The G-protein is a heterotrimer composed of which subunits?

• A. Delta, Epsilon, and Zeta
• B. Mu, Pi, and Sigma
• C. Alpha, Beta, and Gamma
• D. Kappa, Lambda, and Theta

Answer: (C)

The main idea is that a G-protein coupled receptor signal uses a heterotrimeric G protein made of three subunit types: alpha, beta, and gamma. In the resting state, the alpha subunit binds GDP and sits with the beta-gamma dimer. When the receptor is activated, GDP on alpha is swapped for GTP, and the alpha subunit dissociates from the beta-gamma pair to regulate downstream effectors like adenylyl cyclase or phospholipase C. Meanwhile, the beta-gamma dimer can also modulate other effectors, such as certain ion channels. Because the canonical G-protein is built from alpha, beta, and gamma subunits, the other groupings listed don’t form this classic heterotrimer.