Protein Sorting: ER or Mitochondria?

[cottonz]

Homework Statement

A protein contains both an N-terminal mitochondrial matrix targeting sequence and an internal
ER start transfer sequence. Where do you think this protein goes?

The Attempt at a Solution

I've been told that it would still go to the ER. I know that when the ER start transfer sequence emerges from the ribosome, the SRP directs it to the ER membrane. Since that sequence is in the middle of the polypeptide, wouldn't the N-terminal mitochondrial matrix targeting sequence, which is right at the front, direct the protein to the receptors on the mitochondria first? Does one sequence have a stronger/higher priority than the other?

Thanks

 

[nate808]

for your question! It is an interesting one and the answer depends on the specific protein in question. Generally, proteins with both an N-terminal mitochondrial matrix targeting sequence and an internal ER start transfer sequence will be targeted to the ER first. This is because the ER start transfer sequence is typically stronger and more efficient at directing the protein to the ER, and the protein will be translated in its entirety before the N-terminal targeting sequence can have an effect. Additionally, the SRP (signal recognition particle) recognizes both the ER start transfer sequence and the ribosome, so it will not be affected by the N-terminal targeting sequence. Therefore, in most cases, the protein will be directed to the ER first and then potentially targeted to the mitochondria at a later stage. However, there may be exceptions to this general rule and further experiments would need to be conducted to determine the exact pathway of this specific protein.