RNA therapeutics have come a long way since the discovery of RNA interference, which earned its discoverers the 2006 Nobel Prize for Physiology and Medicine. This natural cellular mechanism has been harnessed to create a powerful class of drugs capable of suppressing disease-related genes. Seven such drugs have already received FDA approval, including inclisiran, a groundbreaking treatment that replaces daily cholesterol-lowering pills with biannual injections. However, despite these clinical successes, the molecular details of how RNA interference machinery executes its cuts have remained elusive.
In a groundbreaking study published in Nature Structural & Molecular Biology, Scripps Research scientists have revealed the first high-resolution structural images of the human RNA interference molecular machinery in its slicing-ready state. This achievement provides a comprehensive understanding of the precise atomic interactions that determine when and where the machinery cuts. The study identifies key protein building blocks and offers a mechanistic explanation for why certain RNA sequences are more effective at cutting their targets.
The research, led by Professor Ian MacRae, focused on Argonaute 2, a critical component of the RNA interference system. Postdoctoral associate Sucharita Sarkar and staff scientist Luca Gebert, co-first authors of the study, successfully determined the high-resolution structure of Argonaute 2 in a cutting-ready state, bound to an siRNA molecule. They achieved this by employing a clever set of mutations that stabilized the active conformation, allowing them to use cryo-electron microscopy to capture the atomic-level arrangement of Argonaute 2 just before it cuts its RNA target.
The findings revealed a surprising distortion in the guide-targeted RNA duplex within Argonaute 2. This deformation positions the crucial chemical bond that must be cleaved directly within the protein's molecular scissors. Two previously overlooked amino acids, Lysine709 and Arginine710, play a pivotal role in driving the cutting reaction. Lysine709 acts as a molecular checkpoint, held away from the active site until extended guide-target pairing triggers the duplex deformation. Arginine710 fine-tunes catalytic efficiency by sensing the identity of a specific position in the target RNA, explaining a long-standing empirical rule in siRNA design.
This breakthrough study has significant implications for the design of siRNA molecules. It suggests that sequences and chemical modifications that allow the paired RNA to adopt the distorted shape are likely to favor activation by Argonaute 2. Conversely, those that rigidify the central region are expected to impair the cutting process. This understanding paves the way for 'rational design'—engineering siRNA sequences based on structural principles rather than trial and error.
As RNA therapeutics continue to evolve, this study provides a crucial foundation for developing more effective drugs. By understanding the molecular mechanisms at play, scientists can design siRNA molecules that are more likely to succeed, potentially expanding the range of treatable diseases. This research marks a significant step forward in the field of RNA therapeutics, offering a more precise and efficient approach to treating various medical conditions.
