Question

multiple choice question
which of the following describes how proteins are targeted for translation to the rough er?
the initial amino acid sequence of these proteins serves as a signal sequence that targets them to the rough er.
the synthesis of all proteins starts on the rough er, but some proteins contain signal sequences that make ribosomes detach from the er.
all proteins are made in the cytosol, but some are modified by attachment of sugars, which targets them to the er.
proteins are always fully made in the cytosol. those that have er signal sequences are translocated into the er by srps.

Explanation:

The signal - recognition particle (SRP) pathway is responsible for targeting proteins to the rough endoplasmic reticulum (ER). Proteins that are destined for the rough ER have an initial amino - acid sequence (signal sequence). This signal sequence is recognized by the SRP. The SRP then binds to the ribosome - nascent chain complex and targets it to the ER membrane.

Let's analyze each option:

• Option 1: The initial amino acid sequence (signal sequence) of these proteins serves as a signal sequence that targets them to the rough ER. This is correct as per the SRP - mediated targeting mechanism.
• Option 2: Protein synthesis does not start on the rough ER for all proteins. Only proteins with a signal sequence are targeted to the rough ER. So, this option is incorrect.
• Option 3: Modification by attachment of sugars (glycosylation) occurs in the ER (for N - linked glycosylation) or Golgi (for O - linked glycosylation). But the targeting to the ER is due to the signal sequence, not sugar attachment. So, this option is incorrect.
• Option 4: Proteins are not always fully made in the cytosol. For proteins targeted to the ER, translation is co - translational (translation and translocation occur simultaneously). So, this option is incorrect.

Answer:

The initial amino acid sequence of these proteins serves as a signal sequence that targets them to the rough ER.