Final answer:
A soluble protein destined for the Golgi lumen would require only an ER signal sequence to direct it to the ER, where it will then be sent to the Golgi apparatus; therefore, the correct answer is A. ER signal sequence.
Step-by-step explanation:
The sorting signal present in a soluble protein destined for the Golgi lumen is an ER signal sequence. When proteins are synthesized, they may contain a signal peptide at their N-terminus, which directs them to the endoplasmic reticulum (ER) for translation. This signal sequence is recognized by the signal recognition particle that guides the ribosome to the ER. Once the protein reaches its destination, the ER signal sequence is removed. In the case of proteins destined for the Golgi lumen, they would typically contain only the ER signal sequence to direct them to the ER, where they begin the secretory pathway eventually leading to the Golgi apparatus.
Therefore, the correct answer is A. ER signal sequence. A stop transfer sequence would not be required for a soluble protein as it would for an integral membrane protein that needs to be embedded in the membrane. Similarly, a nuclear localization signal is not relevant to a protein targeting the Golgi, and KDEL sequences are retention signals for ER-resident proteins rather than those destined for the Golgi.