Nucleocytoplasmic RNA transport in retroviral replication.

Retroviral replication is highly dependent on post-transcriptional regulation because a single primary transcript directs synthesis of many viral proteins. The identification and characterization of two post-transcriptional regulatory systems (Rev/RRE and CTE) revealed the efficient use of cellular transport pathways by retroviruses to achieve production of infectious progeny virus. The Rev/RRE system of HIV-1 consists of the viral Rev protein which binds to its target sequence on incompletely spliced RNAs and channels these into the CRM1-dependent export pathway, which is normally used for export of cellular proteins and RNAs (U snRNAs and 5 S rRNA). The CTE, on the other hand, directly recruits the cellular mRNA export receptor TAP to the viral RNA. Both systems have in common that they recruit a key player of a specific cellular export pathway and this recruitment appears to out-compete the respective cellular target molecules. The fact that CTE can functionally substitute for Rev/RRE, yielding a replication-competent virus, indicates that very short sequence elements are sufficient for post-transcriptional control. The presence of short dominant export signals could relieve the selective pressure on the remainder of the genome to maintain a sequence that is easily exported. The resultant increase in permitted sequence space may increase the potential for immune escape, thereby providing a selective advantage for the virus. Replication of the CTE-dependent HIV-1 variant is significantly impaired compared with the wild-type virus. Considering that post-transcriptional control in the case of HIV is also used to provide a temporal switch from the early phase of regulatory protein expression to the late phase of virion production, one may suggest that the CRM1 export pathway is advantageous for the rapid delivery of large amounts of cargo (i.e. HIV RNA). This would be in accordance with its normal function because CRM1 has been shown to direct the nuclear export of cellular regulatory proteins which must be accomplished rapidly as well. In summary, retroviruses have evolved fascinating ways to deal with their cellular environment and to make use of cellular transport pathways, allowing nuclear export of intron-containing RNAs which are normally restricted to the nucleus. Specific signals on the viral RNAs recruit key factors of cellular export, thus bypassing these restrictions and ensuring efficient viral replication.