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

Abstract

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 for bacterial survival at high temperatures. HumHtrA2 is also homologous to human HtrA1, also known as L56/HtrA, which is differentially expressed in human osteoarthritic cartilage and after SV40 transformation of human fibroblasts. HumHtrA2 is upregulated in mammalian cells in response to stress induced by both heat shock and tunicamycin treatment. Biochemical characterization of humHtrA2 shows it to be predominantly a nuclear protease which undergoes autoproteolysis. This proteolysis is abolished when the predicted active 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. HumHtrA2 cleaves b-casein with an inhibitor profile similar to that previously described for E. coli HtrA, in addition to an increase in b-casein turnover when the assay temperature is raised from 37 to 45 8C. The biochemical and sequence similarities between humHtrA2 and its bacterial homologues, in conjunction with its nuclear location and upregulation in response to tunicamycin and heat shock suggest that it is involved in mammalian stress response pathways.

Omi/HtrA2 is a positive regulator of autophagy that facilitates the degradation of mutant proteins involved in neurodegenerative diseases

“…Maintenance of autophagy at a basal level in most cells contributes to the routine turnover of cytoplasmic components, and autophagy is rapidly upregulated when cells need to generate intracellular nutrients and energy. As previously reported, ER stress can activate autophagy 18 and increase Omi expression 4 (confirmed in Supplementary Figure S2a), indicating that endogenous Omi might be involved in regulating constitutive basal autophagy. We therefore examined whether endogenous Omi levels were related to stress-induced autophagy activation.…”

“…Omi is reported to be upregulated by severalfold after heat shock or tunicamycin treatment. 4 In our report, both mRNA and protein levels increased under tunicamycin treatment (Figure 2a; Supplementary Figure S2b). In a mouse model of kidney ischemia/ reperfusion, protease activity of Omi obviously increased.…”

“…3 Most Omi resides in the intermembrane space of the mitochondria; however, endoplasmic reticulum (ER) and Golgi localization have also been reported. 4 Under apoptotic stimulation, a mature processed form of Omi is released into the cytosol where it binds to the cytosolic inhibitor of apoptosis proteins (IAPs) and inactivates their caspase-inhibitory activity 5 by direct cleavage. 3 Mature Omi that lacks the mitochondrial targeting sequence induces dramatic apoptosis-like morphological changes and cell death that is not blocked by caspase inhibitors.…”

“…These mice died within 30 days after birth, similarly to mnd2 mice. 10 Furthermore, many other neurodegenerative disease proteins, such as presenilin-1 4,11 or amyloid precursor protein, 12 were reported to be associated with Omi.…”

Serine protease Omi/HtrA2 targets WARTS kinase to control cell proliferation

“…The serine protease Omi (also known as high temperature requirement (HtrA2)) was identified first as an interactor of mxi2 or presenilin in human cells (Faccio et al, 2000;Gray et al, 2000) Thereafter, Omi was found to interact with XIAP(X-linked inhibitor of apoptosis). As XIAP was thought to be a direct inhibitor of caspases, the apoptotic function of Omi was extensively investigated.…”