Serine proteinase. Releases tripeptides from the free amino terminus of proteins. Has a requirement for Pro in the P1 position, but is inactivated by Pro in the P1' position. Hydrolysis of Xaa-Xaa-Pro-|-Yaa- releasing the N-terminal tripeptide of a peptide with Pro as the third residue (position P1) and where Yaa is not proline. Strongly inhibited by diisopropyl fluorophosphate and Pefabloc. Weakly inhibited by 3,4-dichloroisocumarin. Not inhibited by phenylmethylsulfonyl fluoride, leupeptin, antipain or prolinal. Activated by iodoacetamide. Optimum pH is 6-8. Stable for at least 12 hours at 25 or 37 degrees Celsius (at pH 7.6). The N-terminus is blocked. Belongs to the peptidase S9B family.