A covalent S-F heterodimer of leucotoxin reveals molecular plasticity of beta-barrel pore-forming toxins.

Staphylococcal leucotoxins, leucocidins, and gamma-hemolysins are bicomponent beta-barrel pore-forming toxins (beta-PFTs). Their production is associated with several clinical diseases. They have cytotoxic activity due to the synergistic action of a class S component and a class F component, which are secreted as water-soluble monomers and form hetero-oligomeric transmembrane pores, causing the lysis of susceptible cells. Structural information is currently available for the monomeric S and F proteins and the homoheptamer formed by the related alpha-hemolysin. These structures illustrate the start and end points in the mechanistic framework of beta-PFT assembly. Only limited structural data exist for the intermediate stages, including hetero-oligomeric complexes of leucotoxins. We investigated the protein-protein interactions responsible for maintaining the final bipartite molecular architecture and describe here the high-resolution crystal structure and low-resolution solution structure of a site-specific cross-linked heterodimer of gamma-hemolysin (HlgA T28C-HlgB N156C), which were solved by X-ray crystallography and small angle X-ray scattering, respectively. These structures reveal a molecular plasticity of beta-PFTs, which may facilitate the transition from membrane-bound monomers to heterodimers.