Human bitter taste receptors undergo internalization in an agonist-selective fashion

The cellular routing of human bitter taste receptors (TAS2Rs) is complex and poorly investigated. However, profound knowledge about this process is essential for elucidating receptor function, regulation, and potential therapeutic targeting of receptor trafficking pathways. Our study investigated the cell surface localization of TAS2Rs and their internalization mechanism upon agonist treatment. To monitor acute receptor cell surface expression in living cells, the HiBiT sequence was fused with the sst3 "export-tag" located at the amino-terminal end of TAS2R constructs used for heterologous expression. The addition of the HiBiT sequence allowed the study of cell surface localization and receptor trafficking with high accuracy and speed. Fluorescence assays to assess receptor function were combined with newly developed luminescence assays for cell surface localization. All tested TAS2Rs exhibited different expression levels on the cell surface. Of several TAS2Rs, TAS2R14 exhibits an agonist-dependent internalization effect in agreement with the recently detected two agonist binding sites in this receptor. The internalization and cell surface expression of TAS2Rs might influence the receptor's role in various physiological processes, both in taste perception and in TAS2R-expressing extraoral tissues. In particular, TAS2R14 has been implicated in cancer progression and treatment due to its high expression level in some malignant tissues and its capacity to induce apoptosis. Knowledge about ligand-specific internalization of this receptor could help devise treatment options for nanomedical delivery approaches.