Structural Biochemistry/Nucleic Acid/RNA/Interference RNA/RISC

=RNA-Induced Silencing Complexes=

RISC: the effector complex for small RNAs
The major role that 20- to 30-nt small RNAs play in regulating gene expression is mediating sequence specific cleavages of their target mRNAs, coined RNA interference. siRNAs have become become powerful as a research tool that also possesses exciting therapeutic promise. MicroRNAs (miRNAs) are a conserved family of endogenous small RNAs. They are to regulate over half of all human genes, fine tuning a diverse amount of biological processes. However, unlike enzymes and ribosomes, small RNAs cannot catylze reactions by themselves. They form effector ribonucleoprotein complexes (RISCs) that can mediate various types of gene silencing. The small RNAs also guide these RISCs to their targets in a sequence specific manner. RISC assembly is a key process for small RNAs to exert their functions.

Argonaute: the core component of RISC
RISCs is a member of the Argonaute (Ago) family of proteins which provides a unique platform for target recognition and silencing. The association of a small RNA with a specific Ago protein dictates its function. The larger Ago family can be traced to the Ago subfamily which includes bindings to siRNAS and miRNAs, and piRNAs. In mammals, each of the 4 Ago proteins (AGO1-4) can repress translation of their target mRNAs, but only AGO2 possesses a target cleavage activity that results in RNAi. Small RNAs are assembled into RISC through an ordered pathway using Ago proteins. Such pathways resemble assembly lines where small RNAs are transferred from one machine to another. At each step of the way, the preRISC is monitored for defects. The small RNA is altered slightly at each step in the process until a mature RISC is produced. Thus to understand the RISC assembly, it must be broken into steps.

RISC loading and unwinding
siRNAs and miRNAs are born double-stranded: the RNase III enzymes, Drosha and Dicer, process precursor RNAs into siRNA duplexes and miRNA duplexes. Because small RNAs in RISC must anneal to their target RNAs, the two strands of the small RNA duplex must be separated. One strand will be discarded called the "passenger strand" and the used strand is called the "guide strand". RISC assembly can be divided into loading and unwinding. (i) RISC-loading is when small RNA duplexes are inserted into Ago proteins. (ii) Unwinding is when the two strands are separated within the Ago protein. An Ago complex bound to an RNA duplex is called pre-RISC. Yet, a complex containing a single guide strand is called a mature RISC.