Final answer:
Adenylate cyclase is activated by G protein-coupled receptors, leading to the production of cAMP that activates various cellular responses.
Step-by-step explanation:
Adenylate cyclase is usually activated by G protein-coupled receptors. When these receptors bind a ligand, they activate an associated G-protein. The activated G-protein then interacts with and activates adenylate cyclase. This enzyme converts ATP to cyclic AMP (cAMP), which acts as a second messenger in the cell. cAMP activates a group of proteins called protein kinases, most notably protein kinase A (PKA). This leads to a cascade of phosphorylation of other proteins, thereby transmitting the signal within the cell, which can cause a variety of cellular responses such as activation of gene transcription.