Specific N-terminal biotinylation of a protein in vitro by a chemically modified tRNA fmet can support the native activity of the translated protein

Abstract

Biotinylation of a protein generally involves chemical modification of a translated protein. Using this methodology, however, biotinylation at a specific position remains difficult. We investigated whether it would be possible to use an Escherichia coli initiator tRNA fmet aminoacylated with methionine biotinylated at the α-amino group to introduce a biotin tag specifically at the N terminus. We report here that a biotin tag could be incorporated into the green fluorescent protein (GFP) at the N-terminal site, in the presence of an E. coli initiator tRNA fmet aminoacylated with methionine biotinylated at the α-amino group. The biotinylated GFP was purified by simple monomeric streptavidin-agarose affinity column chromatography. Based on the total amount of GFP molecules, the purification yield and the biotin labelling efficiency of this system were approximately 7% and 10–20%, respectively, according to the densitometric analysis of Western blots. Judging from the results of a fluorescence imaging experiment, almost all the purified GFP molecules retained the native fluorescence activity. Importantly, the present results support the hypothesis that the E. coli initiator tRNA fmet aminoacylated with a relatively large substituent can be recognized by an E. coli ribosome and adequately placed at the P site to initiate translation.