Final answer:
Activation of the G protein following the first messenger's binding entails an exchange of GDP for GTP on the α subunit, causing fragmentation of the G protein to the α subunit and βγ complex. These components interact with other membrane proteins, generating a cascade of intracellular effects. Ultimately, the GTP is hydrolyzed to GDP, deactivating and reassociating the G protein components.
Step-by-step explanation:
Upon the activation of a G protein by the first messenger - a signaling molecule which binds to the G-protein-coupled receptor, a series of events occurs that are central to the process of signal transduction. The G protein is composed of three subunits, and the binding of the first messenger prompts the exchange of GDP for GTP on the α subunit. This results in the dissociation of the G protein into two parts: the α subunit and the βγ complex.
After the GTP-GDP exchange on the α subunit, this activated α subunit, now bound to GTP, can then go on to interact with and activate other proteins in the cell, such as enzymes or ion channels embedded in the cell membrane. This leads to the generation of a second messenger, such as cyclic AMP (cAMP), or the opening of ion channels, further propagating the signal within the cell. The activation of these effector proteins by the G protein α subunit and βγ complex leads to a series of cellular responses that constitute the next steps in cell signaling pathways.
Eventually, the GTP on the active α subunit is hydrolyzed back to GDP, deactivating the G protein. The α subunit reassociates with the βγ complex, forming the inactive G protein, readying the cell for another round of signal transduction when a new first messenger arrives.