High Temperature Requirement Factor A3 (HTRA3)

The long isoform, HTRA3L, contains 453 amino acids and has a predicted molecular mass of 49 kD. It contains an N-terminal signal peptide, followed by an insulin/IGF-binding domain, a Kazal-type S protease inhibitor domain, a trypsin protease domain, and a PDZ domain. The short isoform, HTRA3S, contains 357 amino acids and has a predicted molecular mass of 38 kD. HTRA3S is truncated at the C terminus relative to HTRA3L and contains 7 unique C-terminal residues; it lacks the PDZ domain. HTRA3L and HTRA3S have domain structures similar to that of HTRA1 (PRSS11), and they share 61% and 66% amino acid identity with HTRA1, respectively. Both HTRA3 isoforms contain active site sequence motifs characteristic of the HTRA serine protease family.