Final answer:
Sar1 becomes GTP-bound when the ER signal peptide activates a Guanine nucleotide exchange factor (GEF), which facilitates GDP release and GTP binding, turning Sar1 active.
Step-by-step explanation:
Sar1 is a GTPase involved in vesicle transport from the endoplasmic reticulum (ER) to the Golgi apparatus, and it changes from its GDP-bound state to the GTP-bound state with the help of a Guanine nucleotide exchange factor (GEF). Specifically, when the ER signal peptide binds to the Sar1-GEF complex, the GEF triggers the release of GDP from Sar1, enabling it to bind GTP. This process turns Sar1 into its active form, which can then participate in the budding of transport vesicles from the ER.
Thus, the correct answer to how Sar1 changes from its GDP to GTP bound state, and what activates it is A. Guanine nucleotide exchange factor (GEF); ER signal.