Structural subunit of the major fimbriae (PubMed:27062925). These long, 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. They play an important role in the invasion of periodontal tissues. Fimbriae and their constituents are major virulence factors. FimA proteins from different strains have highly divergent sequences, and this has been used for classification. The sequence-based classification correlates with pathogenicity. Fimbriae are composed of a major, structural subunit (FimA) and the minor components FimC, FimD and FimE (By similarity). Head-to-tail oligomerization of FimA molecules mediates assembly of the fimbrium stalk, while the minor components probably form the fimbrium tip (Probable). Linear, head-to-tail oligomerization of FimA is mediated by a conformation change, facilitating the insertion of a C-terminal beta-strand into a groove in the N-terminal domain of the following subunit (PubMed:27062925). 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 (PubMed:27062925). Synthesized as palmitoylated precursor. The lipidated propeptide is removed during processing to the mature protein. Synthesized as palmitoylated precursor. Efficient export to the outer membrane and integration into fimbriae requires lipidation and subsequent proteolytic removal of the lipidated propeptide. Strain ATCC BAA-308 / W83 seems to lack fimbriae, probably due to very low expression of the fimA gene. 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 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 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. FimA/Mfa1 family.