Two-dimensional crystallization on lipid monolayers and three-dimensional structure of sticholysin II, a cytolysin from the sea anemone Stichodactylahelianthus

Sticholysin II (Stn II), a potent cytolytic protein isolated from the sea a nemone Stichodactyla helianthus, has been crystallized on lipid monolayers. With Fourier-based methods, a three-dimensional (3D) model of Stn II, up t o a resolution of 15 Angstrom, has been determined. The two-sided plane gro up is p22(1)2, with dimensions a = 98 Angstrom, b = 196 Angstrom. The 3D mo del of Stn II displays a Y-shaped structure, slightly flattened, with a sma ll curvature along its longest dimension (51 Angstrom). This protein, with a molecular mass of 19.2 kDa, is one of the smallest structures reconstruct ed with this methodology. Two-dimensional (2D) crystals of Stn II on phosph atidylcholine monolayers present a unit cell with two tetrameric motifs, wi th the monomers in two different orientations: one with its longest dimensi on lying on the crystal plane and the other with this same axis leaning at an angle of similar to 60 degrees with the crystal plane.