The amino-dipeptidyl peptidases DPP8 and DPP9: Purification and enzymatic assays.

Proline residues highly impact protein stability when present either in the first or second N-terminal position. While the human genome encodes for more than 500 proteases, only few proteases are capable of hydrolyzing a proline-containing peptide bond. The two intra-cellular amino-dipeptidyl peptidases DPP8 and DPP9 are exceptional as they possess the rare ability to cleave post-proline. By removing N-terminal Xaa-Pro dipeptides, DPP8 and DPP9 expose a neo N-terminus of their substates, which can consequently alter inter- or intra-molecular interactions of the modified protein. Both DPP8 and DPP9 play key roles in the immune response and are linked to cancer progression, emerging as attractive drug targets. DPP9 is more abundant than DPP8 and is rate limiting for cleavage of cytosolic proline-containing peptides. Only few DPP9 substrates have been characterized; these include Syk, a central kinase for B-cell receptor mediated signaling; Adenylate Kinase 2 (AK2) which is important for cellular energy homeostasis; and the tumor suppressor Breast cancer type 2 susceptibility protein (BRCA2) that is critical for repair of DNA double strand breaks. N-terminal processing of these proteins by DPP9 triggers their rapid turn-over by the proteasome, highlighting a role for DPP9 as upstream components of the N-degron pathway. Whether N-terminal processing by DPP9 leads to substrate-degradation in all cases, or whether additional outcomes are possible, remains to be tested. In this chapter we will describe methods for purification of DPP8 and DPP9 as well as protocols for biochemical and enzymatic characterization of these proteases.
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