FIG. 1.

Schematic domain structures of HAT and TMPRSS2. HAT and TMPRSS2 are synthesized as single-chain zymogens that consist of an N-terminal transmembrane domain (TM), a stem region containing, e.g., sea urchin sperm protein, enterokinase, and agrin domain (SEA) for HAT or low-density lipoprotein receptor class A domain (LDLRA), and scavenger receptor cysteine-rich domain (SRCR) for TMPRSS2, and a C-terminal trypsin-like serine protease domain that contains the catalytic triad consisting of histidine (H), aspartic acid (D), and serine (S). The zymogens are activated by proteolytic cleavage at arginine residues R186 for HAT and R255 for TMPRSS2 (indicated by arrows). For immunochemical detection of the recombinant proteases HAT and TMPRSS2 are expressed with a C-terminally fused FLAG tag peptide (DYKDDDDK).