Target immobilization on glass microfiber membranes as a label-free strategy for hit identification.

The discovery of novel chemical entities targeting G protein-coupled receptors (GPCRs) is usually guided by their receptor affinity. However, traditional affinity assay methods and hit identification procedures are usually laborious and expensive. In this work, the type-2 vasopressin receptor (VR) was chosen as a prototypical GPCR. Membrane fragments from cells highly expressing SNAP-VR were immobilized on the surface of a glass microfiber (GMF) coated with O-benzylguanine (BG). This was achieved by transferring the benzyl group of BG to the active site of the SNAP-tag through a nucleophilic substitution reaction. As a result, a biofilm called SNAP-VR@GMF-BG was produced that showed good specificity and stability. The adsorption ratio for each VR ligand treated with SNAP-VR@GMF-BG was determined by HPLC and exhibited a good linear correlation with the K value determined by displacement assays. Furthermore, a K prediction assay was performed by comparing the data with that generated by a homogeneous time-resolved fluorescence (HTRF) assay. SNAP-VR@GMF-BG was also used to screen hit compounds from natural products. After SNAP-VR@GMF-BG was incubated with the total extract, the ligand that binds to VR could be separated and subjected to LC‒MS analysis for identification. Baicalein was screened from Clerodendranthus spicatus and verified as a potential VR antagonist. This VR-immobilized GMF platform can help determine the affinity of VR-binding hit compounds and screen the compounds efficiently and accurately.