Figure 11.

Labeling of membrane proteins such as GPCRs by metal ions recognizing small amino acid tags on live cells. A) A tetracysteine fragment (CCXXCC; X=A, G, or P; green) is genetically fused to the N terminus of a GPCR (gray). Addition of FlAsH (fluorescein arsenical hairpin binder) transfers the bisarsenic fluorescein onto the free thiols of the cysteine stretch, thus enhancing the fluorescence of the dye. B) The His₆‐tag (green), fused to the N terminus of a membrane protein like a GPCR (gray), can be labeled with Ni²⁺‐loaded tris‐NTA (N‐nitrilotriacetic acid). The metal ion is selectively chelated by clustered imidazole side chains of the oligo‐histidine sequence. To visualize the POI, the tris‐NTA is equipped with a respective reporter moiety like an organic dye.