Photoactivatable fluorescent proteins for diffraction-limited and super-resolution imaging.

Photoactivatable fluorescent proteins (PA-FPs) are molecules that switch to a new fluorescent state in response to activation to generate a high level of contrast. Over the past eight years, several types of PA-FPs have been developed. The PA-FPs fluoresce green or red, or convert from green to red in response to activating light. Others reversibly switch between 'off' and 'on' in response to light. The optical "highlighting" capability of PA-FPs has led to the rise of novel imaging techniques providing important new biological insights. These range from in cellulo pulse-chase labeling for tracking subpopulations of cells, organelles or proteins under physiological settings, to super-resolution imaging of single molecules for determining intracellular protein distributions at nanometer precision. This review surveys the expanding array of PA-FPs, including their advantages and disadvantages, and highlights their use in novel imaging methodologies.