Crystal structure of the catalytic domain of DESC1, a new member of the type II transmembrane serine proteinase family.

DESC1 was identified using gene-expression analysis between squamous cell carcinoma of the head and neck and normal tissue. It belongs to the type II transmembrane multidomain serine proteinases (TTSPs), an expanding family of serine proteinases, whose members are differentially expressed in several tissues. The biological role of these proteins is currently under investigation, although in some cases their participation in specific functions has been reported. This is the case for enteropeptidase, hepsin, matriptase and corin. Some members, including DESC1, are associated with cell differentiation and have been described as tumor markers. TTSPs belong to the type II transmembrane proteins that display, in addition to a C-terminal trypsin-like serine proteinase domain, a differing set of stem domains, a transmembrane segment and a short N-terminal cytoplasmic region. Based on sequence analysis, the TTSP family is subdivided into four subfamilies: hepsin/transmembrane proteinase, serine (TMPRSS); matriptase; corin; and the human airway trypsin (HAT)/HAT-like/DESC subfamily. Members of the hepsin and matriptase subfamilies are known structurally and here we present the crystal structure of DESC1 as a first member of the HAT/HAT-like/DESC subfamily in complex with benzamidine. The proteinase domain of DESC1 exhibits a trypsin-like serine proteinase fold with a thrombin-like S1 pocket, a urokinase-type plasminogen activator-type S2 pocket, to accept small residues, and an open hydrophobic S3/S4 cavity to accept large hydrophobic residues. The deduced substrate specificity for DESC1 differs markedly from that of other structurally known TTSPs. Based on surface analysis, we propose a rigid domain association for the N-terminal SEA domain with the back site of the proteinase domain.

Catalytic domain structures of MT-SP1/matriptase, a matrix-degrading transmembrane serine proteinase.

The type II transmembrane multidomain serine proteinase MT-SP1/matriptase is highly expressed in many human cancer-derived cell lines and has been implicated in extracellular matrix re-modeling, tumor growth, and metastasis. We have expressed the catalytic domain of MT-SP1 and solved the crystal structures of complexes with benzamidine at 1.3 A and bovine pancreatic trypsin inhibitor at 2.9 A. MT-SP1 exhibits a trypsin-like serine proteinase fold, featuring a unique nine-residue 60-insertion loop that influences interactions with protein substrates. The structure discloses a trypsin-like S1 pocket, a small hydrophobic S2 subsite, and an open negatively charged S4 cavity that favors the binding of basic P3/P4 residues. A complementary charge pattern on the surface opposite the active site cleft suggests a distinct docking of the preceding low density lipoprotein receptor class A domain. The benzamidine crystals possess a freely accessible active site and are hence well suited for soaking small molecules, facilitating the improvement of inhibitors. The crystal structure of the MT-SP1 complex with bovine pancreatic trypsin inhibitor serves as a model for hepatocyte growth factor activator inhibitor 1, the physiological inhibitor of MT-SP1, and suggests determinants for the substrate specificity.