Dr. Lefkowitz said that although the notion of cell receptors went back more than a century, “when I kind of started my work in the area in the early ’70s, there was still a lot of skepticism as to whether there really was such a thing.” By attaching radioactive iodine to a hormone, Dr. Lefkowitz was able to track the movement of the hormone and explore the behavior of these receptors. Over the years, he was able to pull out the receptor proteins and show they were specific molecules.

In the 1980s, his group, which included Dr. Kobilka as a postdoctoral researcher, searched for and found the gene that produced one of these protein receptors. The genetic blueprint indicated that the shape of the protein included long spirals that wove through the cell membrane seven times. Meanwhile, other researchers had discovered a class of proteins, called G proteins, inside the cell that, when activated, set off a Rube Goldberg cascade of molecular machinery.

The receptor was the last missing piece. “If you have something like adrenaline, it sticks in there, turns the key, changes the shape of the receptor, and now the receptor, having changed shape, is able to tickle the G protein,” Dr. Lefkowitz said.

There was a “eureka moment,” Dr. Lefkowitz said, when he realized that his receptor was the same as another receptor that had been found in another part of the body — the light receptor rhodopsin in the retina. “We said, ‘Well, wait a moment, maybe anything which couples to a G protein looks like this,’ ” he said.

Within a year, they were able to decode the genetic blueprints for several other similar receptors, and they were right.

About 1,000 of these receptors, known as G protein-coupled receptors, are now known, residing on the surface of cells and reacting to a host of hormones and neurotransmitters.