RNA-induced silencing complex

from Wikipedia, the free encyclopedia
Argonaut protein (gray) with lead strand (red) and target strand to be degraded (green)

The RNA-induced silencing complex (RISC) is a complex of RNA and proteins . The RNA is a small interfering RNA (siRNA) or microRNA (miRNA) and the protein portion is proteins of the Argonaut family and other proteins. The function of the RISC is to switch off the production of specific proteins ( gene knockout ) or to reduce it ( gene knockdown ) by breaking down the mRNA coding for these proteins or by inhibiting their translation . This process is known as RNA interference .

Formation of the complex

The composition of the complex varies depending on the interfering RNA and RNA interference path used and can also differ from species to species. Proteins of the Argonaut family are responsible for the basic functions of the RNA-induced silencing complex . The formation of the RNA-induced silencing complex itself is a multi-stage process and takes place in the so-called P-bodies in the cytosol .

The active RISC consists of the Argonaut protein Ago2, other proteins and the single-stranded siRNA.

In the first step, the 19 to 23 base pairs of double-stranded siRNA or miRNA molecules formed by Dicer are transferred to the argonaut proteins of the RNA-induced silencing complex . This complex loaded with double-stranded RNA is also known as pre-RISC. It is assumed that the enzymes responsible for the formation of interfering RNA interact with the RNA-induced silencing complex and mediate the transfer. In turn, the interfering RNA can indirectly control which Argonaut proteins it is transferred to. Using the example of the fruit fly Drosophila melanogaster, it could be shown that at least in this the double-stranded and mostly not perfectly paired miRNA from Dicer DCR1 with the double-stranded RNA binding domain Loquacious (LOQS) is transferred to the argonaut protein AGO1 . In Drosophila melanogaster, however, double-stranded perfectly paired siRNA is transferred from Dicer DCR2 with the double-stranded RNA binding domain R2D2 to the argonaut protein AGO2 . Thanks to the direct transfer of interfering RNA, it can also be ensured that no single-stranded degradation products of the mRNA get into the RNA-induced silencing complex and thus cause a malfunction of the RNA interference.

In a second step, the double strands of the bound siRNA or miRNA are untied and cleaved within the pre-RISC that was formed beforehand. The RNA single strand known as the guide strand remains in the RNA-induced silencing complex , which in this state is called Holo-RISC, while the other strand leaves the complex and is broken down. The choice of the guiding line is highly specific. In the RNA-induced silencing complex , of the two possible strands of double-stranded interfering RNA, the strand that has a lower thermodynamic stability at its 5 'end remains.

In the third step, a target mRNA complementary to the lead strand is incorporated into the RNA-induced silencing complex . For the binding of the target mRNA, the thermodynamic stability of the leading strand-mRNA complex at the 5 'end of the leading strand is of particular importance. The uptake of the mRNA in the complex can also be influenced by ribonucleoproteins bound to the mRNA . These can block binding sites of the mRNA or otherwise release binding sites that are otherwise inaccessible due to secondary structures.

Breakdown of the RNA

The siRNA or miRNA of the RISC binds, as already mentioned, to the corresponding target mRNA. Then, by activating the Argonaut protein, which acts as a ribonuclease , the target mRNA is broken down if paired perfectly. This is often the case with siRNAs. Another possibility is the inhibition of translation , which happens when the pairing of the target RNA does not match perfectly with that of the leading strand, as is often the case with miRNAs. No ribonuclease activity is required for this function. This reduces the available amount of certain mRNA, so that the corresponding protein can only be produced in small amounts or not at all. So you could say that the transport of information is inhibited. The RNA-protein complex acts as an enzyme , so it can intercept and break down mRNAs several times in a row and in this way reduce or switch off the effectiveness of a gene.

literature

  • Michael T. McManus, Phillip Allen Sharp : Gene silencing in mammals by small interfering RNAs . In: Nature Rev. Genet. , 3 (2002), pp. 737-747.

Individual evidence

  1. ^ Dianne S. Schwarz, György Hutvágner, Tingting Du, Zuoshang Xu, Neil Aronin: Asymmetry in the Assembly of the RNAi Enzyme Complex . In: Cell . tape 115 , no. 2 , October 17, 2003, ISSN  0092-8674 , p. 199-208 , doi : 10.1016 / S0092-8674 (03) 00759-1 , PMID 14567917 ( cell.com [accessed March 25, 2020]).
  2. ↑ Anastasia Khvorova, Angela Reynolds, Sumedha D. Jayasena: Functional siRNAs and miRNAs Exhibit Strand Bias . In: Cell . tape 115 , no. 2 , October 17, 2003, ISSN  0092-8674 , p. 209-216 , doi : 10.1016 / S0092-8674 (03) 00801-8 , PMID 14567918 ( cell.com [accessed March 25, 2020]).
  3. ↑ Stefan Ludwig Ameres, Javier Martinez, Renée Schroeder: Molecular Basis for Target RNA Recognition and Cleavage by Human RISC . In: Cell . tape 130 , no. 1 , July 13, 2007, ISSN  0092-8674 , p. 101–112 , doi : 10.1016 / j.cell.2007.04.037 , PMID 17632058 ( cell.com [accessed March 25, 2020]).