Removes residual C-terminal Arg or Lys remaining after initial endoprotease cleavage during prohormone processing. Release of C-terminal arginine or lysine residues from polypeptides. Binds 1 zinc ion per subunit. Associated with the secretory granule membrane through direct binding to lipid rafts in intragranular conditions. Expressed in brain, pituitary and islet organs, but not in other tissues which do not secrete peptide hormones. Belongs to the peptidase M14 family.