Localization and function of the accessory protein <scp>M</scp>fa3 in <i>Porphyromonas gingivalis </i><scp>M</scp>fa1 fimbriae

<jats:title>Summary</jats:title><jats:p>The fimbriae of <jats:italic>Porphyromonas gingivalis</jats:italic>, the causative agent of periodontitis, have been implicated in various aspects of pathogenicity, such as colonization, adhesion and aggregation. <jats:italic>Porphyromonas gingivalis </jats:italic><jats:styled-content style="fixed-case">ATCC</jats:styled-content> 33277 has two adhesins comprised of the FimA and Mfa1 fimbriae. We characterized the <jats:styled-content style="fixed-case">PGN</jats:styled-content>0289 (Mfa3) protein, which is one of the three accessory proteins of Mfa1 fimbriae in <jats:italic>P. gingivalis</jats:italic>. The Mfa3 protein was present in two different sizes, 40 and 43 kDa, in the cell. The 43‐kDa and 40‐kDa Mfa3 were detected largely in the inner membrane and the outer membrane, respectively. Purified Mfa1 fimbriae contained the 40‐kDa Mfa3 alone. Furthermore, the 40‐kDa Mfa3 started with the Ala<jats:sup>44</jats:sup> residue of the deduced amino acid sequence, indicating that the N‐terminal region of the nascent protein expressed from the <jats:italic>mfa3</jats:italic> gene is processed in the transport step from the inner membrane into fimbriae. Immuno‐electron microscopy revealed that Mfa3 localized at the tip of the fimbrial shaft. Interestingly, deletion of the <jats:italic>mfa3</jats:italic> gene resulted in the absence of other accessory proteins, <jats:styled-content style="fixed-case">PGN</jats:styled-content>0290 and <jats:styled-content style="fixed-case">PGN</jats:styled-content>0291, in the purified Mfa1 fimbriae, suggesting that Mfa3 is required for integration of <jats:styled-content style="fixed-case">PGN</jats:styled-content>0290 and <jats:styled-content style="fixed-case">PGN</jats:styled-content>0291 into fimbriae. A double mutant of <jats:italic>mfa3</jats:italic> and <jats:italic>fimA</jats:italic> genes (phenotype Mfa1 plus, FimA minus) showed increased auto‐aggregation and biofilm formation similar to a double mutant of <jats:italic>mfa1</jats:italic> and <jats:italic>fimA</jats:italic> genes (phenotype Mfa1<jats:sup>–</jats:sup>, FimA<jats:sup>–</jats:sup>). These findings suggest that the tip protein Mfa3 of the Mfa1 fimbriae may function in the integration of accessory proteins and in the colonization of <jats:italic>P. gingivalis</jats:italic>.</jats:p>