Chemical modifications of pokeweed antiviral protein: effects upon ribosome inactivation, antiviral activity and cytotoxicity

High resolution X-ray structure of potent anti-HIV pokeweed antiviral protein-III

Pokeweed antiviral protein III (PAP-III), a naturally occurring protein isolated from late summer leaves of the pokeweed plant (Phytolacca americana), has potent anti-HIV activity by an as yet undetermined molecular mechanism. PAP-III belongs to a family of ribosome-inactivating proteins that catalytically deadenylate ribosomal and viral RNA. The chemical modification of PAP-III by reductive methylation of its lysine residues significantly improved the crystal quality for X-ray diffraction studies. Trigonal crystals of the modified PAP-III, with unit cell parameters a=b=80.47A, c=76.21A, were obtained using 30% PEG400 as the precipitant. These crystals contained one enzyme molecule per asymmetric unit and diffracted up to 1.5A, when exposed to a synchrotron source. Here we report the X-ray crystal structure of PAP-III at 1.6A resolution, which was solved by molecular replacement using the homology model of PAP-III as a search model. The fold typical of other ribosome-inactivating proteins is conserved, despite several differences on the surface and in the loop regions. Residues Tyr(69), Tyr(117), Glu(172), and Arg(175) are expected to define the active site of PAP-III. Molecular modeling studies of the interactions of PAP-III and PAP-I with a single-stranded RNA heptamer predicted a more potent anti-HIV activity for PAP-III due to its unique surface topology and more favorable charge distribution in its 20A-long RNA binding active center cleft. In accordance with the predictions of the modeling studies, PAP-III was more potent than PAP-I in depurinating HIV-1 RNA.

Expression of recombinant trichosanthin, a ribosome-inactivating protein, in transgenic tobacco

Trichosanthin (TCS) is an antiviral plant defense protein, classified as a type-I ribosome-inactivating protein, found in the root tuber and leaves of the medicinal plant Trichosanthes kirilowii. It is processed from a larger precursor protein, containing a 23 amino acid amino (N)-terminal sequence (pre sequence) and a 19 amino acid carboxy (C)-terminal extension (pro sequence). Various constructs of the TCS gene were expressed in transgenic tobacco plants to determine the effects of the amino- and carboxy-coding gene sequences on TCS expression and host toxicity in plants. The maximum TCS expression levels of 2.7% of total soluble protein (0.05% of total dry weight) were obtained in transgenic tobacco plants carrying the complete prepro-TCS gene sequence under the Cauliflower mosaic virus 35S RNA promoter. The N-terminal sequence matched the native TCS sequence indicating that the T. kirilowii signal sequence was properly processed in tobacco and the protein translation inhibitory activity of purified rTCS was similar to native TCS. One hundred-fold lower expression levels and phenotypic aberrations were evident in plants expressing the gene constructs without the C-terminal coding sequence. Transgenic tobacco plants expressing recombinant TCS exhibited delayed symptoms of systemic infection following exposure to Cucumber mosaic virus and Tobacco mosaic virus (TMV). Local lesion assays using extracts from the infected transgenic plants indicated reduced levels of TMV compared with nontransgenic controls.

X-ray crystallographic analysis of pokeweed antiviral protein-II after reductive methylation of lysine residues

Pokeweed antiviral protein II (PAP-II) is a naturally occurring protein isolated from early summer leaves of the pokeweed plant (Phytolacca americana). PAP-II belongs to a family of ribosome-inactivating proteins which catalytically deadenylate ribosomal and viral RNA. The chemical modification of PAP-II by reductive methylation of its lysine residues significantly improved the crystal quality for X-ray diffraction studies. Hexagonal crystals of the modified PAP-II, with unit cell parameters a = b = 92.51 A, c = 79.05 A, were obtained using 1.8 M Na/K phosphate as the precipitant. These crystals contained one enzyme molecule per asymmetric unit and diffracted up to 2.4 A, when exposed to a synchroton source.

RNase inhibition of human immunodeficiency virus infection of H9 cells

Onconase and bovine seminal RNase, two members of the RNase A superfamily, inhibit human immunodeficiency virus type 1 replication in H9 leukemia cells 90-99.9% over a 4-day incubation at concentrations not toxic to uninfected H9 cells. Two other members of the same protein family, bovine pancreatic RNase A and human eosinophil-derived neurotoxin, have no detectable antiviral activity, demonstrating a strikingly selective antiviral activity among homologous ribonucleases. The antiviral RNases do not appear to affect viral particles directly but inhibit replication in host cell cultures. Onconase, already in clinical trials for cancer therapy, and bovine seminal RNase have potential as antiviral therapeutics.

Yucca leaf protein (YLP) stops the protein synthesis in HSV-infected cells and inhibits virus replication

Yucca leaf protein (YLP), an inhibitor of tobacco mosaic virus isolated from the leaves of Yucca recurvifolia Salisb., exhibited potent activity against herpes simplex virus type 1 (HSV-1) with no cytotoxicity below 300 micrograms/ml. The inhibitory dose was varied with the time of addition; 50% effective concentrations (ED50) of YLP were 3, 19 and 95 micrograms/ml when YLP exposure was begun 3 h before virus infection, 0 h and 3 h after infection, respectively. This protein also inhibited the multiplication of herpes simplex virus type 2 and human cytomegalovirus. YLP has been shown to have a weak virucidal activity at higher concentrations. Analysis of early events following infection showed that YLP affected viral penetration in HeLa cells but did not interfere with adsorption to the cells. YLP was found to exert strong inhibition of protein synthesis in virus-infected cells but not in uninfected cells. This selective effect can be considered to attribute mainly to the antiviral activity of YLP.

Poliovirus-mediated entry of pokeweed antiviral protein

Infection of HeLa cells with poliovirus results in cell permeabilization to pokeweed antiviral protein. Cell permeabilization was dependent on the integrity of virus capsid proteins and directly proportional to the multiplicity of infection. This study demonstrates that virus adsorption is sufficient for the entry of pokeweed antiviral protein into poliovirus-infected cells.