SpsA, a novel pneumococcal surface protein with specific binding to secretory Immunoglobulin A and secretory component

The interaction of pathogenic bacteria with host serum and matrix proteins is a common strategy to enhance their virulence. Streptococcus pneumoniae colonizes the human upper respiratory tract in healthy individuals and is also able to cause invasive diseases. Here, we describe a novel pneumococcal surface protein, SpsA, capable of binding specifically to human secretory immunoglobulin A (SlgA). The dissociation constant of SlgA binding to SpsA was 9.3 x 10(-9)M. Free secretory component (SC) also binds to S. pneumoniae, whereas serum IgA does not, suggesting that pneumococcal binding to SlgA is mediated by the SC. To our knowledge, this is the first defined interaction of SC with a prokaryotic protein. The spsA gene encodes a polypeptide of 523 amino acids with a predicted molecular mass of 59151 Da. The SlgA-or SC-binding domain is located in the N-terminal part of SpsA and exhibits no significant homology to any other proteins. The purified SlgA-binding domain of SpsA could completely inhibit the binding of SlgA to pneumococci. SpsA was expressed by 73% of the tested S. pneumoniae isolates and was substantially conserved between different serotypes. The interaction between S. pneumoniae and SC via SpsA represents a novel biological interaction that might increase virulence by the impairment of bacterial clearance.