RNAs export to the cytoplasm
Cellular nucleus retains unspliced or incompletely spliced RNAs where they are rapidly degraded, and this system has to be circumvented by some nuclear viruses. For example HIV-1 encodes for the Rev protein which promotes nuclear exit of HIV-1 unspliced genomic RNAs.
HBV genome encodes conserved splicing sites, but the majority of its mRNAs exits the nucleus unspliced. This is presumably because HBV mRNAs comprise a nuclear exit post-transcriptional regulatory element (PRE) (Huang et al. 1993) Rating=2. The PRE sequence shares similarities with the REV-responsive element (RRE) from HIV-1, although it does not utilize the same export pathway. Indeed, nuclear export inhibitor leptomycin prevents REV/RRE mediated export but not the PRE-dependent pathway (Otero et al. 1998) Rating=2. The host cellular factor PTB has been suggested to participate in PRE-dependent HBV RNA export (Zang et al. 2001) Rating=1.