Cleaves the N-terminal leader peptide from preflagellins. 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. Belongs to the peptidase A24 family. Archaeal preflagellin peptidase subfamily. Truncated N-terminus.