More than two millennia ago, the Roman natural philosopher Pliny the Elder wrote of a "slime" that could be obtained from marine creatures and used to make objects glow green. Now, molecular and cellular biologists have turned this marvel into a powerful research tool used to illuminate the workings of genes and follow the wanderings of protein molecules inside a living cell. The source of the green glow is a unique protein, called green fluorescent protein (GFP), found in a Pacific Northwest jellyfish.
National Science Foundation-supported biologists James Remington of the University of Oregon and Roger Tsien of the University of California at San Diego have determined the structure of an altered form of the protein and revealed the source of the green glow. "The unusual molecule responsible for the glow resembles a chinese finger puzzle: a barrel-shaped structure with a coil of amino acids corked in the center," explains Kamal Shukla, program director in NSF's division of molecular and cellular biosciences, which funded the research. "The green color results from the collapse of one turn of the coil to form a ring of three amino acids." The researchers chose to study the modified GFP because it is more useful for cell biologists, glowing much more brightly than the natural variant.
In a second experiment, the researchers deliberately changed one amino acid in contact with the green pigment, in hopes that change would make GFP glow yellow, rather than the green of the original protein. It worked, giving biologists a new tool to track the location in a living cell of two proteins simultaneously, and to determine whether two genes are "turned on" at the same time. The researchers hope to make more colors, including orange and red, by introducing other mutations into the protein.
The uses for GFP are almost unlimited, they say. In gene therapy, doctors could inject GFP along with the therapy substance, and by checking for fluorescence, determine whether the therapy had been properly delivered. By "tuning" the green dye to different colors, scientists studying vision may be able to gain insight into how the human eye is able to respond to so many different colors. Says Kamal Shukla, "Pliny the Elder probably never would have guessed where his original observations would lead."