Allosteric collaboration between elongation factor G and the ribosomal L1 stalk directs tRNA movements during translation

Determining the mechanism by which transfer RNAs (tRNAs) rapidly and precisely transit through the ribosomal A, P and E sites during translation remains a major goal in the study of protein synthesis. Here, we report the real-time dynamics of the L1 stalk, a structural element of the large ribosomal subunit that is implicated in directing tRNA movements during translation. Within pre-translocation ribosomal complexes, the L1 stalk exists in a dynamic equilibrium between open and closed conformations. Binding of elongation factor G (EF-G) shifts this equilibrium towards the closed conformation through one of at least two distinct kinetic mechanisms, where the identity of the P-site tRNA dictates the kinetic route that is taken. Within post-translocation complexes, L1 stalk dynamics are dependent on the presence and identity of the E-site tRNA. Collectively, our data demonstrate that EF-G and the L1 stalk allosterically collaborate to direct tRNA translocation from the P to the E sites, and suggest a model for the release of E-site tRNA.

💡 Research Summary

The paper investigates how transfer RNAs (tRNAs) move rapidly and accurately through the ribosomal A, P, and E sites during protein synthesis, focusing on the dynamic behavior of the large‑subunit L1 stalk. Using real‑time single‑molecule FRET, the authors monitored distance changes between fluorescently labeled L1 stalk and tRNAs in both pre‑translocation (PRE) and post‑translocation (POST) ribosomal complexes. In PRE complexes the L1 stalk fluctuates between open and closed conformations, establishing a dynamic equilibrium that is modulated by the identity of the P‑site tRNA. For instance, Met‑tRNA^Met favors the open state, whereas Pro‑tRNA^Pro shifts the equilibrium toward the closed state, indicating that tRNA structural features influence stalk‑tRNA contacts and the associated energy landscape.

Binding of elongation factor G (EF‑G) drives the stalk toward the closed conformation through at least two distinct kinetic routes. The first route involves the EF‑G·GDP·Pi ternary complex making a direct contact with the stalk, causing a rapid closure; this pathway predominates when Met‑tRNA^Met occupies the P site. The second route operates after GTP hydrolysis, when EF‑G·GDP interacts more indirectly, inducing a gradual closure; this route is favored with bulkier P‑site tRNAs such as Pro‑tRNA^Pro. In both cases, stalk closure provides a mechanical “push” that moves the P‑site tRNA toward the E site.

In POST complexes, the presence and type of the E‑site tRNA dictate L1 stalk dynamics. Without an E‑site tRNA the stalk remains largely open, reflecting complete tRNA release. When an E‑site tRNA (e.g., Lys‑tRNA^Lys) is retained, the stalk alternates between open and closed states, suggesting a “gripping” mode that holds the tRNA until it is ready for dissociation. These observations support an allosteric collaboration model: EF‑G binding triggers stalk closure, which in turn guides the P‑to‑E translocation and subsequently regulates E‑site tRNA release.

The study refines existing models of ribosomal translocation by demonstrating that EF‑G and the L1 stalk act as a coordinated mechanical engine whose activity is tuned by the specific tRNAs present. This mechanistic insight explains how the ribosome achieves high fidelity and speed, and it opens avenues for antibiotic development targeting the EF‑G–L1 stalk interface, as well as for engineering synthetic ribosomes with altered translocation dynamics.