Final answer:
The free beta-gamma complex activates cell signaling pathways by interacting with effector proteins, thereby generating secondary messengers like cAMP or Ca2+ which then propagate cellular responses. The alpha subunit separately activates enzymes such as adenylate cyclase, contributing to the pathway. The G-protein cycle is reset by reassociation of the subunits after the alpha subunit hydrolyzes GTP to GDP.
Step-by-step explanation:
The free beta-gamma complex directly activates cell signaling pathways by triggering a series of downstream effects within a cell. When a hormone or other ligand binds to a plasma membrane receptor, it causes a conformational change that activates a G-protein. The G-protein, composed of alpha, beta, and gamma subunits, undergoes a GDP/GTP exchange, leading to the dissociation of the alpha subunit.
The free beta-gamma complex can then interact with various effector proteins such as ion channels or enzymes like phospholipase C to generate secondary messengers. These secondary messengers, such as cAMP, calcium ions (Ca2+), or others, then propagate the cellular response, such as gene transcription or muscle contraction.
In the example provided, the alpha subunit would be responsible for activating a transmembrane enzyme like adenylate cyclase, leading to the production of cAMP and activation of Protein Kinase A (PKA), which could lead to various responses based on the cell type. The beta-gamma complex may play a role in activating different enzymes or channels within the same signaling pathway.
Eventually, to reset the signaling, the alpha subunit re-associates with beta and gamma subunits in the presence of GDP, forming an inactive G-protein complex ready for the next signaling event.