Human HTRA1 Recombinant Protein. Secreted human HtrA1 serine protease is supposed to form oligomers of identical protein chains as revealed by X-ray analysis for the bacterial HtrA protein DegP. Polypeptide chains of human HtrA1 consist of several domains: An N-terminal insulin-like growth factor domain is followed by a Kazal-type serine protease inhibitor domain, a linker region, a trypsin-like protease domain and a PDZ domain. The function of HtrA1 appears closely linked to signaling by proteins of the TGFβ family. During mouse embryo development HtrA1 is localized in specific areas where signaling by TGFβ family proteins occurs. HtrA1 binds TGFβ, BMP4, Gdf5 and activin. It inhibits signaling by these factors. For inhibitio