cDNA cloning and characterization of a ribosome inactivating protein of a hemi-parasitic plant (Viscum album L.) from North-Western Himalaya (India)

Abstract

Mistletoe ribosome inactivating proteins (RIPs) are excellent immunomodulators obtained from a hemi-parasitic plant Viscum album L. In the present study, we have characterized and cloned a 65 kDa heterodimeric RIP from Himalayan V. album. Himalayan mistletoe ribosome inactivating protein (HmRIP) possessed unique sugar affinity for l-Rhamnose, Meso-inositol and l-Arabinose besides Galactose and N-acetyl-Galactosamine. The lectin activity was stable to a broad range of temperature (4-65°C) and pH (2.5-12.5). cDNA cloning showed that HmRIP is 500 amino acids long and shortest among mistletoe RIPs. Amino acid sequence analyses revealed important differences at the functionally significant sites. In the lectin subunit, two critical residues forming base of the 2γ sugar-binding cleft were deleted. Such differences lead to a different conformation of sugar-binding cleft giving rise to unique sugar-binding properties of HmRIP. Toxin subunit also showed a sizeable deletion of four residue segment in the antigenic epitope (83-103) determining its antigenecity. Due to striking differences at the functional sites associated with medicinal properties, HmRIP is a novel type II RIP. In the phylogenetic tree based on amino acid sequence of type II RIPs from six dicot families, mistletoe RIPs branched out from the RIPs of all other taxa and formed a distinct and distant group supporting independent evolution of Viscaceae among the angiosperms.