Antiviral activity of prickly pear (Opuntia ficus-indica (L.) Miller) extract: Opuntin B, a second antiviral protein

We previously showed that aqueous extract of prickly pear (Opuntia ficus-indica) cladode has significant antiviral activity against Cucumber mosaic virus (CMV: family Bromoviridae). Antiviral activity of the extract was retained for a period of 9 days on treated faba bean leaves. This activity was considerably decreased at concentrations lower than 30 mg mL−1, and by washing the treated leaves of faba bean plants 2 h after application of cactus extract, and was ineffective when applied 24 h after virus inoculation. No inhibitory effect was found against agro-inoculated strain of Beet curly top virus (BCTV: family Geminiviridae) on faba bean plants. Prickly pear extract reduced the efficiency of CMV transmission by Aphis gossypii on treated cucumber seedlings. An antiviral protein designated Opuntin B was isolated from the extract by heat treatment and differential centrifugation, and bulk protein precipitation by ammonium sulfate followed by cation and anion exchange chromatography. SDS-PAGE analysis in the presence of 2-ME revealed that Opuntin B is made of four protein components, in the molecular weight (Mr) range of 29-32 kDa, while in the absence of 2-ME, only two bands in the range of 26-28 kDa were visible. Native PAGE of the protein also showed 2 bands. Purified Opuntin B completely inhibited CMV on faba bean and zucchini plants at concentration of 40 μg mL−1 and had an IC50 of 0.75 μg mL−1 for inhibition of the virus on faba bean leaves. The isolated antiviral protein exhibited inhibitory activity against Zucchini yellow mosaic virus (ZYMV: family Potyviridae) Squash mosaic virus (SqMV: family Secouviridae) and Tobacco mosaic virus, (TMV: family Virgaviridae), as well.