25. In signal transduction trimeric G protein with α,β and γis involved. Which subunit will activate adenylate cylase
(1) α subunit        (2) β subunit
(3) γ  subunit       (4) All three

Role of G-Protein α Subunit in Activating Adenylate Cyclase During Signal Transduction

Meta Description: Learn how the α subunit of the heterotrimeric G protein activates adenylyl cyclase to produce cyclic AMP (cAMP), amplifying cellular signals in G protein-coupled receptor pathways.

Introduction

In cellular signal transduction, trimeric G proteins, composed of α, β, and γ subunits, play a central role in transmitting messages from cell-surface receptors to intracellular effectors. Among their several functions, one critical role is the regulation of the enzyme adenylyl cyclase, which synthesizes the second messenger cyclic AMP (cAMP).

The Role of G-Protein Subunits

• α Subunit: Upon activation, the Gα subunit exchanges GDP for GTP. This activated GTP-bound Gα dissociates from the βγ complex and interacts with target proteins, notably adenylyl cyclase, to modulate its activity.

  • Specifically, the Gαs (stimulatory) subtype activates adenylyl cyclase, increasing cAMP levels.

  • Conversely, Gαi (inhibitory) subtypes inhibit adenylyl cyclase activity.

• β and γ Subunits: Usually form a dimer (Gβγ) that can regulate other signaling pathways but generally do not directly activate adenylyl cyclase.

Why the α Subunit Activates Adenylate Cyclase

• The α subunit possesses guanine nucleotide binding and hydrolysis activity and undergoes a conformational change upon GTP binding.

• This change allows it to directly bind to adenylyl cyclase, altering its catalytic activity.

• The interaction stimulates conversion of ATP to cyclic AMP, a key messenger that propagates cellular responses.

Summary

Given the subunit options, the correct answer is:

(1) α subunit

The α subunit of the heterotrimeric G protein activates adenylyl cyclase, making it the primary regulator of cAMP production during G protein-coupled receptor signaling.