Turning Off A Protein By Turning On A Light

If biochemists could dial in more or less of a certain protein in a cell, they could then watch how the cell responds to determine that protein’s role. Now researchers report a simple dimmer switch that lowers levels of a protein inside a cell using blue light (ACS Chem. Biol. 2013, DOI: 10.1021/cb400755b).

Most systems that use light to alter protein levels require two or more genetic tweaks to cells, says Kimberly M. Bonger, now at Radboud University, in Nijmegen, the Netherlands. She and her colleagues wanted to develop a method that worked with just one—the insertion of a light-sensitive domain into their gene of choice—to make the method easier to apply to a wide range of proteins.

The key to this simplification was a four-amino-acid sequence called a degron, which tells the proteasome—an enzyme complex that acts as the cell’s garbage disposal—to chew up the protein labeled with that sequence. To degrade a protein of interest, Bonger, while in the laboratory of Thomas J. Wandless at Stanford University, linked a degron to a light-sensitive protein domain called LOV2. In the dark, LOV2 folds up, and that hides the degron. But, when excited with blue light, the domain unfolds, exposing the degron and unleashing the wrath of the proteasome. Blue light doesn’t harm cells, so the switch shouldn’t affect any cellular function on its own.

To test the method, the researchers expressed the gene for yellow fluorescent protein with the LOV2-degron tag in mouse fibroblast cells. In the dark, the fluorescent signal from the cells was strong. After exposing the cells to blue light for two hours, the fluorescence decreased by 80 to 90%, signaling the protein’s demise.

Bonger plans to use the method to turn genes off and on by attaching the light-sensitive protein domain to transcription factors.