Practical Synthesis of N-Formylmethionylated Peptidyl-tRNA Mimics

Hydrolysis-resistant RNA-peptide conjugates that mimic peptidyl-tRNAs are frequently needed for structural and functional studies of protein synthesis in the ribosome. Such conjugates are accessible by chemical solid-phase synthesis, allowing for the utmost flexibility of both the peptide and the RNA sequence. Commonly used protection group strategies, however, have severe limitations with respect to generating the characteristic N^α-formylmethionyl terminus because the formyl group of the conjugate synthesized at the solid support is easily cleaved during the final basic deprotection/release step. In this study, we demonstrate a simple solution to the problem by coupling appropriately activated N^α-formyl methionine to the fully deprotected conjugate. The structural integrity of the obtained N^α-formylmethionyl conjugate─and hence the chemoselectivity of the reaction─were verified by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry sequence analysis. Additionally, we confirmed the applicability of our procedure for structural studies by obtaining two structures of the ribosome in complex with either fMAI-nh-ACCA or fMFI-nh-ACCA in the P site and ACC-PMN in the A site of the bacterial ribosome at 2.65 and 2.60 Å resolution, respectively. In summary, our approach for hydrolysis-resistant N^α-formylated RNA-peptide conjugates is synthetically straightforward and opens up new avenues to explore ribosomal translation with high-precision substrate mimics.