Heterogeneous post-translational modification of Actinobacillus actinomycetemcomitans fimbrillin

Fresh isolates of Actinobacillus actinomycetemcomitans produce bundle-forming fimbriae. The exact molecular mass of A. actinomycetemcomitans fimbrillin, a structural subunit of fimbriae, was determined by liquid chromatography-electrospray ionization mass spectrometry. Three major molecular species with 6,226.0, 6,366.0, and 6,513.0 Da were detected in a purified fimbrial fraction from the strain 310-a. These molecular masses were significantly higher than the molecular weight (5,118 Da) calculated from nucleotide sequence data of the fimbrillin gene, flp, suggesting that the fimbrial peptides were post-translationally modified. Modification of the fimbrial peptides was also suggested by an N-terminal amino acid sequence analysis of fimbrillin peptic fragments, with the modified amino acids being due to seven serine or asparagine residues located in the C-terminal region. A periodate oxidation/biotin-hydrazide labeling assay of fimbrillin suggested that it might be glycosylated.