Green fluorescent protein has done wonders in biology labs to track all kinds of things in vivo. Being a pretty bulky protein itself, though, makes it difficult to use GFP to label other proteins. Now MIT researchers have developed a method called PRIME (PRobe Incorporation Mediated by Enzymes) that uses a much smaller blue fluorescent probe (7-hydroxycoumarin) that doesn’t seem to interfere with natural function of proteins it’s been attached to.
Unlike GFP, the new probe is not joined to the target protein as it’s produced inside the cell. Instead, the probe is attached later on by a new enzyme that the researchers also designed.
For this to work, the researchers must add the gene for the new enzyme, known as a fluorophore ligase, to each cell at the same time that they add the gene for the protein of interest. They also add a short tag (13 amino acids) to the target protein, and this tag allows the enzyme to recognize the protein. When the blue fluorescent probe (7-hydroxycoumarin) is added to the cell, the enzyme attaches it to the short tag on the target protein.
With this method, proteins such as actin can move freely throughout the cell and cross into the nucleus even when tagged with the fluorescent probe.
The researchers also demonstrated that they can label proteins in specific parts of the cell, such as the nucleus, cell membrane [see image] or cytosol (the interior of the cell), by tagging the enzyme with genetic sequences that direct it to specific locations. That way, the enzyme attaches the fluorescent probe only to proteins in those locations.
MIT press release: Giving proteins a new glow…
Abstract in PNAS: A fluorophore ligase for site-specific protein labeling inside living cells
