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The crystal structure of viscotoxin A3 (VT A3) extracted from European mistletoe (Viscum album L.) has been solved using the anomalous diffraction of the native S atoms measured in-house with Cu Kα radiation to a resolution of 2.2 Å and truncated to 2.5 Å. A 1.75 Å resolution synchrotron data set was used for phase expansion and refinement. An innovation in the dual-space substructure-solution program SHELXD enabled the individual S atoms of the disulfide bonds to be located using the Cu Kα data; this resulted in a marked improvement in the phasing compared with the use of super-S atoms. The VT A3 monomer consists of 46 amino acids with three disulfide bridges and has an overall fold resembling the canonical architecture of the α- and β-thionins, a capital letter L. The asymmetric unit consists of two monomers related by a local twofold axis and held together by hydrophobic interactions between the monomer units. One phosphate anion (confirmed by 31P-NMR and MS) is associated with each monomer.

Supporting information

PDB reference: viscotoxin A3, 1okh, r1okhsf

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