Flagellin and several other external components of the bacterial flagellum are thought to be exported, not by the general N-terminal signal peptide-dependent pathway, but by a flagellum-specific pathway involving a central channel in the flagellum itself. We have constructed a variety of mutant alleles of the Escherichia coli flagellin gene. Mutant flagellins with large internal deletions or truncations of their C-terminal region could still be exported, even though they could not assemble into filament. The most extreme example was a fragment containing only the N-terminal 183 residues of the 497-residue wild-type flagellin. This result suggests that the N-terminal region of flagellin contains a signal that enables the protein to be recognized and exported by the flagellum-specific pathway.

Kuwajima, G., Kawagishi, I., Homma, M., Asaka, J., Kondo, E., & Macnab, R. M. (1989). Export of an N-terminal fragment of Escherichia coli flagellin by a flagellum-specific pathway. Proceedings of the National Academy of Sciences, 86(13), 4953–4957.