Final answer:
G-proteins, consisting of α, β, and γ subunits, are crucial for cell signaling. Upon ligand binding to GPCR, GDP is exchanged for GTP on the α subunit, triggering the dissociation of the subunits and initiation of signaling pathways. Both the activated α subunit and βγ dimer contribute to the signaling process, which is terminated when the GTP is hydrolyzed back to GDP.
Step-by-step explanation:
G-protein Signal Transduction
Contrary to the student's initial assertion that the beta/gamma subunits (Bγ) have no signal transduction capability, it is essential to understand that heterotrimeric G-proteins play a crucial role in cell signaling. They consist of three subunits: alpha (α), beta (β), and gamma (γ). In the inactive state, the α subunit is bound to GDP and is associated with the βγ subunits.
Upon the binding of a signaling molecule (ligand) to a G-protein-coupled receptor (GPCR), the α subunit releases GDP and binds GTP. This exchange induces a conformational change leading to the dissociation of the α subunit from the βγ subunits. Following the dissociation, the α subunit, now activated by GTP, as well as the βγ dimer, can initiate different signaling pathways, thus both contributing to the cell's response.
Lastly, the signal is terminated by the hydrolysis of GTP to GDP on the α subunit, rendering it inactive. The inactive α subunit re-associates with the βγ subunits, resetting the G-protein complex to its original state, ready for another cycle of signal transduction. This cycle is critical for many physiological processes like vision, taste, immune response, and inflammation.