Characterization of human HtrA2, a novel serine protease involved in the mammalian cellular stress response

Human HtrA2 is a novel member of the HtrA serine protease family and shows extensive homology to the Escherichia coli HtrA genes that are essential fo r bacterial survival at high temperatures. HumHtrA2 is also homologous to h uman HtrA1, also known as L56/HtrA, which is differentially expressed in hu man osteoarthritic cartilage and after SV40 transformation of human fibrobl asts. HumHtrA2 is upregulated in mammalian cells in response to stress indu ced by both heat shock and tunicamycin treatment. Biochemical characterizat ion of humHtrA2 shows it. to be predominantly a nuclear protease which unde rgoes autoproteolysis. This proteolysis is abolished when the predicted act ive site serine residue is altered to alanine by site-directed mutagenesis. In human cell lines, it is present as two polypeptides of 38 and 40 kDa. H umHtrA2 cleaves beta-casein with an inhibitor profile similar to that previ ously described for E. coli HtrA, in addition to an increase in beta-casein turnover when the assay temperature is raised from 37 to 45 degrees C. The biochemical and sequence similarities between humHtrA2 and its bacterial h omologues, in conjunction with its nuclear location and upregulation in res ponse to tunicamycin and heat shock suggest that it is involved in mammalia n stress response pathways.