Eugene Skripkin

Dimerization of retroviral genomic RNAs: Structural and functional implications

Retroviruses are a family of widespread small animal viruses at the origin of a diversity of diseases. They share common structural and functional properties such as reverse transcription of their RNA genome and integration of the proviral DNA into the host genome, and have the particularity of packaging a diploid genome. The genome of all retroviruses is composed of two homologous RNA molecules that are non-covalently linked near their 5 end in a region called the dimer linkage structure (DLS). There is now considerable evidence that a specific site (or sites) in the 5 leader region of all retroviruses, located either upstream or/and downstream of the major splice donor site, is involved in the dimer linkage. For MoMuLV and especially HIV-1, it was shown that dimerization is initiated at a stem-loop structure named the dimerization initiation site (DIS). The DIS of HIV-1 and related regions in other retroviruses corresponds to a highly conserved structure with a self-complementary loop sequence, that is involved in a typical loop-loop kissing complex which can be further stabilized by long distance interactions or by conformational rearrangements. RNA interactions involved in the viral RNA dimer were postulated to regulate several key steps in retroviral cycle, such as: i) translation and encapsidation: the arrest of gag translation imposed by the highly structured DLS-encapsidation signal would leave the RNA genome available for the encapsidation machinery; and ii) recombination during reverse transcription: the presence of two RNA molecules in particles would be necessary for variability and viability of virus progeny and the ordered structure imposed by the DLS would be required for efficient reverse transcription.

Dimerization of HIV-1 genomic RNA of subtypes A and B: RNA loop structure and magnesium binding.

Retroviruses encapsidate their genome as a dimer of homologous RNA molecules noncovalently linked close to their 5 ends. The dimerization initiation site (DIS) of human immunodeficiency virus type 1 (HIV-1) RNA is a hairpin structure that contains in the loop a 6-nt self-complementary sequence flanked by two 5 and one 3 purines. The self-complementary sequence, as well as the flanking purines, are crucial for dimerization of HIV-1 RNA, which is mediated by formation of a kissing-loop complex between the DIS of each monomer. Here, we used chemical modification interference, lead-induced cleavage, and three-dimensional modeling to compare dimerization of subtype A and B HIV-1 RNAs. The DIS loop sequences of these RNAs are AGGUGCACA and AAGCGCGCA, respectively. In both RNAs, ethylation of most but not all phosphate groups in the loop and methylation of the N7 position of the G residues in the self-complementary sequence inhibited dimerization. These results demonstrate that small perturbations of the loop structure are detrimental to dimerization. Conversely, methylation of the N1 position of the first and last As in the loop were neutral or enhanced dimerization, a result consistent with these residues forming a noncanonical sheared base pair. Phosphorothioate interference, lead-induced cleavage, and Brownian-dynamics simulation revealed an unexpected difference in the dimerization mechanism of these RNAs. Unlike subtype B, subtype A requires binding of a divalent cation in the loop to promote RNA dimerization. This difference should be taken into consideration in the design of antidimerization molecules aimed at inhibiting HIV-1 replication.

Mechanisms of Inhibition of in Vitro Dimerization of HIV Type I RNA by Sense and Antisense Oligonucleotides

Retroviruses display a strong selective pressure to maintain the dimeric nature of their genomic RNAs, suggesting that dimerization is essential for viral replication. Recently, we identified the cis-element required for initiation of human immunodeficiency virus type I (HIV-I) RNA dimerization in vitro. The dimerization initiation site (DIS) is a hairpin structure containing a self-complementary sequence in the loop. We proposed that dimerization is initiated by a loop-loop kissing interaction involving the self-complementary sequence present in each monomer. We tested the ability of sense and antisense oligonucleotides targeted against the DIS to interfere with a preformed viral RNA dimer. Self-dimerization and inhibition properties of the tested oligonucleotides are dictated by the nature of the loop. An RNA loop is absolutely required in the case of sense oligonucleotides, whereas the nature and the sequence of the stem is not important. They form reversible loop-loop interactions and act as competitive inhibitors. Antisense oligonucleotides are less efficient in self-dimerization and are more potent inhibitors than sense oligonucleotides. They are less sensitive to the nature of the loop than the antisense oligonucleotides. Antisense hairpins with either RNA or DNA stems are able to form highly stable and irreversible complexes with viral RNA, resulting from complete extension of base pairing initiated by loop-loop interaction.

The use of chemical modification interference and inverse PCR mutagenesis to identify the dimerization initiation site of HIV-1 genomic RNA

The retroviral genome consists of two identical RNA molecules physically linked together close to their 5 end, in a region called the Dimer Linkage Structure (DLS). Recent findings suggest that dimerization is involved in encapsidation, regulation of translation and reverse transcription. Previous in vitro studies localized the DLS of HIV-1 in a region downstream of the splice donor (SD) site. More recently, we showed that dimerization of HIV-1 RNA also involves sequences upstream of the SD site. Modification interference experiments and site-directed mutagenesis were used to identify the nucleotides required in the dimerization process of HIV-1 RNA. Our results point out a self-complementary sequence located in a hairpin loop, between the Primer Binding Site (PBS) and the SD site, as the Dimerization Initiation Site.