How tuberculosis enzyme tells initiator and elongator tRNA apart

Protein synthesis is a central process that many antibiotics target, and understanding how Mycobacterium tuberculosis (Mtb) builds proteins can point to new anti-tubercular strategies. A key enzyme in that process is methionyl-tRNA synthetase (MetRS), which has roles in both the initiation and elongation phases of protein synthesis by attaching methionine to tRNA. A critical step is how MetRS reads the CAU anticodon on tRNA, but until now there have been no experimental structures showing how Mtb MetRS binds these tRNAs. To fill that gap, Rukmankesh Mehra and colleagues created models of Mtb MetRS bound to both the initiator and elongator tRNAs and explored their behavior using molecular dynamics simulations totaling 6 µs. By focusing on the molecular recognition of the CAU anticodon and the contacts between tRNA and MetRS domains, the team aimed to reveal whether and how initiator and elongator tRNAs are handled differently by the enzyme during charging.

The study used modeled complexes of Mtb MetRS with initiator and elongator tRNAs and ran molecular dynamics simulations for a combined 6 µs to observe binding behavior over time. The simulations showed that the elongator tRNA formed a stable association with the protein, whereas the initiator tRNA was more transiently bound, though major intra-tRNA interactions remained intact in both cases. In the simulated complexes, tRNA contacted the MetRS active site and the anticodon domain. Electrostatic attractions between tRNA and the protein’s catalytic domain likely promoted charging with methionine. Meanwhile, a mix of repulsive and attractive forces between tRNA and the protein’s connective peptide domain and KMSKS loop appeared to trigger opening of the binding pocket, enabling the chemical reaction and product release. Strong binding of tRNA to the anticodon domain also facilitated the sequence of events. At the residue level, tRNA formed salt-bridges with positively charged Arg and Lys, while negatively charged Asp and Glu produced repulsive interactions. Together these observations led the authors to propose a plausible mechanism for how Mtb MetRS differentiates initiator versus elongator tRNA during charging.

These findings matter because protein synthesis is an essential target for anti-tubercular drug design, and MetRS sits at a key decision point in that process. The work suggests that initiator tRNA may be charged more rapidly and transiently, while elongator tRNA engages the enzyme in a more stable manner, a difference that could be exploited when designing inhibitors that disrupt tRNA recognition or charging. Mapping how electrostatic interactions, the connective peptide domain and the KMSKS loop coordinate pocket opening, reaction and product release gives a step-by-step picture of the charging pathway. Even though the study is based on modeled complexes rather than experimental structures, the simulated behavior provides a concrete hypothesis for how CAU anticodon recognition and domain movements drive methionine transfer. Those mechanistic details could help prioritize regions of MetRS to target with small molecules or other interventions that aim to block protein synthesis in Mtb.