Information essential for cell-cycle-dependent secretion of the 591-residue Caulobacter hook protein is confined to a 21-amino-acid sequence near the N-terminus.

Recent findings suggest that axial flagellar proteins and virulence proteins of Gram-negative bacteria are exported from the cytoplasm via conserved translocation systems. To identify residues essential for secretion of flagellar axial proteins we examined the 591-residue Caulobacter crescentus flagellar hook protein. Western blot assays of the culture media of strains producing mutant hook proteins show that only residues 38-58 are essential for its secretion to the cell surface. We discuss the observation that this unprocessed 21-residue sequence is not conserved in other axial proteins and does not correspond to the SGL-, ANNLAN- and heptad repeat motifs that are located just upstream of the essential secretion information in the hook protein and are conserved near the N-termini of other axial proteins. These motifs, for which an essential role in export or assembly has been proposed, are required for motility. However, we also demonstrate that hook protein can only be secreted when the flagellar basal body is present in the cell envelope. The cell-cycle regulation of hook protein secretion confirms the specificity of the assay used in these studies and suggests that the basal body itself may serve as a secretion channel for the hook protein.