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Molecular steps involved in the creation of gene-silencing MicroRNAs identified

June 23, 2005

Connections to HIV and cancer seen

PHILADELPHIA - First discovered only a few brief years ago, microRNAs are small, remarkably powerful molecules that appear to play a pivotal role in gene silencing, one of the body's main strategies for regulating its genome. A scant 22 nucleotides in length, miRNAs appear to work by binding to and somehow interfering with messenger RNA, itself responsible for translating genes into proteins.

But how do miRNAs arise? And what can we learn about their biological function from their origins? In a study published last year in Nature, researchers at The Wistar Institute identified a two-protein complex, called the microprocessor complex, which controls the earliest steps in the creation of miRNAs in the cell nucleus.

Now, in a new study published online by Nature today, the Wistar group has identified a three-protein complex that picks up the process in the cell cytoplasm and carries it through to the maturation of the finished miRNAs.

Taken together, the two Nature studies trace the generation of miRNAs from the genes that give rise to long primary RNA molecules through to the mature miRNAs that target messenger RNA. Significantly, the research also shows that the finished miRNAs are associated with a protein called Argonaute 2, known to be involved in inactivating messenger RNA.

"In this study, we were able to link processing of the miRNAs directly through to the molecules responsible for silencing genes," says Ramin Shiekhattar, Ph.D., an associate professor at Wistar and senior author on both Nature studies. "The miRNAs provide specificity for those molecules, which do the actual work of gene silencing."

Intriguingly, the research also links the process of creating miRNAs with aspects of the HIV life cycle and with tumor suppression. The study identifies three proteins that work together in the cytoplasm to create finished miRNAs. Individually, each of the proteins was known previously, but their joint role in producing miRNAs is newly delineated here. Equally as important, however, is the fact that while two of the proteins had been associated with miRNAs in earlier work, the third protein, TRBP, had not been. And TRBP is clearly a protein of interest to scientists.

"TRBP was first observed as a protein that binds to HIV during transcription of the virus," says Shiekhattar. "The tantalizing implication is that the RNA interference pathway may play a significant role in HIV replication. TRBP has also been identified as a tumor suppressor, which suggests still other connections to be explored."

The lead authors on the Nature study are Thimmaiah Chendrimada, Richard I. Gregory, and Easwari Kumaraswamy, with each contributing equally to the work. The remaining co-authors are Jessica Norman, Neil Cooch, and Wistar professor Kazuko Nishikura, Ph.D. Corresponding author Shiekhattar is an associate professor in two programs at Wistar, the gene expression and regulation program and molecular and cellular oncogenesis program. Support for the research was provided by the National Institutes of Health, the American Cancer Society, and the Jane Coffin Child Memorial Fund for Medical Research.

The Wistar Institute