Viruses and RNA interference in mammalian cells

Figure 4. Dicer components. 2 helicase domains, Domian of Unknown Function – DUF, PAZ domain, 2
ribonuclease III domains, double-stranded RNA binding domain (motif). (Jaronczyk et al., 2005)
RISC is a multiprotein complex in RNAi, into which a siRNA strand is assembled to become
effective in gene silencing (Hong, 2008). This complex uses the siRNA guide to identify
mRNAs with a complementary sequence to the siRNA. It also cleaves the mRNA in the
middle of the mRNA–siRNA duplex. RISC is formed in an ATP-dependent manner.
(Meister and Tuschl, 2004). It consists of subunits: helicase, exonuclease, endonuclease, and
homology searching domains (Nykanen et al., 2001). Endonuclease domains are proteins,
which belong to the argonaute family, which cleave the target mRNA strictly
complementary to their siRNA. Argonaute-2 (AGO2) has been identified as the catalytic
center of RISC, which cleaves the target mRNA strand complementary to their bound
siRNA, as they have endonuclease activity (Gregory et al., 2005) (Figure 5). AGO 2 protein
performs the siRNA anti-guide strand cleavage, using one strand of the siRNA duplex as a
guide to find mRNAs containing complementary sequences. Then at a specific site
measured from the guide strand’s 5’ end, cleaves the phosphodiester backbone (Rand et al.,
2005).
MicroRNA (miRNA) is another important class of small RNAs in cells 20–25 nucleotides in
length. They regulate gene expression during development and defense against viruses.
They are processed from long ssRNA (Grosshans and Filipowicz, 2008). miRNAs are
generated from primary transcripts (pri-miRNAs). Their length ranges from several
hundreds to several thousands bases. Pre-miRNAs are produced from pri-miRNAs by
RNase III-like enzyme –Drosha (van Rij and Andino, 2006). They are approximately 75 nt
long and contain the active, mature miRNA component (Hammond, 2006). Then, pre-
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