Viruses and RNA interference in mammalian cells

miRNAs are taken from the nucleus to the cytoplasm by Ran-GTP and the export receptor
Exportin-5 (Yi et al., 2003; Lund et al., 2004). Pre-miRNAs are processed by Dicer into
miRNAs. Hence, miRNAs are produced endogenously (Carmell and Hannon, 2004).
Drosha - RNase III, nuclease that executes the initiation step of miRNA processing in the
nucleus: processes pri-miRNA to pre-miRNA (Lee et al., 2003) (Figure 5). This nuclear
RNase III enzyme, which was first discovered in humans and subsequently in Drosophila
and C. elegans, is implicated in initiation of the miRNA pathway (Tang, 2005).
Molecular mechanism of RNAi in mammalian cells
In mammalian cells, siRNAs are also produced with the help of Dicer from dsRNA.
Mammals encode only one Dicer gene, while for instance the plant Arabidopsis thaliana,
encodes four different Dicer-like (DCL) genes (van Rij and Andino, 2006). Dicer is
complexed with the TAR-RNA binding protein (TRBP), which binds to the transactivation
response element (TAR) of RNAs through different sites (Dorin et al., 2002), and delivers
the siRNAs to the RISC. Its core components are the Argonaute family proteins. Only Ago-
2 possesses an active catalytic domain for cleavage activity in humans. The Ago-2 protein
cleaves the target mRNAs between 10 and 11 bases, relative to the 5′ end of the antisense
siRNA strand (Aagaard and Rossi, 2007)
Figure 5. Human AGO2 and Drosha. Ago proteins contain PAZ domains. Ago proteins carry an extra PIWI
domain. The PIWI domain is a protein domain homologous to PIWI proteins. RNAse III domains (RIIIa,
RIIIb) are the parts of Drosha. Drosha also contains double-standed RNA binding domains (dsRBDs). (Meister
and Tuschl, 2004).
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