Two structural transitions in membrane pore formation by pneumolysin, the pore-forming toxin of Streptococcus pneumoniae

The human pathogen Streptococcus pneumoniae produces soluble pneumolysin mo nomers that bind host cell membranes to form ring-shaped, oligomeric pores. We have determined three-dimensional structures of a helical oligomer of p neumolysin and of a membrane-bound ring form by cryo-electron microscopy. F itting the four domains from the crystal structure of the closely related p erfringolysin reveals major domain rotations during pore assembly. Oligomer ization results in the expulsion of domain 3 from its original position in the monomer. However, domain 3 reassociates with the other domains in the m embrane pore form. The base of domain 4 contacts the bilayer, possibly alon g with an extension of domain 3. These results reveal a two-stage mechanism for pore formation by the cholesterol-binding toxins.