Heterotrimeric G proteins: which subunit binds and hydrolyzes guanine nucleotides (GDP/GTP)?

Introduction:
Heterotrimeric G proteins transmit signals from G protein–coupled receptors (GPCRs) by cycling between GDP-bound inactive and GTP-bound active states.

Given Data / Assumptions:

• Components: Gα, Gβ, Gγ.
• Ligand-activated GPCRs act as GEFs (guanine nucleotide exchange factors) for Gα.

Concept / Approach:

Only the Gα subunit binds and hydrolyzes GDP/GTP. Upon GTP loading, Gα dissociates from Gβγ to engage effectors; intrinsic GTPase activity resets the cycle.

Step-by-Step Solution:

Verification / Alternative check:

Biochemical assays show nucleotide binding/hydrolysis centered on Gα; Gβγ modulates effectors but does not bind GTP.

Why Other Options Are Wrong:

Gβ and Gγ do not bind/hydrolyze GTP; “delta” is not a heterotrimeric subunit; Gβγ dimer does not directly bind GTP.

Common Pitfalls:

Assuming Gβγ holds nucleotides; mixing up small GTPases (Ras-like) with heterotrimeric G proteins.

Final Answer:

Alpha (Gα)