Crystal Structures and Electron Micrographs of Fungal Volvatoxin A2

Membrane adhesion and insertion of protein are essential to all organisms, but the underlying mechanisms remain largely unknown. Membrane pore-forming toxins (PFTs) are potential model systems for studying these mechanisms. We have determined the crystal structures of volvatoxin A2 (VVA2), a fungal PFT from Volvariella volvacea, using Br-multiple-wavelength anomalous diffraction (MAD). The VVA2 structures obtained at pH 4.6, pH 5.5 and pH 6.5 were refined to resolutions of 1.42 Å, 2.6 Å and 3.2 Å, respectively. The structures reveal that the VVA2 monomer contains a single α/β domain. Most of the VVA2 surface is occupied by its oligomerization motif and two putative heparin-binding motifs. Residues Ala91 to Ala101 display several conformations at different pH values, which might be under the control of His87. We also found that the shape of one putative heparin-binding motif in VVA2 appears similar to those found in fibroblast growth factors, and the other one displays a linear polypeptide. Our results suggest several possible intermediates of protein assembly in solution and protein adhering to cell membranes before conformational changes. The electron micrographs of VVA2 molecules in solution, at a protein concentration of 1 μg ml−1, show that they can assemble into filament-like or braid-like oligomers in a pH-dependent way. In addition, the arc-shaped VVA2 structure obtained at pH 6.5 suggests that VVA2 could form a two-layered helical oligomer with 18 subunits per turn. The structures presented here could be used to elucidate the pore-formation mechanisms of VVA2 and its structural neighbors, Cyt toxins from Bacillus thuringiensis.