Human HTRA1 Recombinant Protein. HtrA1 inhibits signaling by proteins of the TGF family; degrades fragments of amyloid precursor protein; and shows increased expression in osteoarthritic cartilage. Truncated, recombinant human HtrA1 is expressed in E.coli cells. The protein consists of amino acids 141-480 of full-length HtrA1, a C-terminal 6x His-tag, and contains the catalytic and PDZ domain.