Poster Presentation Australian Society for Microbiology Annual Scientific Meeting 2014

Phenotypic Characterisation of Streptokinase Variants from Streptococcus pyogenes:Implications for Disease Pathogenesis (#235)

Simon M Cook 1 , Amanda Skora 1 , Martina Sanderson-Smith 1 , Mark Walker 2 , Jason McArthur 1
  1. Biological Sciences/Illawarra Health and Medical Research Institute, University of Wollongong, Wollongong, NSW, Australia
  2. Australian Infectious Diseases Research Centre School of Chemistry and Molecular Biosciences, University of Queensland, Brisbane, QLD, Australia

Streptococcus pyogenes (group A streptococcus, GAS) is a human-specific pathogen responsible for a wide range of disease states. Streptokinase (SK) is a secreted plasminogen (Plg) activator and GAS virulence factor. SK binds to Glu-Plg and induces conformational changes in the molecule that result in the formation of a proteolytic active site. Active SK-Plg complex can sequester substrate Plg and convert these molecules to the broad serine protease plasmin. SK-plasmin(ogen) complexes produce unregulated soluble and cell bound plasmin activity which allows GAS to circumvent host defence mechanisms and facilitates bacterial invasion of normally sterile tissues to produce systemic infections. GAS SK gene sequences are polymorphic and group into two distinct sequence clusters (termed cluster type-1 and cluster type-2) with cluster type-2 further divided into sub-clusters type-2a and type-2b. In a process similar to the well-characterised therapeutic SK variant from group C streptococcus, type-1 and type-2a SK variants formed a functional activator complex with Glu-Plg which is resistant to inhibition by host α2-antiplasmin. In contrast, type-2b SK variants displayed reduced affinity for Glu-Plg and cannot generate an active site in Glu-Plg through the hallmark non-proteolytic mechanism of SK mediated Plg activation. While type-2b SK can form a functional activator complex with plasmin directly, these activator complexes are inhibited by α2-antiplasmin. Interestingly, pre-binding Glu-Plg with fibrinogen, fibrinogen fragment D, fibrin or PAM (a Plg-binding M protein expressed on the surface of a subset of GAS strains) allowed formation of type-2b SK activator complexes. However, these complexes were still inhibited by α2-antiplasmin, unless Glu-Plg was bound to fibrin or a combination of PAM/fibrinogen. Taken together, these data suggest type-2b SK mediated Plg activation may be restricted to specific micro-environments within the host such as fibrin deposits or the bacterial cell surface through the action of α2-antiplasmin. Furthermore, using a mouse model of GAS infection we demonstrate that distinct Plg activation properties displayed by SK variants can affect the invasive pathogenesis of this pathogen.