Structure of the Pseudomonas Aeruginosa Transamidosome Reveals Unique Aspects of Bacterial tRNA-dependent Asparagine Biosynthesis
Many prokaryotes lack a tRNA synthetase to attach asparagine to its cognate tRNAAsn, and instead synthesize asparagine from tRNAAsn-bound aspartate. This conversion involves two enzymes: a nondiscriminating aspartyl-tRNA synthetase (ND-AspRS) that forms Asp-tRNAAsn, and a heterotrimeric amidotransferase GatCAB that amidates Asp-tRNAAsn to form Asn-tRNAAsn for use in protein synthesis. ND-AspRS, GatCAB, and tRNAAsn may assemble in an ∼400-kDa complex, known as the Asn-transamidosome, which couples the two steps of asparagine biosynthesis in space and time to yield Asn-tRNAAsn. We report the 3.7-Å resolution crystal structure of the Pseudomonas aeruginosa Asn-transamidosome, which represents the most common machinery for asparagine biosynthesis in bacteria. We show that, in contrast to a previously described archaeal-type transamidosome, a bacteria-specific GAD domain of ND-AspRS provokes a principally new architecture of the complex. Both tRNAAsn molecules in the transamidosome simultaneously serve as substrates and scaffolds for the complex assembly. This architecture rationalizes an elevated dynamic and a greater turnover of ND-AspRS within bacterial-type transamidosomes, and possibly may explain a different evolutionary pathway of GatCAB in organisms with bacterial-type vs. archaeal-type Asn-transamidosomes. Importantly, because the two-step pathway for Asn-tRNAAsn formation evolutionarily preceded the direct attachment of Asn to tRNAAsn, our structure also may reflect the mechanism by which asparagine was initially added to the genetic code.
Proceedings of the National Academy of Sciences
Suzuki, T.; Nakamura, A.; Kato, K.; Soll, D.; Tanaka, I.; Sheppard, K.; Yao, M., Structure of the Pseudomonas aeruginosa transamidosome reveals unique aspects of bacterial tRNA-dependent asparagine biosynthesis. Proc. Natl. Acad. Sci. U. S. A. 2015, 112 (2), 382-387.