Final answer:
Heterotrimeric G proteins consist of three subunits (alpha, beta, and gamma) that are involved in transmitting signals from G-protein-coupled receptors into cells, leading to various cellular responses. The signal transduction process is terminated by the hydrolysis of GTP to GDP, allowing the G protein to return to its inactive state.
Step-by-step explanation:
Heterotrimeric G proteins play a pivotal role in cellular signaling, translating extracellular signals into diverse intracellular responses. Comprising alpha (α), beta (β), and gamma (γ) subunits, these proteins are crucial components of G-protein-coupled receptor (GPCR) signaling pathways.
The signaling cascade initiated by GPCRs begins with the binding of a signaling molecule to the receptor on the cell surface. This event triggers a conformational change in the receptor, facilitating the exchange of GDP for GTP on the alpha subunit of the associated G protein. Subsequently, the alpha subunit separates from the beta-gamma pair, allowing both entities to interact with downstream effectors in the cell.
The dissociation of the alpha subunit and the beta-gamma pair enables them to modulate the activity of various intracellular signaling proteins. For example, the alpha subunit might activate or inhibit specific enzymes or ion channels, while the beta-gamma pair can also participate in signaling events independently. These interactions transmit the signal and orchestrate a range of cellular responses, including changes in gene expression, alterations in enzyme activity, or adjustments in ion channel conductance.
The termination of the signaling cascade occurs when the intrinsic GTPase activity of the alpha subunit hydrolyzes GTP to GDP. This hydrolysis event promotes the reassociation of the alpha subunit with the beta-gamma pair, restoring the inactive heterotrimeric G protein state. This cycle of activation, signal propagation, and deactivation provides a finely tuned mechanism for regulating cellular responses to extracellular signals, ensuring precise control and coordination of various physiological processes.