G protein-coupled receptors (GPCRs) are primarily known for what function?

G protein-coupled receptors (GPCRs) are primarily recognized for their role in transmitting signals from the external environment to internal cellular processes. When a ligand, such as a hormone or neurotransmitter, binds to a GPCR on the cell membrane, it triggers a conformational change in the receptor. This change activates the associated G protein, which then dissociates and interacts with other intracellular signaling pathways. These pathways can lead to a cascade of intracellular events, including enzyme activation, ion channel modulation, and changes in gene expression.

The significance of GPCRs lies in their ability to convert extracellular signals into a functional response within the cell, affecting various physiological processes. This makes them crucial in mediating many biological responses, such as sensory perception, immune response, and mood regulation.

Other options refer to functions that are not the primary role of GPCRs. While they may influence intracellular enzyme activity indirectly, they do not serve as transcription factors themselves, nor do they directly produce primary messengers. Their main function is indeed to relay signals from the external environment into the cell, making option B a clear representation of their primary role.