Construction of Protein-Based Biosensors Using Ligand-Directed Chemistry for Detecting Analyte Binding

Receptor-based fluorescent sensors constructed from ribonucleopeptide

Receptor-based fluorescent sensors are the representative tool for quantitative detection of target ligands. The high substrate-selectivity originated from biomacromolecule receptor is one of the advantages of this tool, but a laborious trial and error is usually required to construct sensors showing satisfactory fluorescence intensity changes without diminishing the function of parent receptor. Ribonucleopeptide (RNP) provides a scaffold of fluorescent sensors to improve such issues. RNP receptors for the ligand of interest are constructed by applying in vitro selection for RNA-derived RNP library. Simple modification of the N-terminal of peptide in RNP by an appropriate fluorophore converts the RNP receptor into the fluorescent sensor with retaining the affinity and selectivity for the substrate. In this chapter, we introduce the protocols for construction of fluorescent RNP sensors through selection from a library of fluorophore-modified RNP complex or by a structure-based modular design. Furthermore, we describe the application of covalently linked RNP sensors for simultaneous detection of multiple ligands.

Ligand-Directed Chemistry of AMPA Receptors Confers Live-Cell Fluorescent Biosensors

AMPA-type glutamate receptors (AMPARs) mediate fast excitatory synaptic transmission in the central nervous system. Dysregulation of AMPAR function is associated with many kinds of neurological, neurodegenerative, and psychiatric disorders. As a result, molecules capable of controlling AMPAR functions are potential therapeutic agents. Fluorescent semisynthetic biosensors have attracted considerable interest for the discovery of ligands selectively acting on target proteins. Given the large protein complex formation of AMPARs in live cells, biosensors using full-length AMPARs retaining original functionality are ideal for drug screening. Here, we demonstrate that fluorophore-labeled AMPARs prepared by ligand-directed acyl imidazole chemistry can act as turn-on fluorescent biosensors for AMPAR ligands in living cells. These biosensors selectively detect orthosteric ligands of AMPARs among the glutamate receptor family. Notably, the dissociation constants of agonists and antagonists for AMPARs were determined in live cells, which revealed that the ligand-binding properties of AMPARs to agonists are largely different in living cells, compared with noncellular conditions. We also show that these sensors can be applied to detecting allosteric modulators or subunit-selective ligands of AMPARs. Thus, our protein-based biosensors can be useful for discovering pharmaceutical agents to treat AMPAR-related neurological disorders.