A highly stable RNA aptamer probe for the retinoblastoma protein in live cells.

Although RNA aptamers can show comparable or better specificity and affinity to antibodies and have the advantage of being able to access different live cell compartments, they are often much less stable in vivo. We report here the first aptamer that binds human retinoblastoma protein (RB) and is stable in live cells. RB is both a key protein in cell cycle control and also a tumour suppressor. The aptamer was selected from an RNA library against a unique 12-residue helical peptide derived from RB rather than the whole protein molecule. It binds RB with high affinity (K d = 5.1 ± 0.1 nM) and is a putative RNA G-quadruplex structure formed by an 18-nucleotide sequence (18E16 - GGA GGG UGG AGG GAA GGG), which may account for its high stability. Confocal fluorescence microscopy of live cells transfected with the aptamer shows it is stable intracellularly and efficient in entering the nucleus where an analogous antibody was inaccessible. The findings demonstrate this aptamer is an advanced probe for RB in live cell applications.