Construction of dopamine sensors by using fluorescent ribonucleopeptide complexes.

A facile strategy of stepwise molding of a ribonucleopeptide (RNP) complex affords fluorescent RNP sensors with selective dopamine recognition. In vitro selection of a RNA-derived RNP library, a complex of the Rev peptide and its binding site Rev Responsive Element (RRE) RNA appended with random nucleotides in variable lengths, afforded RNP receptors specific for dopamine. The modular structure of the RNP receptor enables conversion of dopamine-binding RNP receptors to fluorescent dopamine sensors. Application of conditional selection schemes, such as the variation of salt concentrations and application of a counter-selection step by using a competitor ligand norepinephrine resulted in isolation of RNP receptors with defined dopamine-binding characteristics. Increasing the salt condition at the in vitro selection stage afforded RNP receptors with higher dopamine affinity, while addition of norepinephrine in the in vitro selection milieu at the counter-selection step reinforced the selectivity of RNP receptors to dopamine against norepinephrine. Thermodynamic analyses and circular dichroismic studies of the dopamine-RNP complexes suggest that the dopamine-binding RNP with higher selectivity against norepinephrine forms a pre-organized binding pocket and that the dopamine-binding RNP with higher affinity binds dopamine through the induced-fit mechanism. These results indicate that the selection condition controls the ligand-binding mechanism of RNP receptors.