An unnatural amino acid has given researchers a switch to turn on a specific enzyme with light. This tool will allow scientists to determine the timing of cellular signaling and identify which parts of a signaling network might be good drug targets.
When organisms or cells receive signals from their surroundings, a cascade of enzymes known as kinases pass those signals along. Each kinase phosphorylates the next one in the chain, until the last one in line receives the signal to perform the desired function. These signaling cascades are complicated networks that can be hard to interpret in real time.
Jason W. Chin and Arnaud Gautier of the Medical Research Council Laboratory of Molecular Biology, in Cambridge, England, and Alexander Deiters of North Carolina State University may have found a way to make such interpretation easier by uncoupling a kinase in the middle of the cascade from its usual upstream signals (J. Am. Chem. Soc., DOI: 10.1021/ja1109979).
They do this by adding a previously reported unnatural photocaged lysine to the portion of the kinase where its phosphate source, adenosine triphosphate (ATP), binds (J. Am. Chem. Soc., DOI: 10.1021/ja910688s). This unnatural lysine keeps the kinase shut down until light removes the photocaging group to reveal normal lysine, thereby turning on the kinase. In addition, the researchers alter the kinase so that it doesn't need its usual inputs. By so doing, they transform a kinase that would normally be in the middle of a signaling network into one that's on the top of its own network.
Chin and coworkers use the enzyme MEK1 as a model system. This kinase is in the middle of a signaling pathway that controls a variety of responses to external stimuli, including cell proliferation. Shining light on cells containing a photocaged MEK1 removes the photocage and turns on the phosphorylation of its downstream kinases within a minute of illumination. Adjusting the illumination time modulated the amount of phosphorylation.
Although Chin and coworkers have demonstrated only one model system, Chin expects that the technique will be general because the same lysine is found in the ATP-binding site of nearly all kinases.
"We've done this for one kinase in the middle, but obviously we could do it for every kinase in the cascade and get information about every elementary step," Chin says. "We put that together and we've got a quantitative model of the whole cascade."
Compared with other methods of taking control of individual kinases in a signaling cascade, Chin's method is rapid and specifically activates, rather than inhibits, a single targeted kinase.
Dario R. Alessi, a researcher in the MRC Protein Phosphorylation Unit at the University of Dundee, in Scotland, says Chin's "technology could be deployed to regulate the activation of any kinase almost instantaneously by light." The challenge, he says, "will be to think of how this technology could be used to solve a significant problem in signal transduction."