Plasminogen (Plg) is the inactive zymogen of serine protease plasmin (Plm), which has a range of physiological functions including blood clot dissolution and extracellular matrix degradation. Physiological activation of Plg involves proteolytic cleavage by activators such as urokinase and tissue-type Plg activators. Due to its high proteolytic activity and abundance, many bacterial pathogens have evolved to secrete their own Plg activators in order to exploit host-derived Plm to facilitate tissue invasion (Parry et al., 2000).

One such example is streptokinase (SK), a Plg activator secreted by group A and C β-hemolytic streptococci. Using a range of cell surface plasminogen receptors and SK, streptococci are armed to evade immune defence and disseminate within the host (Lahteenmaki et al, 2001).  Interestingly, SK does not activate Plg through proteolysis as it does not have an intrinsic enzyme activity. Instead, SK binds and causes structural rearrangement of Plg serine protease domain, effectively converting the latter into an active protease. Although the kinetics of SK-mediated Plg activation has been studies extensively to date, the molecular mechanisms of SK-Plg interaction remains unknown. 

Our research aims to elucidate the structural details of SK-Plg protein complex using small-angle X-ray scattering and X-ray crystallography. However, active SK-Plg complex is highly autolytic and thus is not suitable for structural studies. Our strategies include (1) using recombinant human Plg with an active site mutation, (2) using non-human Plg and (3) using SK secreted from non-human isolates of streptococci strains. Strategies 2 and 3 were designed based on a previous study which suggested that Plg activation by SK is a species-specific event (Cabarello et al., 1999). Ultimately, our structural studies will have several implications: we hope to provide insights into the invasive mechanism of pathogenic streptococci as well as to improve the therapeutic potential of SK as an effective clot-buster in thrombotic diseases.