Identification of dipeptide repeats and a cell wall sorting signal in the fimbriae-associated adhesin, Fap1, of Streptococcus parasanguis

Fap1, a fimbriae-associated protein, is involved in fimbriae assembly and a dhesion of Streptococcus parasanguis FW213 (Wu et al., 1998). In this study , the sequence of the fap1 gene was resolved using a primer island transpos ition system. Sequence analysis Indicated that fap1 was composed of 7659 nu cleotides. The predicted Fap1 protein contains an unusually long signal seq uence (50 amino acid residues), a cell wall sorting signal and two repeat r egions, Repeat regions I and II have a similar dipeptide composition (E/V/I )S, composed of 28 and 1000 repeats respectively The two regions combined a ccounted for 80% of the Fap1 coding region. The experimental amino acid com position and isoelectric point (pI) of Fap1 were similar to that predicted from the deduced Fap1 protein. Results of Northern analyses revealed that t he fap1 open reading frame (ORF) was transcribed as a 7.8 kb monocistronic message, insertional inactivation at the 3' end, downstream of the fap1 ORF , did not affect Fap1, fimbrial expression or bacterial adhesion. Insertion al inactivation of fap1 immediately upstream of the repeat region II abolis hed expression of Fap1 and fimbriae, and was concurrent with a diminution i n adhesion of FW213. Inactivation of the cell wall sorting signal of fap1 a lso eliminated long fimbrial formation and reduced the ability of FW213 to bind to SHA. Fap1 was no longer anchored on the cell surface. Large quantit ies of truncated Fap1 were found in the growth medium instead. These result s suggest that the fap1 ORF alone is sufficient to support Fap1 expression and adhesion, and demonstrate that anchorage of Fap1 on the cell surface is required for long fimbriae formation. These data further document the role of long fimbriae in adhesion of S. parasanguis FW213 to SHA.