The RISC complex, or RNA-induced silencing complex, is a multi-protein complex that plays a crucial role in the RNA interference (RNAi) pathway. RISC is responsible for gene silencing through the degradation of target messenger RNA (mRNA) or the inhibition of its translation. This process is mediated by small RNA molecules, such as small interfering RNA (siRNA) or microRNA (miRNA), which guide the RISC complex to complementary mRNA sequences.
The RISC complex is assembled through the following steps:
- Processing of small RNA molecules: Double-stranded RNA (dsRNA) is cleaved by the enzyme Dicer into siRNA or miRNA duplexes, which are approximately 21-23 nucleotides in length.
- RISC loading: The siRNA or miRNA duplexes are incorporated into the RISC complex. The core component of the RISC complex is an Argonaute protein, which binds the small RNA duplex.
- Strand selection: Within the RISC complex, the siRNA or miRNA duplex is unwound. The guide strand (antisense strand) is retained, while the passenger strand (sense strand) is degraded. The guide strand is responsible for target recognition and base-pairing with the complementary mRNA sequence.
- Target recognition and silencing: The guide strand within the RISC complex binds to the target mRNA molecule through complementary base pairing. This interaction leads to one of two outcomes, depending on the type of small RNA and the degree of complementarity between the guide strand and the target mRNA: a. mRNA cleavage: If the guide strand is an siRNA with perfect or near-perfect complementarity to the target mRNA, the Argonaute protein within the RISC complex cleaves the mRNA, leading to its degradation and effective gene silencing. b. Translational repression: If the guide strand is an miRNA with partial complementarity to the target mRNA, the RISC complex inhibits translation of the mRNA without cleaving it. This can also lead to mRNA destabilization and degradation.
The RISC complex is an essential component of the RNAi pathway, which is conserved across many eukaryotic organisms. RNAi plays a crucial role in gene regulation, development, and the maintenance of genome integrity. In research, RNAi is a valuable tool for studying gene function, validating drug targets, and developing gene-specific therapeutics.