G protein-coupled receptors (GPCRs) are molecular sensors embedded in the cell membrane. They allow cells to respond to hormones, neurotransmitters, and many other signals, and are among the most important targets for modern medicines. These receptors are often pictured as switches that turn on when a signaling molecule binds. In a living cell, however, they move through a crowded membrane and must encounter the right signaling partner (G protein), before a signal can be transmitted. How this membrane environment shapes signaling remains poorly understood.

In this talk, I will show how single-molecule fluorescence methods in cultured human cells can reveal this hidden spatial layer of GPCR signaling. Using the muscarinic M1 and adenosine A2A receptors as complementary model systems, I target individual molecules to connect receptor motion, assembly, local confinement, and proximity to G proteins. For M1, mobility and oligomerization change with receptor density, ligand condition, and membrane fluidity, showing that receptor assembly is dynamically shaped by its surroundings. Comparing receptor–G protein pairs reveals a further contrast: A2A and Gαs are enriched within shared confined regions even before stimulation, whereas M1 and Gα11 show limited basal association. This difference parallels the strong ligand-independent signaling of A2A and the weaker case of M1.

Finally, I will introduce a combined Förster resonance energy transfer and single-particle tracking approach that distinguishes broad spatial overlap from nanometre-scale receptor–G protein proximity. Together, these results present the plasma membrane as a dynamic reaction landscape that helps determine where, when, and how efficiently cellular signaling occurs.