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Monomer-Monomer Interactions Propagate Structural Transitions Necessary for Pore Formation by the Cholesterol-dependent Cytolysins*

Background: The cholesterol-dependent cytolysins (CDCs) undergo a complex set of structural transitions to form the homo-oligomeric pore complex. Results: Structural transitions are propagated between monomers of the oligomeric complex. Conclusion: Specific structural changes establish the geometry of the oligomeric pore complex and promote the completion of existing oligomers. Significance: CDCs use membrane binding and ordered intermolecular interactions to drive assembly of their β-barrel pore. The assembly of the cholesterol-dependent cytolysin (CDC) oligomeric pore complex requires a complex choreography of secondary and tertiary structural changes in domain 3 (D3) of the CDC monomer structure. A point mutation was identified in the archetype CDC, perfringolysin O, that blocks detectable D3 structural changes and traps the membrane-bound monomers in an early and reversible stage of oligomer assembly. Using this and other mutants we show that specific D3 structural changes are propagated from one membrane-bound monomer to another. Propagation of these structural changes results in the exposure of a β-strand in D3 that allows it to pair and form edge-on interactions with a second β-strand of a free membrane-bound monomer. Pairing of these strands establishes the final geometry of the pore complex and is necessary to drive the formation of the β-barrel pore. These studies provide new insights into how structural information is propagated between membrane-bound monomers of a self-assembling system and the interactions that establish the geometry of the final pore complex.