RNA-induced silencing complex

RNA-induced silencing complex, or RISC, is a multi-protein siRNA complex which cleaves (incoming viral) dsRNA and binds short antisense RNA strands which are then able to bind complementary strands. When it finds the complementary strand, it activates RNAse activity and cleaves the RNA. This process is important both in gene regulation by micro-RNAs and in defense against viral infections, which often use double-stranded RNA as an infectious vector.

Strands
The RNA endonuclease Dicer plays a role in aiding RISC action by providing the initial RNA material to activate the complex as well as the first RNA substrate molecule. When Dicer, which has endonuclease activity against dsRNA and pre-miRNAs, cleaves a pre-miRNA stem-loop or a dsRNA, a 20-25 base pair double-stranded RNA fragment is formed with a 2 nucleotide 3' overhang at each end.


 * One strand is integrated into the RISC complex. This strand is known as the guide strand and is selected by the argonaute protein, the catalytically active RNase in the RISC complex, on the basis of the stability of the 5' end.


 * The remaining strand, known as the anti-guide or passenger strand, is degraded as a RISC complex substrate.

Binding
The RISC complex with a bound siRNA recognizes complementary messenger RNA (mRNA) molecules and degrades them, resulting in substantially decreased levels of protein translation and effectively turning off the gene. It is as yet unclear how the activated RISC complex locates the mRNA targets in the cell, though it has been shown that the process is not coupled to ongoing protein translation from the mRNA. Endogenously expressed miRNA is usually imperfectly complementary to a large number of nuclear genes and has a modulating effect on these genes' levels of expression via translational repression.