Proteases are an important class of enzymes involved in a diverse range of physiological processes. The modulation of proteolytic activity is an established means of therapeutic intervention with currently marketed products for afflictions as diverse as type 2 diabetes, hypertension and viral infections. The human protease tree is comprised of at least 676 diverse proteins that have been systematically organized into clans and families based on similarity in sequence, structure, and function [1
]. Although the structural basis of catalytic mechanism, substrate specificity and rational drug design has been identified for numerous protease families, there has been no structural description of the S28 family of proteases that form a distinct branch of the serine carboxypeptidase clan.
The S28 family of peptidases consists of two enzymes, PRCP and DPP7. DPP7 is also called dipeptidyl peptidase 2 and quiescent cell proline dipeptidase [2
]. PRCP is a lysosomal, serine carboxypeptidase that cleaves hydrophobic C-terminal amino acids adjacent to proline [5
]. In contrast, DPP7 is a serine dipeptidyl aminopeptidase that cleaves N-terminal amino acids adjacent to proline and is localized to intracellular vesicles [3
Human PRCP and human DPP7 share 39.6% sequence identity and 55.4% sequence similarity. At the sequence level, the two enzymes are unrelated to other proteases; the next closest human homologues are PEP (8.4% sequence identity and 13.9% sequence similarity) and DPP4 (6.5% sequence identity and 11.2% sequence similarity). The S28 proteases PRCP and DPP7 are therefore unique within the protease superfamily.
PRCP was originally discovered as an angiotensinase [7
] and has since been implicated in vasodilatory, proinflammatory, and metabolic pathways [6
]. For example, angiotensin II, III and prekallikrein are all inactivated by PRCP, implicating a role for the enzyme in hypertension, tissue proliferation and smooth muscle growth. PRCP is also reported to inactivate α-melanocyte-stimulating hormone, a neuropeptide that plays a role in regulating appetite [10
]. DPP7 has been implicated in apoptosis in quiescent lymphocytes [3
Here we report the crystal structure of human PRCP. The enzyme consists of an α/β hydrolase domain that contains a unique structural domain insertion that caps the active site. Comparison with the recently released coordinates of DPP7 illuminates the structural basis for the different substrate specificities of PRCP and DPP7. The results lay the foundation for understanding the structural basis of PRCP activity and for the structure-guided discovery of PRCP modulators for target validation and disease modification.