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This is the first report of the structural studies of a novel ribosome-inactivating protein (RIP) obtained from the Himalayan mistletoe (Viscum album) (HmRip). HmRip is a type II heterodimeric protein consisting of a toxic enzyme (A-­chain) with an active site for ribosome inactivation and a lectin subunit (B-chain) with well defined sugar-binding sites. The crystal structure of HmRip has been determined at 3.8 Å resolution and refined to a crystallographic R factor of 0.228 (Rfree = 0.271). A comparison of this structure with other type II RIPs reveals the presence of distinct structural features in the active site of the A-chain and in the 2γ sugar-binding site of the B-­chain. The conformation of the side chain of Tyr110, which is a conserved active-site residue in the A subunit, is strikingly different from those observed in other mistletoe RIPs, indicating its unique substrate-binding preference. The deletion of two important residues from the kink region after Ala231 in the 2γ subdomain of the B-chain results in a significantly different conformation of the sugar-binding pocket. A ribosome-recognition site has also been identified in HmRip. The site is a shallow cavity, with the conserved residues Arg51, Asp70, Thr72 and Asn73 involved in the binding. The conformations of the antigenic epitopes of residues 1–20, 85–103 and 206–223 differ from those observed in other type II RIPs, resulting in the distinct antigenicity and pharmacological properties of HmRip.

Supporting information

PDB reference: HmRip, 1pc8, r1pc8sf


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