Cleaves the N-terminal leader peptide from preflagellins. The processing of preflagellins is necessary for assembly of flagellins into a flagellum structure. Cleaves the signal peptide of 3 to 12 amino acids from the N-terminal of preflagellin, usually at Arg-Gly-|- or Lys-Gly-|-, to release flagellin. Optimum pH is 8.5. Optimum temperature is 37 degrees Celsius. Cells lacking this gene are non-motile and non-flagellated. The flagellins of the mutant have larger molecular weights than their wild-type counterparts, as expected if they retain their 11- to 12-amino-acid leader peptide. Belongs to the peptidase A24 family. Archaeal preflagellin peptidase subfamily.