Sequence, purification, and cloning of an intracellular serine protease, quiescent cell proline dipeptidase

We recently observed that specific inhibitors of postproline cleaving amino dipeptidases cause apoptosis in quiescent lymphocytes in a process independ ent of CD26/dipeptidyl peptidase IV. These results led to the isolation and cloning of a new protease that we have termed quiescent cell proline dipep tidase (QPP), QPP activity was purified from CD26(-) Jurkat T cells. The pr otein was identified by labeling with [H-3]diisopropylfluorophosphate and s ubjected to tryptic digestion and partial amino acid sequencing. The peptid e sequences were used to identify expressed sequence tag clones. The cDNA o f QPP contains an open reading frame of 1476 base pairs, coding for a prote in of 492 amino acids. The amino acid sequence of QPP reveals similarity wi th prolylcarboxypeptidase. The putative active site residues serine, aspart ic acid, and histidine of QPP show an ordering of the catalytic triad simil ar to that seen in the post-proline cleaving exopeptidases prolylcarboxypep tidase and CD26/dipeptidyl peptidase IV. The post-proline cleaving activity of QPP has an unusually broad pH range in that it is able to cleave substr ate molecules at acidic pH as web as at neutral pH, QPP has also been detec ted in nonlymphocytic cell lines, indicating that this enzyme activity may play an important role in other tissues as well.