Structural basis of Substrate and Antibody recognition by Group A streptococcal anti-chemotactic proteases

Streptococcus pyogenes, also known as group A streptococci (GAS), is a disease-causing bacterium contributing to more than half a million deaths globally each year. It is a major cause of invasive infections and can lead to rheumatic heart disease, an autoimmune condition resulting from recurrent streptococcal infections, especially throat infections. To survive, this bacterium uses two enzymes, SpyCEP and ScpA, to prevent the immune system white blood cells (neutrophils) from being recruited to the infection site. SpyCEP and ScpA are cell surface serine proteases (CEPs) which enzymatically cut critical regions from host proteins that activate the immune system, thereby disabling them. SpyCEP targets and degrades the entire family of CXC immune signalling molecules, while ScpA dismantles C5a, as well as C3a. Despite their importance, there is striking lack of a detailed understanding of how these proteins disarm immune system components and current models are insufficient to define their precise mechanisms of action. Furthermore, SpyCEP and ScpA could be key players in developing a multicomponent vaccine as they trigger protective immune responses, however, there is no vaccine for Streptococcus pyogenes yet. These important knowledge gaps are hindered by our lack of comprehensive structural information on these enzymes assembled with their target immune molecules and antibodies. To better understand how these molecular systems work and help design a potential vaccine, we plan to employ a sophisticated structural biology approach combining cryo-electron microscopy (cryo-EM) and Nuclear Magnetic Resonance (NMR). By employing both methods synergistically, we can reach atom level detail and understanding that is not available from each technique alone. Specifically, we aim to determine how these CEP proteins recognise their host immune system targets and change their shape in the process. We will also test observations from our structural insight with incisive biochemical and cellular experiments to derive more accurate functional models for the CEPs. We also aim to visualise precisely how antibodies recognise the CEPs and determine which regions are most immunogenic. With a better understanding of how these proteins carry out their roles and react with antibodies, we hope to create a smarter vaccine. We will combine important immunogenic features of both enzymes in novel antigens that can be tested in vaccination studies. This would provide a powerful defence strategy for our immune system to offer protection against this harmful bacterium. In summary, this new research proposal addresses the urgent need for a vaccine against Streptococcus pyogenes. By exploiting advanced structural and biochemical techniques, we aim to gain deeper insights into the virulence mechanisms of SpyCEP and ScpA, paving the way for the development of an effective vaccine to combat this significant global health threat.