Catalyzes the removal of a penultimate prolyl residue from the N-termini of peptides, such as Leu-Pro-Ala (PubMed:25609706, PubMed:28476889). Also shows low activity towards peptides with Ala or Ser at the P1 position (PubMed:28476889). Promotes TNFRSF1B-mediated phosphorylation of MAPK8/JNK1 and MAPK9/JNK2, suggesting a function as an adapter protein for TNFRSF1B; the effect is independent of XPNPEP3 peptidase activity. May inhibit apoptotic cell death induced via TNF-TNFRSF1B signaling. Release of any N-terminal amino acid, including proline, that is linked to proline, even from a dipeptide or tripeptide. Binds 2 manganese ions per subunit. Homodimer (PubMed:28476889). Isoform 1 interacts with TNFRSF1B/TNFR2 (activated) and TRAF2 (PubMed:25609706). Mainly mitochondrial. Translocates to the cytoplasm following TNFRSF1B activation. Isoform 1 and isoform 2 are widely expressed, with isoform 1 being more abundant. The disease is caused by variants affecting the gene represented in this entry. Belongs to the peptidase M24B family.