Genetically Encoded Initiator for Polymer Growth from Proteins

Despite the importance of protein−polymer bioconjugates, there is no general method for producing homogeneous recombinant protein that contains polymer initiators at defined sites. To address this deficiency, we designed the amino acid 4-(2′-bromoisobutyramido)phenylalanine (1) as an initiator in atom-transfer radical polymerization (ATRP) that would provide a stable linkage between the protein and growing polymer. We synthesized 1 and evolved a Methanococcus jannaschii tyrosyl-tRNA synthetase/tRNA_CUA pair to genetically encode this initiator in response to an amber codon. To demonstrate the utility of this initiator, we produced green fluorescent protein (GFP) with 1 site-specifically incorporated on its surface (GFP−1). Purified GFP−1 was then used as an initiator under standard ATRP conditions with a monomer, oligo(ethylene oxide) monomethyl ether methacrylate, efficiently producing a polymer−GFP bioconjugate where the polymer is connected at our selected site on GFP.