A Simplified Protocol to Incorporate the Fluorescent Unnatural Amino Acid ANAP into Xenopus laevis Oocyte-Expressed P2X7 Receptors

The long intracellular P2X7 C-terminus accounts for diverse downstream effects of P2X7 activation. Although the recent determination of the cryo-EM structure of the full-length P2X7 receptor finally revealed the structure and several unexpected features of the large cytoplasmic domain, its molecular function remains enigmatic. Incorporation of unnatural amino acids (UAA) via an amber Stop codon has been a powerful tool for structure-function analysis of proteins. Voltage clamp fluorometry (VCF) with the fluorescent unnatural amino acid L-3-(6-acetylnaphthalen-2-ylamino)-2-aminopropanoic acid (ANAP) provides a means to study intracellular domain movements of ion channel receptors. In the Xenopus laevis oocyte expression system, site-specific introduction of this environment-sensitive fluorophore can be achieved by the nuclear injection of cDNA encoding an orthogonal amber suppressor tRNA/aminoacyl-tRNA synthetase pair and subsequent cytoplasmic injection of ANAP together with the respective cRNA containing the amber Stop codon. Here, we describe this protocol for expression of ANAP-labeled P2X7. In addition, we provide a simplified alternative protocol, in which we coinject cRNAs encoding the tRNA synthetase and mutant P2X7 together with the synthesized amber suppressor tRNA and ANAP in one step into the cytosol. We found that the new protocol yielded more reproducible results and was less harmful for the oocytes. By selective fluorescence labeling of the ANAP-labeled P2X7 protein in the oocyte plasma membrane and VCF recordings, we show that this method results in comparable levels of functional ANAP-labeled P2X7 protein.