Effects of polyethylene glycol on reverse transcriptase and other polymerase activities

The Host Cell Metabolite Inositol Hexakisphosphate Promotes Efficient Endogenous HIV-1 Reverse Transcription by Stabilizing the Viral Capsid

HIV-1 infection requires reverse transcription of the viral genome. While much is known about the biochemistry of reverse transcription from simplified biochemical reactions, reverse transcription during infection takes place within a viral core. However, endogenous reverse transcription reactions using permeabilized HIV-1 virions or purified viral cores have been inefficient. Using viral cores purified from infectious HIV-1 particles, we show that efficient reverse transcription is achieved in vitro by addition of the capsid-stabilizing metabolite inositol hexakisphosphate. The enhancement of reverse transcription was linked to the capsid-stabilizing effect of the compound, consistent with the known requirement for an intact or semi-intact viral capsid for HIV-1 infection. Our results establish a biologically relevant system for dissecting the function of the viral capsid and its disassembly during reverse transcription. The system should also prove useful for mechanistic studies of capsid-targeting antiviral drugs.

A defining activity of retroviruses is reverse transcription, the process by which the viral genomic RNA is converted into the double-stranded DNA required for virus replication. Reverse transcriptase (RT), the viral enzyme responsible for this process, was identified in 1970 by assaying permeabilized retrovirus particles for DNA synthesis in vitro. Such reactions are inefficient, with only a small fraction of viral genomes being converted to full-length double-stranded DNA molecules, possibly owing to disruption of the structure of the viral core. Here, we show that reverse transcription in purified HIV-1 cores is enhanced by the addition of the capsid-binding host cell metabolite inositol hexakisphosphate (IP6). IP6 potently enhanced full-length minus-strand synthesis, as did hexacarboxybenzene (HCB), which also stabilizes the HIV-1 capsid. Both IP6 and HCB stabilized the association of the viral CA and RT proteins with HIV-1 cores. In contrast to the wild type, cores isolated from mutant HIV-1 particles containing intrinsically hyperstable capsids exhibited relatively efficient reverse transcription in the absence of IP6, further indicating that the compound promotes reverse transcription by stabilizing the viral capsid. We also observed that the capsid-destabilizing antiviral compound PF74 inhibited endogenous reverse transcription with a potency that mirrors its ability to inhibit reverse transcription during infection. Our results show that the stabilization of the HIV-1 capsid permits efficient reverse transcription in HIV-1 cores, providing a sensitive experimental system for analyzing the functions of viral and host cell molecules and the role of capsid disassembly (uncoating) in the process.

Porcine retrovirus: optimal conditions for its biochemical detection

The assay of reverse transcriptase (RT) activity was used to detect the presence of retrovirus in porcine cells. A set of optimal assay conditions was determined to design a sensitive, quantitative and reproducible RT assay for porcine systems. The template-primer poly(rA).oligo(dT) was an absolute requirement. The presence of Mn++ was indispensable, with an optimal concentration of 0.25 mM. Monocations (K+, Na+) at 50 mM greatly enhanced, but their high doses inhibited the reaction. The pH of the medium influenced very much the reaction, especially with non-purified virus samples, with which the RT activity was inhibited at pHs above 8.2. Non-ionic detergents at 1% enhanced several-fold the RT activity. It was also shown that porcine retrovirus could be spontaneously reactivated in porcine cell lines by in vitro long-term propagation and transmitted to pigs by inoculation with virus-producing cells.

A solid phase reverse transcriptase micro-assay for the detection of human immunodeficiency virus and other retroviruses in cell culture supernatants

A simple and rapid solid-phase reverse transcriptase assay was developed based on the use of poly(rA):oligo(dT)12-18 as template primer immobilized on DEAE cellulose paper squares to detect human immunodeficiency virus (HIV) and/or other retroviruses in cell culture supernatants. It was found that PEG (per se) -up to 4% concentrations (w/v)--did not inhibit reverse transcriptase activity. Optimal conditions of the assay were determined. This solid-phase technique is much faster and more convenient than the methods described previously.

Characterization of the AIDS-associated retrovirus reverse transcriptase and optimal conditions for its detection in virions

The RNA-dependent DNA polymerase of the AIDS-associated retrovirus (ARV) gives highest activity with the synthetic template, poly(rA)oligo(dT) and prefers Mg2+ over Mn2+ as a divalent cation. It can use other template-primer combinations but with poly(rCm)oligo(dG), it prefers Mn2+ over Mg2+. Detection of ARV reverse transcriptase in culture fluids is substantially increased with an optimal KCl concentration and a special combination of EGTA and reducing agents. Our results indicate some distinguishing characteristics of the ARV reverse transcriptase and optimal conditions for its detection.