Final answer:
Guanine nucleotide exchange factors facilitate the exchange of GDP for GTP on G proteins, but do not hydrolyze GTP to GDP. It is the G protein α subunit that hydrolyzes GTP to GDP, which inactivates the G protein and ends the signal.
Step-by-step explanation:
The statement that the Guanine nucleotide exchange factor hydrolyzes GTP to GDP in the nucleoplasm is false. In reality, the Guanine nucleotide exchange factor (GEF) facilitates the exchange of GDP for GTP on the G protein α subunit, which is an essential step in the activation of G proteins. However, it is actually the G protein α subunit itself that possesses GTPase activity and hydrolyzes GTP back to GDP, therefore deactivating the G protein and terminating the signal.
Activation of G proteins involves a GDP/GTP exchange, triggered when a signaling molecule binds to a G-protein-coupled receptor. Once activated by GTP binding, the α subunit separates from the β and γ subunits and goes on to activate downstream effectors. This process continues until GTP is hydrolyzed to GDP, at which point the α subunit reassociates with the β and γ subunits, returning the G protein to its inactive state.