Since the 1990s, molecular biologists have tagged biomolecules with proteins that fluoresce when exposed to light to observe and decipher fundamental cellular processes.

Although green fluorescent protein (GFP), a well-known tracer, has long been used for protein imaging, most GFP-like proteins are too large, slow to fluoresce, and unstable for many applications.

Researchers in the journal PNAS describe a protein tag dubbed Yellow Fluorescence-Activating and absorption-Shifting Tag (Y-FAST), which is much smaller in size than GFP and fluoresces instantaneously upon activation by a cell-permeant and nontoxic ligand.

The authors engineered Y-FAST from photoactive yellow protein using directed evolution, a laboratory technique that mimics the process of natural selection to produce proteins with specific characteristics.

The engineered traits allow Y-FAST to label targets with a high degree of selectivity and in a reversible manner, thus providing a fluorescence on/off switch.

According to the authors, Y-FAST compares favorably to other fluorescent proteins in terms of brightness and stability and is less likely to perturb the normal function of its binding targets. 

http://www.pnas.org/content/early/2015/12/23/1513094113