Fap1-like serine-rich proteins are a new family of bacterial adhesins found in a variety of streptococci and staphylococci that have been implicated in bacterial pathogenesis. A gene cluster encoding glycosyltransferases and accessory Sec components is required for Fap1 glycosylation and biogenesis in Streptococcus parasanguinis. Here we report that the glycosylation-associated protein, Gap1, contributes to glycosylation and biogenesis of Fap1 by interacting with another glycosylation-associated protein, Gap3. Gap1 shares structural homology with glycosyltransferases. The gap1 mutant, like the gap3 mutant, produced an aberrantly glycosylated Fap1 precursor and failed to produce mature Fap1, suggesting that Gap1 and Gap3 might function in concert in the Fap1 glycosylation and biogenesis. Indeed, Gap1 interacted with Gap3 in vitro and in vivo. A Gap1 N-terminal motif, within a highly conserved domain of unknown function (DUF1975) identified in many bacterial glycosyltransferases, was required for the Gap1-Gap3 interaction. Deletion of one, four and nine amino acids within the conserved motif gradually inhibited the Gap1-Gap3 interaction and diminished production of mature Fap1 and concurrently increased production of the Fap1 precursor. Consequently, bacterial adhesion to an in vitro tooth model was also reduced. These data demonstrate that the Gap1-Gap3 interaction is required for Fap1 biogenesis and Fap1-dependent bacterial adhesion. © 2008 The Authors.