Allelic Variation in the highly polymorphic pspC locus of Streptococcus pneumoniae

PspC, also called SpsA, CbpA, PbcA, and Hic, is a surface protein of Streptococcus pneumoniae studied for its antigenic properties, its capability to bind secretory IgA, C3 and complement factor H, and its activity as an adhesin. In this work we characterized the pspC locus of 43 pneumococcal strains by DNA sequencing of PCR fragments. Using PCR primers designed on two unrelated open reading frames, flanking the pspC locus, it was possible to amplify the pspC locus of each of the 43 strains of S. pneumoniae. In 37 out of 43 strains there was a single copy of the pspC gene, while two tandem copies of pspC were found in the other six strains. The sequence of the pspC locus was different in each of the 43 strains. Insertion sequences were found in the pspC locus of 11 out of 43 strains. Analysis of the deduced amino acid sequence of the PspC variants showed a common organization of the molecules: (i) a 37 amino acid leader peptide which is conserved in all proteins, (ii) an N-terminal portion which is essentially alpha-helical, and is the result of assembly of eight major sequence blocks, (iii) a proline-rich region, and (iv) a C-terminal anchor responsible for the cell surface attachment. By sequence comparison we identified 11 major groups of PspC proteins. Proteins within one group displayed only minor variations of the amino acid sequence. An unexpected finding was that PspC variants could differ in the anchor sequence. While 32 of the PspC proteins displayed the typical choline binding domain of pneumococcal surface proteins, 17 other PspCs showed the LPXTG motif, which is typical of surface proteins of other gram-positive bacteria. This major difference in the anchor region was also observed in the adjacent proline-rich regions which differed considerably in size and composition.