Brian Kobilka knew that his postdocs didn't like him peeking at their experiments until they were finished. But he couldn't resist a quick look — after all, he and his entire field had been waiting for this result for more than 20 years.
As Kobilka peered through the microscope, the dream finally came into focus. Nestled in a drop of viscous liquid were tiny crystals, each trapping millions of copies of a fragile protein complex. The structure of this complex could finally reveal how one of biology's most important signalling mechanisms, G-protein-coupled receptors (GPCRs), do their job. This structure, published online in Nature1 by a team led by Kobilka at Stanford University in California and Roger Sunahara at the University of Michigan in Ann Arbor, now reveals the complete three-dimensional atomic structure of an activated GPCR — the β2 adrenergic receptor (β2AR) — in a complex with its G protein. GPCRs sit in the membranes of cells throughout the body, where they detect signals from the outside world — such as light, odours and flavours — and signals from within the body, such as hormones and neurotransmitters. These signals are transmitted to the inside of the cell where they activate intracellular G proteins, which then trigger a variety of biochemical pathways.