Anchoring subunit of the major fimbriae. Regulates fimbrial length (PubMed:20530728). These filamentous pili are attached to the cell surface; they mediate biofilm formation, adhesion onto host cells and onto other bacteria that are part of the oral microbiome. Fimbriae of P.gingivalis are major virulence factors. FimB is not part of the fimbrium itself, but anchors the fimbrium in the outer membrane. Linear, head-to-tail oligomerization of fimbrial subunits mediates assembly of the fimbrium stalk, while the minor components FimC, FimD and FimE probably form the fimbrium tip. The anchoring subunit FimB limits fimbrium length and is important for solid fimbrium attachment to the outer membrane. In its absence, the major fimbriae become very long and are easily detached from the membrane. The name (major fimbrium subunit) does not indicate the abundance of the protein, but is derived from the greater length of the major fimbriae. In strain ATCC 33277 and strain ATCC BAA-1703 / FDC 381, major fimbriae are 300 - 1600 nM in length and about 5 nm in diameter. In contrast, minor fimbriae are only about 80 - 120 nm long. This length difference is observed only in a small number of strains, including strain ATCC 33277 and strain ATCC BAA-1703 / FDC 381, and is due to a loss of function mutation in FimB, a protein that restricts fimbrial length in other strains. Belongs to the bacteroidetes fimbrillin superfamily. FimB/Mfa2 family.