Two strategies for extracellular labeling of TRPV1. (A) TRPV1 expression in DRG neurons is distributed across the plasma membrane and intracellular compartments (top). If the extracellular side of TRPV1 can be labeled at a click-chemistry site, labeling the plasma membrane population of TRPV1 is possible (bottom). (B, i and ii) Amber codon suppression techniques permit the substitution of a ncAA at a single residue in the TRPV1 sequence. The addition of an ncAA with click-chemistry properties in an extracellular loop of TRPV1 allows surface labeling of the ion channel (PDB accession no. 7LQY; B, i). By introducing the HaloTag domain (blue; PDB accession no. 6U32) to an extracellular loop of TRPV1, cell-impermeable HaloLigands can be specifically targeted to the population of channels that are expressed on the plasma membrane (B, ii).