Structure of viscotoxin A3: disulfide location from weak SAD data

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 Kalpha radiation to a resolution of 2.2 Angstrom and truncated to 2.5 Angstrom. A 1.75 Angstrom 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 Kalpha 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 alpha- and beta-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 P-31-NMR and MS) is associated with each monomer.