Identification of a common structural motif in the disordered N-terminal region of bacterial flagellins. Evidence of a new class of fibril-forming peptides

Abstract

Flagellin proteins lacking the N- or C-terminus form polymers of reduced filament stability and straight morphology, in contrast to the coiled native flagella. In the present study, the N-terminal amino acid sequence of flagellins of the anaerobic beer spoilage bacteria Pectinatus cerevisiiphilus and Pectinatus frisingiensis as well as Enterobacter aerogenes and Pseudomonas sp. were determined. Sequence similarity was revealed between these and the N-termini of all known eubacterial flagellins. Synthetic peptides corresponding to the first 15 amino acid residues of the flagellins of Pectinatus, Campylobacter jejuni, E. aerogenes or Proteus mirabilis flagellins had a spontaneous tendency under physiological conditions to form 4-6 nm broad, 1-2 μm long fibrillar structures that had a tendency to form clusters. In contrast, the Pectinatus peptide missing residues 1-3 did not form fibrils. The peptide missing residues 13-15 formed fibrils less easily and the peptide missing residues 11-15 formed fibrils almost without clustering, In electron micrographs, the fibrillisation of the bacterial flagellar peptides resembled that of β-amyloid and prion peptides. 1H-NMR and infrared spectroscopy studies with homology analysis indicate that although the flagellar N-terminal peptides are flexible with many conformational minima, they have a significant tendency to form β-type structures and a loop in the middle of the peptide. The hydrophobic character of the N-terminus together with the property of forming a conserved β-strand - loop - β-strand motif may be related to a mechanism involved in attaining the proper morphology and stability of the flagellar filament, by providing a device for facilitating the attachment of the flagellin monomers to each other. The flagellar peptides represent a new class of fibril-forming peptides.