Final answer:
The correct statement regarding the ER signal sequence on soluble proteins is that it functions to open the protein translocator (C). As the protein is synthesized, the signal sequence engages with the translocon of the ER membrane, initiating co-translational translocation. Once inside the ER lumen, the signal peptide is cleaved off and often remains associated with the ER membrane.
Step-by-step explanation:
The true statement about the ER signal sequence on soluble proteins is that it functions to target the protein to the endoplasmic reticulum (ER) and is cleaved from the polypeptide during the translocation process. As the soluble protein is being synthesized, the N-terminal signal sequence is recognized by the signal recognition particle which then directs the ribosome to the ER membrane. Once the protein is targeted to the ER, the signal sequence initiates the translocation process by interacting with the translocon.
The translocation of the growing polypeptide into the ER lumen is co-translational, which means it occurs simultaneously as the protein is being synthesized by the ribosome. When the N-terminal signal sequence enters the ER membrane it opens the protein translocator channel. As the protein passes through the channel into the lumen of the ER, a signal peptidase cleaves the signal sequence. This cleaved signal peptide often remains associated with the ER membrane rather than being released into the lumen or cytosol.
The correct answer to the student's question is C: It will function to open the protein translocator. The signal sequence functions both to direct the soluble protein to the ER and to open the translocation channel, enabling the protein to enter the ER lumen where it can fold and undergo post-translational modifications.