New and improved fluorescent protein tags

Fluorescent tags have been used as molecular markers for decades. They have been supremely useful in literally making particular proteins light up within cells. However, they do have drawbacks, most notably that they are too big to use in certain applications. Alice Ting and her colleagues from MIT have developed a new, smaller fluorescent tag that should eliminate this problem.

The first molecular probe, known simply as green fluorescent protein (GFP), has been helping biologists identify cells and cell functions since its isolation from jellyfish in 1962. To mark proteins with GFP, the GFP gene is fused to the gene for the protein of interest. A hybrid protein is thus created which will fluoresce under the right conditions, allowing scientists to watch it go about its business in the cell. Unfortunately, at a bulky 238 amino acids, GFP can interfere with the functioning of some proteins. In addition, all parts of the cell that contain the hybrid protein will fluoresce, making it impossible to differentiate between different cell regions.

To solve these problems, the MIT team created not only a new fluorescent probe, but also a whole new method of tagging interesting proteins (PRIME-- PRobe Incorporation Mediated by Enzymes). Their new protein tagging system contains three elements. First, they developed a new smaller blue fluorescent probe (7-hydroxycoumarin). Second, they genetically altered the protein they wished to label (in this case, actin) to include a special 13 amino acid sequence. And third, they created a new enzyme (fluorophore ligase) that attaches the blue probe to the 13 amino acid protein tag. The three ingredients are mixed together in a cell, and voila! Fluorescent blue protein.

To make the new probe even more useful, Ting and her lab have been able to direct their probe to only label proteins in specific parts of the cell. They do this by attaching genetic instructions to the ligase, limiting it to certain areas of the cell.

MIT researchers have designed a fluorescent probe that can be targeted to different locations within a cell.
Left, the probe only labels proteins in the cell membrane
Middle, the probe labels proteins in the cell nucleus
Right, the probe labels proteins in the cytosol.
Images: Katharine White and Tao Uttamapinant