Packaging of MLV genomes requires four cis-acting stem-loops. Stem-loops A and B are self-complementary and bind Gag in their dimeric form, while the C and D elements mediate loop-loop interactions that facilitate RNA dimerization. Packaging also requires nuclear export of viral genomes, and their cytoplasmic transport toward the plasma membrane. For MLV, this is mediated by Gag and Env, and occurs on endosomal vesicles. Here, we report that MLV Psi acts at several steps during the transport of genomic RNAs. First, deletion of stem-loop B or C leads to the accumulation of genomic RNAs in the nucleus, suggesting that these elements are involved in export. Second, in chronically infected cells, mutation of the C and D loops impairs endosomal transport. This suggests that RNA dimerization is essential for vesicular transport, consistent with its proposed requirement for Gag binding. Surprisingly, deletion of stem-loop A blocks vesicular transport, whereas removal of stem-loop B has no effects. This suggests that stem-loop A has unique functions in packaging, not predicted from previous in vitro analyses. Finally, in packaging cells that do not express any Psi-containing RNA, endosomal RNA transport becomes sequence-independent. This non-specific activity of Gag likely promotes packaging of cellular mRNAs.
The packaging signal of MLV is an integrated module that mediates intracellular transport of genomic RNAs
Basyuk, E.; Boulon, S.; Skou Pedersen, F.; Bertrand, E.; Vestergaard Rasmussen, S.
Journal of molecular biology
2005-11-25 / vol 354 / pages 330-9
Signal Transduction; Molecular Sequence Data; Base Sequence; Mutation; Nucleic Acid Conformation; *Genome, Viral; Biological Transport; Sequence Deletion; Dimerization; *Virus Assembly; Cytoplasm; Gene Products, env/physiology; Gene Products, gag/physiology; Moloney murine leukemia virus/chemistry/*genetics/*metabolism; Recombinant Proteins/genetics/isolation & purification/metabolism; RNA, Viral/chemistry/genetics/*metabolism