title: Ligatoxin B, a new cytotoxic protein with a novel helix-turn-helix DNA-binding domain from the mistletoe Phoradendron liga.
abstract: A new basic protein, designated ligatoxin B, containing 46 amino acid residues has been isolated from the mistletoe Phoradendron liga (Gill.) Eichl. (Viscaceae). The protein's primary structure, determined unambiguously using a combination of automated Edman degradation, trypsin enzymic digestion, and tandem MS analysis, was 1-KSCCPSTTAR-NIYNTCRLTG-ASRSVCASLS-GCKIISGSTC-DSGWNH-46. Ligatoxin B exhibited in vitro cytotoxic activities on the human lymphoma cell line U-937-GTB and the primary multidrug-resistant renal adenocarcinoma cell line ACHN, with IC50 values of 1.8 microM and 3.2 microM respectively. Sequence alignment with other thionins identified a new member of the class 3 thionins, ligatoxin B, which is similar to the earlier described ligatoxin A. As predicted by the method of homology modelling, ligatoxin B shares a three-dimensional structure with the viscotoxins and purothionins and so may have the same mode of cytotoxic action. The novel similarities observed by structural comparison of the helix-turn-helix (HTH) motifs of the thionins, including ligatoxin B, and the HTH DNA-binding proteins, led us to propose the working hypothesis that thionins represent a new group of DNA-binding proteins. This working hypothesis could be useful in further dissecting the molecular mechanisms of thionin cytotoxicity and of thionin opposition to multidrug resistance, and useful in clarifying the physiological function of thionins in plants.
authors.affiliation: Division of Pharmacognosy, Department of Medicinal Chemistry, Uppsala Biomedical Center, PO Box 574, SE-751 23, Uppsala, Sweden.
authors.forename: Shi-Sheng
authors.initials: SS
authors.lastname: Li
authors.forename: Joachim
authors.initials: J
authors.lastname: Gullbo
authors.forename: Petra
authors.initials: P
authors.lastname: Lindholm
authors.forename: Rolf
authors.initials: R
authors.lastname: Larsson
authors.forename: Eva
authors.initials: E
authors.lastname: Thunberg
authors.forename: Gunnar
authors.initials: G
authors.lastname: Samuelsson
authors.forename: Lars
authors.initials: L
authors.lastname: Bohlin
authors.forename: Per
authors.initials: P
authors.lastname: Claeson
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002470:Cell Survival / Q000187:drug effects*; D002851:Chromatography, High Pressure Liquid; D003603:Cytotoxins / Q000737:chemistry / Q000302:isolation & purification / Q000633:toxicity; D004247:DNA / Q000737:chemistry / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000737:chemistry / Q000302:isolation & purification* / Q000633:toxicity; D019077:Helix-Turn-Helix Motifs; D006801:Humans; D008223:Lymphoma; D013058:Mass Spectrometry; D014784:Mistletoe / Q000737:chemistry*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000737:chemistry; D010940:Plant Proteins / Q000737:chemistry / Q000302:isolation & purification* / Q000633:toxicity; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D014407:Tumor Cells, Cultured
chemical_list: D003603:Cytotoxins; D004268:DNA-Binding Proteins; D010446:Peptide Fragments; D010940:Plant Proteins; D004247:DNA; C488038:viscotoxin