Question

multiple choice question
for proteins that are targeted to the er, where is the signal sequence usually located?
o in an untranslated portion of the mrna, usually near the 5 end
o at the beginning of the protein (first several amino acids added during translation)
o in the middle of the protein sequence
o at the end of the protein (last several amino acids added during translation)

Explanation:

The signal sequence for proteins targeted to the ER is a short peptide sequence. It is recognized by the signal recognition particle (SRP) during translation. The SRP binds to the signal sequence as it emerges from the ribosome. This binding pauses translation and targets the ribosome - nascent polypeptide complex to the ER membrane. Since it needs to be recognized early in the translation process, it is located at the beginning of the protein (first several amino acids added during translation).
The untranslated portion of mRNA does not code for the protein sequence. A signal sequence in the middle or at the end of the protein would not allow for early recognition by the SRP for ER - targeting.

Answer:

At the beginning of the protein (first several amino acids added during translation)