Bacteria and eukaryotes have evolved a set of pore-forming proteins that are used as lethal biological weapons against other bacteria, protozoa, or virus-infected cells. These proteins, known as pore-forming toxins (PFTs) are potent virulence factors that use water-soluble, secreted monomers as elements for the subsequent assembly of a membrane-integrated pore. This stands in contrast to the vast majority of membrane proteins, which are inserted into membranes within cells as they are synthesized. Assembled PFTs act by perforating holes that destroy the membrane’s permeability barrier or by delivering toxic compounds through the pores they form. To date, PFTs have been classified into two main categories: alpha-PFTs and beta-PFTs, depending on whether membrane integration is mediated by alpha-helical or beta-sheet elements.
A long-standing question about PFTs has revolved around the mechanism of spontaneous membrane insertion, assembly, and pore formation. Mueller and colleagues sought to elucidate some insights on these issues and took a step forward in that direction by solving the X-Ray structure of 400kDa dodecameric transmembrane pore formed by Cytolysin A (ClyA), an alpha-PFT found in numerous Escherichia coli and Salmonella enterica highly pathogenic strains, at 3.3 Angstrom resolution. These PFTs are secreted by the bacteria and are responsible for the hemolytic phenotypes associated with cytotoxicity of mammalian cells, induced apoptosis of macrophages, and tissue death.
The crystal structure of the transmembrane pore presents evidence for the remarkable conformational changes that the monomeric form of ClyA must undergo in order to partition and insert into the membrane, and assemble into its dodecameric form. The conversion from ClyA soluble monomer to transmembrane protomer involves rearrangements of up to 140 Angstroms of parts of the soluble form involving more than half of all residues, followed by reorganization of the hydrophobic core, and transformation of disordered loops and beta-sheet regions into alpha-helical structures. These changes suggest that alpha-PFT pore assembly occurs in a sequential mechanism that transforms a ClyA secreted monomeric protein resembling a hunched, malleable tube into a ClyA erect protomer that provides a network of interfaces that contribute to protomer-protomer contacts required for oligomerization of the deadly transmembrane pore.
- Mueller et al, The Structure of a cytolytic alpha-helical toxin pore reveals its assembly mechanism, Nature 459:726 (2009).
- Photo credit: Mueller & Ban, Enhanced SnapShot: Pore-Forming Toxins, Cell 142:334 (2010).