The Catalytic Determinants of Streptococcal Pneumoniae IgA1 Protease are Formed by Multiple Domains

Streptococcus pneumoniae (SPN) is a gram-positive bacterium which causes non-invasive infections, such as otitis media, as well as invasive diseases like pneumonia and meningitis. Despite the development of bacterial antibiotics and vaccines, exposure to SPN still leads to diseases in young, elderly, and immunocompromised individuals. Current vaccines have serious limitations due to the serotype variability and genomic plasticity of the bacterium and therefore, an ever-increasing frequency of multidrug-resistant strains have been reported. Immunoglobulin A1 (IgA1) represents 90% of the IgA within the human respiratory tract and multiple bacterial pathogens, including SPN, can produce an IgA1-specific protease (IgA1P) to inactivate this major component of mucosal immunity. IgA1P cleaves IgA1 at the hinge region and leads to removal of the Fc domain of IgA1, which is recognized by host clearance mechanisms. Because of the universal expression among different serotypes of SPN, IgA1P has been shown to be a potential target for new vaccine development. SPN IgA1P is unique with no sequence conservation to any other known protein, other than a conserved HExxH Zn-binding motif found in many metalloproteases. We have discovered that the SPN IgA1P released from cells is active and our biochemical studies have revealed that the SPN IgA1P contains a well-folded C-terminal domain (CTD) that houses the third Zn coordinating residue, Glu1628. Furthermore, our data illustrates that the N-terminal domain (NTD) is primarily responsible for engaging IgA1 and the CTD does not have detectable affinity for IgA1 by itself. However, the CTD does play a role in facilitating the binding and is essential for the catalytic activity. Our discoveries shed light on the mechanistic details of this novel metalloprotease and potentially help in developing better vaccines.