Crystal structure of Streptococcus pneumoniae pneumolysin provides key insights into early steps of pore formation.

Sara L Lawrence,Terrence D Mulhern,Susanne C Feil,Rodney K Tweten,Craig J Morton,Allison J Farrand,Kristin R Wade,Michael W Parker,Michael A Gorman

doi:10.1038/srep14352

Abstract

Pore-forming proteins are weapons often used by bacterial pathogens to breach the membrane barrier of target cells. Despite their critical role in infection important structural aspects of the mechanism of how these proteins assemble into pores remain unknown. Streptococcus pneumoniae is the world’s leading cause of pneumonia, meningitis, bacteremia and otitis media. Pneumolysin (PLY) is a major virulence factor of S. pneumoniae and a target for both small molecule drug development and vaccines. PLY is a member of the cholesterol-dependent cytolysins (CDCs), a family of pore-forming toxins that form gigantic pores in cell membranes. Here we present the structure of PLY determined by X-ray crystallography and, in solution, by small-angle X-ray scattering. The crystal structure reveals PLY assembles as a linear oligomer that provides key structural insights into the poorly understood early monomer-monomer interactions of CDCs at the membrane surface.

Highlights

Domain 4 (D4) is the membrane-sensing domain of CDCs and, for many CDCs, membrane-bound cholesterol appears to be the receptor[8]
This loop is a key element in the allosteric pathway that couples membrane binding in domain 4 to the conformational changes that have to occur in domain 3 for the conversion of the prepore to pore[13]
The binding site for CD59 was identified to be on one face of D4 by mutagenesis studies[18] that were subsequently confirmed by the crystal structure of ILY complexed to CD5919

Summary

Introduction

Domain 4 (D4) is the membrane-sensing domain of CDCs and, for many CDCs, membrane-bound cholesterol appears to be the receptor[8]. Tryptophan fluorescence studies suggest cholesterol binds in a 1:1 complex with PLY and the cholesterol-binding site is close to the Trp-rich loop[12] This loop is a key element in the allosteric pathway that couples membrane binding in domain 4 to the conformational changes that have to occur in domain 3 for the conversion of the prepore to pore[13]. All CDCs must form a cholesterol-dependent interaction to function, recent work on PLY suggests that it can bind yet another type of receptor: certain glycans including Lewis histo-blood group antigens[20]. The PLY structure shows the formation of linear head-to-head oligomers that are consistent with previous experimental data showing that PLY and PFO can be trapped at an early stage of interaction at the membrane surface by preventing domain 3 structural changes.

Methods

Results

Conclusion