When a scientist labels a membrane protein with a dye, she has to wash the cells a few times to remove dye molecules that aren’t bound to the protein. Without this rinse cycle, the free dye produces a background signal that can swamp the signal from the protein. Unfortunately, washing takes time and mostly limits these studies to cells in culture. “You can’t wash inside a mouse,” says Howard Riezman at the University of Geneva.
He and his coworkers skipped the washing step by using the dye Nile Red, a hydrophobic molecule that glows bright in greasy environments, such as inside cell membranes, but emits almost no light in water. They tethered the dye to a benzyl guanine group that allows it to covalently link to a protein the researchers previously designed called SNAP-tag.
To use the probe, researchers first must add the DNA sequence for SNAP-tag to the gene for their membrane protein of interest. With this genetic modification, SNAP-tag sits on the protein’s outer surface. Once researchers add the probe to cells expressing the hybrid gene, the modified dye attaches to the membrane protein, inserts itself into the membrane, and starts to glow.
Using the probe, Riezman and his coworkers labeled human insulin receptors expressed with SNAP-tag in hamster cells in a half hour. They next plan to use the probe to light up cell division in living fruit flies. Another useful property of the probe, Riezman says, is that the wavelength of light the probe emits reveals how tightly the membrane lipids pack around the protein. Scientists want to understand how this packing influences cellular processes.