Molecule of the Month: Fluorescent RNA Aptamers

Scientists are constantly looking for new tools to explore cells in greater and greater detail. Green fluorescent protein is an example of a tool that opened entirely new doors. With it, we can tag specific proteins and then watch what they are doing inside living cells. Recently, scientists have been developing a new tool that allows us to watch RNA in a similar way. RNA itself is not fluorescent, so the trick is to design a short RNA that can bind to a fluorophore (a small fluorescent molecule) and enhance its fluorescence. Then, we can engineer this RNA into a natural RNA, such as a ribosome. When the fluorophore is added to the cell, it binds to the modified ribosome and we can watch where it goes.

SELEX (systematic evolution of ligands by exponential enrichment) has been used to discover these useful fluorophore-binding RNA molecules. The process begins mixing a fluorophore with many random RNA sequences, and then isolating any that bind. These are then randomly modified, and the best ones again selected. After several more rounds of modification and selection, an “aptamer” is found that binds to the fluorophore and enhances its fluorescence. The aptamer shown here, named “Spinach”, was discovered by this process using a flourophore similar to the one in green fluorescent protein.

The Spinach aptamer is a long hairpin, with an intricately-folded section at the center that surrounds the fluorophore. RNA is always difficult to crystalize, so two tricks were used to determine its structure. In PDB entry 4kzd (shown in the illustration), the loop at one end was engineered to bind to an antibody, which assists with forming a stable crystal lattice. In PDB entry 4ts2 (shown in the JSmol below), the loop was clipped off, making it easier for the molecules to pack end-to-end in to form a crystal.

Other fluorescent aptamers have been discovered, in a rainbow of colors. This is very useful since it allows us to tag several different types of RNA molecules in one cell, using different colors to distinguish them. Also, techniques like FRET can be used to monitor distances between different flourophores in living cells. Three examples are shown here: the yellow Corn aptamer (PDB entry 5bjp), orange Mango-II (PDB entry 6c63), and red DIR2s (PDB entry 6db8).