Question 5

shortly after translation begins, two pathways for routing
the protein products diverge: cotranslational import
- what is it?
-what uses it?
-what happens to these?

Answer 5

• transfer of polypeptides into the ER
• utilized by ribosomes synthesizing polypeptides destined for export from the cell
• such ribosomes become attached to the ER membranes early in translation, and polypeptide chains are transferred across the ER membrane as synthesis takes place

Question 6

shortly after translation begins, two pathways for routing
the protein products diverge: posttranslational import
- who uses it?
-what happens here after translation is complete?
-what does this process require?

Answer 6

• employed for polypeptides destined for the cytosol or mitochondria, chloroplast, peroxisomes, or nuclear interior
• after translation is complete, the polypeptides are released from ribosomes, and remain free in the cytosol , or are taken up by the appropriate organelle
• the uptake by organelles of such complex polypeptides requires the presence of special targeting signals