THE STAPHYLOCOCCUS-AUREUS ALPHA-TOXIN CHANNEL COMPLEX AND THE EFFECT OF CA2+ IONS ON ITS INTERACTION WITH LIPID LAYERS

Using the techniques of two-dimensional crystallization on supported lipid bilayers together with computer image processing, two distinct two-dimensional crystal types of staphylococcal α-toxin complex are formed depending on the presence or absence of Ca2+ ions. Without Ca2+, these are hexagonally packed (in Å, a = b = 89.5 ± 2.5 Å; Φ = 119.7°) With Ca2+ present, rectangular crystal packing is seen (in Å, a = 114.8 ± 1.6 Å, b = 140.2 ± 0.7Å; Φ = 89.1°). A third, banded crystal type is also seen which is interpreted as a side-to-side packing of regular tubules. We use these tubular crystals for cross-correlation searches with top and side-on views of the complex from single particle reconstructions, and with the repeating units from the two-dimensional crystal types. The results lead us to propose a model in which the different two-dimensional crystal types are formed as a result of α-toxin hexamers packing in different orientations. In the hexagonal crystals the hexamers lie end-on with a 6-fold axis in projection. On the addition of Ca2+, the hexamers reorient to lie tilted with respect to the support, thus giving rise to a rectangular projection. © 1992.