Final answer:
An inactive G protein is a trimeric complex with three subunits (α, β, and γ), where the GDP-bound α subunit is associated with the β and γ subunits. It remains inactive until a ligand-induced activation of a GPCR facilitates the exchange of GDP for GTP, leading to the dissociation of the subunits and subsequent cellular response.
Step-by-step explanation:
In its inactive form, a trimeric GTP-binding protein, commonly known as a G protein, is associated with a guanosine diphosphate (GDP). It consists of three subunits: the alpha (α), beta (β), and gamma (γ) subunits. The α subunit is bound to GDP, and together with the β and γ subunits, forms a stable complex.
This complex remains inactive until a ligand binds to a G-protein-coupled receptor (GPCR) on the cell surface. The binding of the ligand to the GPCR causes a conformational change in the receptor, which, in turn, prompts the G protein's α subunit to release GDP and bind guanosine triphosphate (GTP) instead.
This exchange of GDP for GTP activates the G protein, and the α subunit then dissociates from the βγ pair, allowing both to interact with other effector proteins in the cell, leading to a cellular response.
Eventually, the GTP on the activated α subunit is hydrolyzed back to GDP, rendering the G protein inactive, and the subunits reassemble into their original inactive trimeric form.