Characterization of the interaction of the pneumococcal surface protein SpsA with the human polymeric immunoglobulin receptor (hpIgR).

BACKGROUND & OBJECTIVES: The polymeric immunoglobulin receptor (pIgR) is produced by mucosal epithelial cells and plays a crucial role in mucosal immunity. At the basolateral surface of mucosal cells, the pIgR binds predominantly polymeric immunoglobulins, such as dimeric IgA and polymeric IgA (pIgA) and mediates their transport across the polarized cells. This results in apical release of secretory component (SC), either free or bound covalently to IgA, forming secretory IgA (SIgA). The choline-binding protein (Cbp) SpsA, also called PspC and CbpA, has been shown to interact with the pIgR. A hexapeptide motif in SpsA was identified as the minimal binding motif required for binding specifically to pIgR and SC. The present study was carried out to show that the hexapeptide motif in SpsA is crucial for the interaction of pneumococci and pIgR-expressing cells. METHODS: Streptococcus pneumoniae were cultured to mid-log phase. Calu-3 cells and MDCK epithelial cells, stably transfected with the hpIgR cDNA in pCB6 were used in in vitro infection experiments. Pneumococcal adherence to and invasion of epithelial cells were assayed. RESULTS: By the use of the N-terminal domain of SpsA and SpsA(201), which exhibits a single amino acid substitution in the pIgR-binding motif, in vitro assays indicated the association of the identified hexapeptide motif, located between amino acid 198 and 203 in SpsA, with pneumococcal adherence to and invasion of hpIgR-expressing cells. INTERPRETATION & CONCLUSION: The present findings demonstrated not only the crucial role of the hexapeptide of SpsA, not only for the SpsA-pIgR interaction, but also for adherence and invasion of hpIgR-expressing cells.

Characterization of plasmin(ogen) binding to Streptococcus pneumoniae.

BACKGROUND & OBJECTIVES: The proteolytic activity of plasmin promotes migration of pathogenic bacteria through the human extracellular matrix. The human pathogen Streptococcus pneumoniae binds both human plasminogen and plasmin via the surface displayed alpha-enolase designated Eno. Electron microscopic studies verified the surface exposition of the glycolytic enzyme alpha-enolase and moreover, its ability to reassociate to the cell surface. Carboxyterminal lysine residues of recently described eukaryotic and prokaryotic plasminogen-binding proteins such as SEN of S. pyogenes are involved in interaction with lysine binding sites of kringle domains of plasminogen. In this study, the role of carboxy terminal lysyl residue of eno in plasminogen binding is further analysed. METHODS: Site-directed mutagenesis of eno gene was done using DNA primers with Hind III-restriction enzyme sites for cloning. Purified Eno fusion proteins were separated by SDS-PAGE and human plasminogen binding assay was performed. Radioiodinated ligand binding was done by competitive inhibition assay. RESULTS: Binding assays performed under reduced conditions indicated also a role of the C-terminal lysyl residues of Eno for plasmin(ogen) binding. Binding of pneumococci to radioiodinated plasminogen was competitively inhibited in the presence of plasminogen, kringle 1-3 (LBS 1) and the lysineanalogon epsilon-amino caproic acid indicating the crucial role of lysine-binding sites of plasminogen. However, binding analysis of plasminogen and LBS 1 to wild type Eno and carboxy terminal modified Eno proteins did not reveal any difference in plasminogen-binding activity under native conditions. INTERPRETATION & CONCLUSION: The present results suggested the presence of a further plasminogenbinding motif in Eno. This hypothesis was confirmed by plasminogen-binding activity of reassociated C-terminal modified enolase to the pneumococcal surface and indicated, therefore, the presence of a further binding motif in Eno for plasminogen binding.