Site-Specific Incorporation of p-Propargyloxyphenylalanine in a Cell-Free Environment for Direct Protein−Protein Click Conjugation

The tyrosine analog p-propargyloxyphenylalanine (pPa), like tyrosine, has limited water solubility. It has been postulated that this limited solubility has contributed to reduced cellular uptake of pPa and thus reduced in vivo incorporation of pPa into proteins. Using a cell-free protein synthesis system (CFPS) to circumvent cellular uptake, pPa has been incorporated site-specifically into proteins with high specificity at yields up to 27 times greater than the highest previously reported yield. The alkyne group present on proteins incorporated with pPa provides a reactive residue for site-specific bioconjugation with the copper(I)-catalyzed azide-alkyne 3 + 2 cycloaddition (CuAAC). Previously, incorporation of another CuAAC-compatible unnatural amino acid p-azido-l-phenylalanine (pAz) was demonstrated with CFPS. However, incorporation of pPa may be preferred over pAz due to the instability of the pAz's aryl-azido moiety upon UV or near-UV light exposure. Also, the ability to incorporate site-specifically both reactants of the CuAAC (the alkyne group of pPa and the azido group of pAz) into proteins enables direct site-specific conjugation of heterologous proteins. We have demonstrated (for the first time to our knowledge) a one-step, linker-less, site-specific, direct protein-to-protein conjugation using CuAAC and unnatural amino acids.