Fluorescent HIV-1 Dimerization Initiation Site: Design, Properties, and Use for Ligand Discovery

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Abstract
The HIV-1 Dimerization Initiation Site (DIS) is an intriguing, yet underutilized, viral RNA target for potential antiretroviral therapy. To study the recognition features of this target and to provide a quantitative, rapid, and real-time tool for the discovery of new binders, a fluorescence-based assay has been constructed. It relies on strategic incorporation of 2-aminopurine, an isosteric fluorescent adenosine analogue, into short hairpin RNA constructs. These oligomers self-associate to form a kissing loop that thermally rearranges into a more stable extended duplex, thereby mimicking the association and structural features of the native RNA sequence. We demonstrate the ability of two fluorescent DIS constructs, DIS272(2AP) and DIS273(2AP), to report the binding of known DIS binders via changes in their emission intensity. Binding of aminoglycosides such as paromomycin to DIS272(2AP) results in significant fluorescence enhancement, while ligand binding to DIS273(2AP) results in fluorescence quenching. These observations are rationalized by comparison to the sequence-analogous bacterial A-site, where the relative emission of the fluorescent probe is dependent on the placement of the flexible purine residues inside or outside the helical domain. Analysis of binding isotherms generated using DIS272(2AP) yields submicromolar EC50 values for paromomycin (0.5 ± 0.2 μM) and neomycin B (0.6 ± 0.2 μM). Other neomycin-family aminoglycosides are less potent binders with neamine, the core pharmacophore, displaying the lowest affinity of 21 ± 1 μM. Screening of additional aminoglycosides and their derivatives led to the discovery of new, previously unreported, aminoglycoside binders of the HIV DIS RNA, among them butirosin A (5.5 ± 0.6 μM) and apramycin (7.6 ± 1.0 μM). A conformationally constrained neomycin B analogue displays a rather high affinity to the DIS (1.9 ± 0.2 μM). Among a series of nucleobase aminoglycoside conjugates, only the uracil derivatives display a measurable affinity using this assay with EC50 values in the 2 μM range. In addition, similarity between the solution behavior of HIV-1 DIS and the bacterial decoding A-site has been observed, particularly with respect to the intra- and extra-helical residence of the conformationally flexible A residues within the bulge. Taken together, the observations reported here shed light on the solution behavior of this important RNA target and are likely to facilitate the design of new DIS selective ligands as potential antiretroviral agents.