Ribosome-inactivating proteins (RIPs) are toxins involved in plant defense. How the plant prevents autotoxicity is not yet fully understood. The present study is the first structural evidence of a naturally inhibited form of RIP from a plant. Himalayan mistletoe RIP (HmRIP) was purified from Viscum album leaves and crystallized with lactose. The structure was determined by the molecular replacement method and refined at 2.8-Å resolution. The crystal structure revealed the presence of high quality non-protein electron density at the active site, into which a pteridine derivative (2-amino 4-isopropyl 6-carboxyl pteridine) was modeled. The carboxyl group of the ligand binds strongly with the key active site residue Arg 162 , nullifies the positive charge required for catalysis, and thereby acts as a natural inhibitor. Lectin subunits of RIPs have two active sugarbinding sites present in 1␣-and 2␥-subdomains. A third functionally active site has been identified in the 1-subdomain of HmRIP. The 1-site is active despite the absence of conserved polar sugar-binding residues. Loss of these residues is compensated by the following: (i) the presence of an extended site where the penultimate sugar also interacts with the protein; (ii) the interactions of galactose with the protein main chain carbonyl and amide nitrogen atoms; (iii) the presence of a well defined pocket encircled by four walls; and (iv) a favorable stacking of the galactose ring with Tyr 66 besides the conserved Phe 75 . The mode of sugar binding is also distinct at the 1␣ and 2␥ sugar-binding sites.
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 A resolution and refined to a crystallographic R factor of 0.228 (R(free) = 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 2gamma 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 2gamma 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.
The crystal structure of the ribosome-inactivating protein (RIP) mistletoe lectin I (ML-I) from Viscum album in complex with adenine has been refined to 1.9 Å resolution. High quality crystals of the ML-I complex were obtained by the method of vapour diffusion using the high density protein crystal growth system (HDPCG) on the international space station, mission ISS 6A. Hexagonal crystals were grown during three months under microgravity conditions. Diffraction data to 1.9Å were collected applying synchrotron radiation and cryo-techniques. The structure was refined subsequently to analyse the structure of ML-I and particularly the active site conformation, complexed by adenine that mimics the RNA substrate binding.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.