Functional and Structural Characterisation of Staphylococcal Superantigen-Like Protein 10 (SSL10)

Abstract

Staphylococcus aureus is a highly versatile gram-positive bacterium that owes its success to the remarkable range of pathogenic factors that it has at its disposal. Staphylococcal superantigen-like protein 10 (SSL10) is a tightly regulated, highly conserved protein exclusive to the arsenal of S.aureus. It is located on the genomic island vSaα alongside ten other related ssl genes. The 2.75 Å crystal structure of SSL10 displays strong structural homology to the superantigen toxins with an N-terminal OB-fold domain linked to a C-terminal β-grasp domain; however they are functionally distinct. SSL10 binds to human IgG and displays striking specificity for the Fc domain of the γ1 subclass. Its strong affinity for IgG only extends to primate species. Kinetic analysis of SSL10 binding to human IgG1 suggests a dissociation constant in the high nanomolar range through a single site 1:1 stoichiometry interaction. Truncation of SSL10 into its C-terminal β-grasp domain abolished its ability to bind IgG1 suggesting the presence of key residues in the N-terminal OB-fold domain. SSL10 competes for binding to IgG1 with cell surface FcγRs on monocytes, and consequently interferes with the phagocytosis of IgG1-opsonised bacteria by neutrophils. This is a key survival strategy for S.aureus given that phagocytic cells play a crucial role in the immune clearance of gram positive bacteria. This small 24 kDa protein displays strong functional diversity through its C-terminal β-grasp domain, disrupting the proteolytic cascades of complement and coagulation. It targets complement C4 and inhibits the classical and mannan-binding lectin pathways of complement. SSL10 also affinity purifies prothrombin, fibrinogen, fibronectin and plasminogen from plasma; key factors involved in the clotting process. It potently delays coagulation prior to thrombin activation suggesting an alternative role for fibrinogen and fibronectin in anchoring SSL10 at the site of infection where it would be most effective. It is clear these bacterial proteins are strategically designed to accommodate a number of circumstances. This study illustrates the incredible complexity of host-pathogen interactions and highlights the need for a comprehensive understanding of this bacterium if we are to successfully develop alternative therapeutics in the future.