Plasminogen (Plg) is the inactive zymogen of plasmin (Plm), a serine protease involved in a range of physiological functions including blood clot dissolution and extracellular matrix degradation. Due to the high abundance and broad specificity of Plm, it is exploited by some bacterial pathogens such as S. pyogenes, S. aureus, H. pylori and Y. pestis for host dissemination and tissue invasion (Parry et al., 2000). Our research focuses on streptokinase (SK), a Plg activator from group A and C β-haemolytic streptococci. SK binds to Plg and induces structural rearrangement of the serine protease domain of Plg and this leads to the formation of an enzymatically active Plg. Although the kinetics of SK-mediated Plg activation has been studied extensively, the molecular mechanisms of SK binding and activation of full-length Plg remain to be elucidated.
Our research aims to elucidate the structural details of SK-Plg protein complex using combinatorial approaches consisting of small-angle X-ray scattering, X-ray crystallography and electron microscopy. Given that active SK-Plg complex is highly proteolytic, it is not suitable for structural studies. We apply strategies such as using recombinant human Plg with an active site mutation to stop the formation of active Plm and studying cross species Plg and SK interactions using Plgs and SKs from different species. Our approaches have allowed us to purify stable and inactive SK-Plg complex suitable for structural studies.
Crystallisation trial and small-angle X-ray scattering experiments are currently underway for SK-Plg complex. By obtaining the structure of SK-Plg, we hope to gain insights on the process of Plg activation by SK.