Two Independent Studies Reveal the Inner Workings of Human RNAi Machinery
"This mechanism has been a black box for a quarter century... Visualizing the mechanism is very important."
Two separate research groups have published landmark studies in June 2026 detailing the molecular assembly and function of the human RNA interference (RNAi) machinery. The findings provide unprecedented structural insights into how the Argonaute2 protein regulates gene expression, offering a roadmap for more effective therapeutic design.
Step-by-Step Assembly of the RISC Complex
A study published in Molecular Cell on May 26, 2026, by researchers at Ohio State University describes the step-by-step process by which the protein Argonaute2 assembles the RNA-induced silencing complex (RISC).
The study identifies a four-step mechanism:
- Loading a double-stranded RNA duplex onto Argonaute2
- Selecting a guide strand
- Unwinding the duplex
- Ejecting the passenger strand
The researchers found that the target messenger RNA (mRNA) assists in ejecting the passenger strand, a step they termed target-assisted passenger ejection (TAPE). The findings were based on biochemical assays and cryogenic-electron microscopy using human Argonaute2 as a model. According to the authors, the mechanism of RISC assembly had not been visualized before this study.
"Our results instead suggest that mRNAs can also bind precursor RISCs and facilitate passenger removal during RISC maturation."
Senior author Kotaro Nakanishi emphasized the significance of the discovery:
"Researchers and pharmaceutical companies have been using siRNAs as a potential therapy to shut down gene expression... So now we can provide a robust structural basis or foundation to design or optimize siRNAs."
The research was supported by the National Institutes of Health and Ohio State's Center for RNA Biology. Co-authors include Huaqun Zhang, Vishal Annasaheb Adhav, Audrey Kehling, Andrew Savidge, Zhangfei Shen, and Tianmin Fu. Giovanna Grandinetti and Yohie Narui assisted with cryo-EM micrographs. Nakanishi reported a financial interest in City Therapeutics, Inc., where he is a co-founder and scientific advisor.
High-Resolution Structures of Slicing-Ready State
A separate study published in Nature Structural & Molecular Biology on June 24, 2026, by scientists at Scripps Research determined the first high-resolution structures of the human RNAi machinery in a state poised to cut RNA. The study identified atomic interactions that determine when and where the machinery cuts.
The study focused on Argonaute 2, the protein that executes RNA slicing. Researchers used cryo-electron microscopy to capture the structure after stabilizing the active conformation with specific mutations.
Key findings reveal that:
- The guide-target RNA duplex is physically distorted, positioning the targeted chemical bond within the protein's active site.
- Two previously overlooked amino acids, Lysine709 and Arginine710, were found to play critical roles in catalysis.
Ian MacRae, senior author, stated that the work provides the structural picture needed to understand what makes a good therapeutic siRNA. Sucharita Sarkar, co-first author, noted the discovery redefines understanding of Argonaute 2 catalysis.
The study was supported by the National Institutes of Health (R35GM127090) and a sponsored research agreement with Eli Lilly and Company.
Implications for Therapeutic Development
The findings from both studies may help develop therapeutic small interfering RNAs (siRNAs) and cleavage-inducing tiny RNAs (cityRNAs) to silence disease-linked genes.
The structures provide a mechanistic explanation for why some siRNA sequences are more effective at cutting their targets.
This advancement could potentially enable rational design of therapeutic siRNAs based on structural principles rather than trial-and-error approaches. Further research is planned to confirm the mechanism for the other three Argonaute proteins.