Crystal structure of a transfer‐ribonucleoprotein particle that promotes asparagine formation

Four out of the 22 aminoacyl‐tRNAs (aa‐tRNAs) are systematically or alternatively synthesized by an indirect, two‐step route requiring an initial mischarging of the tRNA followed by tRNA‐dependent conversion of the non‐cognate amino acid. During tRNA‐dependent asparagine formation, tRNA^Asn promotes assembly of a ribonucleoprotein particle called transamidosome that allows channelling of the aa‐tRNA from non‐discriminating aspartyl‐tRNA synthetase active site to the GatCAB amidotransferase site. The crystal structure of the Thermus thermophilus transamidosome determined at 3 Å resolution reveals a particle formed by two GatCABs, two dimeric ND‐AspRSs and four tRNAs^Asn molecules. In the complex, only two tRNAs are bound in a functional state, whereas the two other ones act as an RNA scaffold enabling release of the asparaginyl‐tRNA^Asn without dissociation of the complex. We propose that the crystal structure represents a transient state of the transamidation reaction. The transamidosome constitutes a transfer‐ribonucleoprotein particle in which tRNAs serve the function of both substrate and structural foundation for a large molecular machine.