Heterotrimeric G proteins: which subunit binds and hydrolyzes guanine nucleotides (GDP/GTP)?
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AAlpha (Gα)
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BBeta (Gβ)
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CGamma (Gγ)
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DDelta (Gδ)
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EBeta-gamma dimer (Gβγ as a unit) directly binds GTP
Answer
Correct Answer: Alpha (Gα)
Explanation
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:
1) GPCR activation promotes GDP release from Gα.2) GTP binds Gα, activating it.3) Gα-GTP and Gβγ signal to downstream effectors.4) Gα hydrolyzes GTP to GDP, reassociating with Gβγ.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)