Protein Complex Controlling Genes Imaged

Scientists at the US Department of Energy (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab) have recently managed another breakthrough that could lead to major changes in the fields of biology and medicine. They have succeeded in imaging one of the most important protein complexes in the human body, a structure that dictates the faith of cells, by expressing or silencing genes. The ensemble, known as the RISC-loading complex, carries a large number of RNA snippets, which, in themselves, control the action of genes.

“We now know how the three main components of the RISC machinery – the Dicer and Argonaute enzymes and the TRBP binding protein – are arranged, and how they interact with one another and are likely to interact as a complex with messenger RNA. Our work should help others in the design of mutants to test the mechanisms of the RNA binding and processing used by the gene-silencing RNA machinery in humans,” Berkeley Lab biochemist Jennifer Doudna explains. She is an international authority on RNA molecular structures, and also one of the team leaders, alongside biophysicist Eva Nogales, also from the Lab.

Details of their accomplishments have been published in a paper entitled “Structural insights into RNA Processing by the Human RISC-Loading Complex,” which appears in the latest issue of the respected scientific journal Nature Structure and Molecular Biology. The experts propose that the RISC (RNA-Induced Silencing Complex) and other “small RNA molecules” are able to target specific messenger RNA (mRNA) molecules, and program them either for silencing or destruction.

“Because of the relatively small size of RISC and the added complications of its not being very stable and having highly mobile parts, imaging this complex was a challenge. We used negative-stain electron microscopy and sophisticated single particle analysis,” Nogales adds. “The results of this study indicate that the TRBP protein is flexibly linked to the Dicer enzyme, which has led to several specific hypotheses about the roles of these three proteins in binding and cleaving RNA during gene silencing by RNA interference,” she continues.

The new investigation, which was supported by grants from the US National Institutes of Health (NIH) and the Human Frontier Science Program (HFSP), will continue to progress until even more details about the RISC-complex are established. “Together with higher-resolution structural studies, we should be able to provide even more new insights into the core machinery of RNA interference,” Doudna concludes.