The flagellum-specific export system is a specialized type III export machinery. Terminally truncated fragments of flagellin (FliC) were used to identify the secretion signal in the main component of flagellar filaments. The first 13 residues were not essential for export, but removal of 29 or more residues destroyed export ability. When an 8 kDa human protein domain was fused to various N-terminal fragments of FliC, the 26-47 sequence alone was sufficient to mediate secretion of this protein module through the flagellum specific export pathway. Neither half of this segment was enough to direct export of the attached protein domain. Our results demonstrate that the 22-residue long 26-47 segment within the disordered N-terminal region of Salmonella flagellin contains the recognition signal for the flagellar export machinery. (c) 2006 Elsevier Inc. All rights reserved.