Human HtrA, an evolutionarily conserved serine protease identified as a differentially expressed gene product in osteoarthritic cartilage

The human homologue of the Escherichia coli htrA gene product was identifie d by the differential display analysis of transcripts expressed in osteoart hritic cartilage. This transcript was identified previously as being repres sed in SV40-transformed fibroblasts (Zumbrunn, J,, and Trueb, B, (1996) FEB S Lett. 398, 187-192). Levels of HtrA mRNA were elevated similar to 7-fold in cartilage from individuals with osteoarthritis compared with nonarthriti c controls, Differential expression of human HtrA protein was confirmed by an immunoblot analysis of cartilage extracts, Human HtrA protein expressed in heterologous systems was secreted and exhibited endoproteolytic activity , including autocatalytic cleavage. Conversion by mutagenesis of the putati ve active site serine 328 to alanine eliminated the enzymatic activity. Ser ine 328 was also found to be required for the formation of a stable complex with alpha(1)-antitrypsin. We have determined that the HtrA gene is highly conserved among mammalian species: the amino acid sequences encoded by Htr A cDNA clones from cow, rabbit, and guinea pig are 98% identical to human. In E. coli, a functional htrA gene product is required for cell survival af ter heat shock or oxidative stress; its role appears to be the degradation of denatured proteins. We propose that mammalian HtrA, with the addition of a new functionality during evolution, i.e. a mac25 homology domain, plays an important role in cell growth regulation.