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Helical filaments of bacterial flagella are built up by a self-assembly process from thousands of flagellin subunits. To clarify how the disordered terminal regions of flagellin interact upon filament formation, polymerization ability of various terminally truncated fragments was investigated. Fragments deprived of 19 N-terminal residues were able to bind to the end of filaments, however, only a single layer was formed. Removal of C-terminal segments or truncation at both ends resulted in the complete loss of binding ability. Our observations are consistent with the coiled-coil model of filament formation, which suggests that the α-helical N- and C-terminal regions of axially adjacent subunits form an interlocking pattern of helical bundles upon polymerization. [Copyright &y& Elsevier]