The <i>Streptococcus pneumoniae</i> adhesin PsrP binds to Keratin 10 on lung cells

Pneumococcal serine-rich repeat protein (PsrP) is a pathogenicity island-encoded adhesin that mediates attachment to lung cells. It is a member of the serine-rich repeat protein family and the largest bacterial protein known. PsrP production by S. pneumoniae was confirmed by immunoblotting and a truncated version of the protein was determined to be glycosylated. Using isogenic psrP mutants complemented with various PsrP constructs and competitive inhibition assays with recombinant proteins, we determined that PsrP requires an extended SRR2 domain for function and that adhesion is mediated through amino acids 273-341 of its basic region (BR) domain. Affinity chromatography, immunoprecipitation, enzyme-linked immunosorbent assay (ELISA), fluorescent-activated cell sorting (FACS) and immunofluorescent colocalization studies determined that PsrP binds to Keratin 10 (K10) on the surface of lung but not nasopharyngeal epithelial cells. Unglycosylated K10 bound to wild type but not psrP deficient pneumococci; suggesting that unlike other serine-rich repeat proteins, PsrP-mediated adhesion is independent of lectin activity. Finally, mice immunized with recombinant (r)PsrP(BR) had significantly less bacteria in their blood and improved survival versus controls following intranasal challenge. We conclude that the BR domain of PsrP binds to K10 in a lectin-independent manner, that K10 is expressed on lung cells and that vaccination with rPsrP(BR) is protective against pneumococcal disease.