Multiple oligomeric states of the Helicobacter pylori vacuolating toxin demonstrated by cryo-electron microscopy

Helicobacter pylori vacuolating toxin (VacA) is a bacterial protein toxin that forms water-soluble oligomeric complexes, and can somehow insert into lipid bilayers to produce anion-selective channels. In this study, we utilize the novel technique of "cryo-negative staining" to examine the morphology of vitrified VacA complexes. Two basic types of oligomeric structures were observed: (i) relatively thick six or seven-sided astral arrays with near-perfect radial symmetry; and (ii) relatively thin astral arrays of six to nine short "rodlets" that display a distinct handedness or "chirality". Additionally, the new technique provided edge-views of the thicker form of VacA oligomer, which appears to be a thin bilayered disc, indicating that the relatively thick six-sided arrays are actually dodecamers. Also observed occasionally in the present cryo-negatively stained VacA preparations were 2D crystalline arrays that appeared to be comprised of interlocked dodecamers. The structural alterations that VacA oligomers must undergo to form these 2D crystals were analyzed, and intermediates in this transition were identified. Additionally, the oligomeric state of acid-activated VacA bound to membranes was visualized by the traditional technique of "deep-etch" electron microscopy, and was found to resemble most closely the top halves of the dodecamers. These results indicate that VacA is able to undergo major conformational changes, accompanied by major changes in its state of oligomerization, under different natural and experimental conditions.