Final answer:
Trimeric GTP-binding proteins are involved in cell signaling; they consist of three subunits and activate upon a ligand binding to a G protein-coupled receptor. Monomeric GTPases are similar in their GDP/GTP binding and act as molecular switches. GPCRs have seven transmembrane domains and trigger diverse signaling pathways by activating G proteins.
Step-by-step explanation:
Trimeric GTP-binding proteins, also known as heterotrimeric G proteins, are composed of three subunits: α (alpha), β (beta), and γ (gamma). When a signaling molecule, such as a hormone, binds to a G protein-coupled receptor (GPCR) on the plasma membrane, a GDP molecule associated with the α subunit is exchanged for GTP. This causes the β and γ subunits to dissociate from the α subunit, and triggers a cellular response, which can be through the α subunit alone or the dissociated βγ pair. The hydrolysis of GTP back to GDP by the α subunit terminates the signaling.
Monomeric GTPases, which also bind GTP, are small G proteins that act as molecular switches in many cellular processes. Just like trimeric G proteins, their activity is regulated by the exchange of GDP for GTP and hydrolysis of GTP to GDP, which turns them 'on' or 'off', respectively. GPCRs consist of seven transmembrane domains and are involved in transmitting the signal from a ligand to initiate the G-protein activation. Different receptors have different specific extracellular domains and G-protein binding sites, accounting for the diversity of signaling pathways they regulate. GPCRs can activate different types of G proteins (Gs, Gi, etc.), which then interact with various downstream effectors like ion channels or enzymes, such as adenylate cyclase that produces cAMP.