When a gene that codes for a protein is active in a cell, its information is transcribed from DNA form into lengths of nucleic acid called messenger RNA (mRNA). If all goes well, these coded mRNA signals make their way to the cell's protein-factories, which use them as templates to synthesize new proteins. RNA-silencing, also called RNA interference (RNAi), is the interception and destruction of these messages -- and as such, is a powerful and specific regulator of cell activity, as well as a strong defender against viral genes.
The silencing process requires not only an Argonaute protein but also a small length of guide RNA, known as a short-interfering RNA or microRNA. The guide RNA fits into a slot on Argonaute and serves as a target recognition device. Like a coded strip of VelcroTM, it latches onto a specific mRNA target whose sequence is the chemical mirror image, or "complement," of its own -- thus bringing Argonaute into contact with its doomed prey.
A team of scientists has now revealed Argonaute2's structure and that it has the same basic set of working parts as bacterial Argonaute proteins, except that they are arranged somewhat differently. Also, key parts of Argonaute2 have extra loops and other structures, not seen in bacterial versions, which may play roles in binding to guide RNA. Finally, Argonaute2 has what appear to be binding sites for additional co-factor proteins that are thought to perform other destructive operations on the target mRNA.