A new function of human HtrA2 as an amyloid-beta oligomerization inhibitor.

Human HtrA2 is part of the HtrA family of ATP-independent serine proteases that are conserved in both prokaryotes and eukaryotes and localizes to the intermembrane space of the mitochondria. Several recent reports have suggested that HtrA2 is important for maintaining proper mitochondrial homeostasis and may play a role in Alzheimer's disease (AD), which is characterized by the presence of aggregates of the amyloid-beta peptide 1-42 (Abeta1-42). In this study, we analyzed the ability of HtrA2 to delay the aggregation of the model substrate citrate synthase (CS) and of the toxic Abeta1-42 peptide. We found that HtrA2 had a moderate ability to delay the aggregation of CS in vitro, and this activity was significantly enhanced when the PDZ domain was removed suggesting an inhibitory role for this domain on the activity. Additionally, using electron microscopy and nuclear magnetic resonance analyses, we observed that HtrA2 significantly delayed the aggregation of the Abeta1-42 peptide. Interestingly, the protease activity of HtrA2 and its PDZ domain were not essential for the delay of Abeta1-42 peptide aggregation. These results indicate that besides its protease activity, HtrA2 also performs a chaperone function and suggest a role for HtrA2 in the metabolism of intracellular Abeta and in AD.

Role of the PDZ domains in Escherichia coli DegP protein.

PDZ domains are modular protein interaction domains that are present in metazoans and bacteria. These domains possess unique structural features that allow them to interact with the C-terminal residues of their ligands. The Escherichia coli essential periplasmic protein DegP contains two PDZ domains attached to the C-terminal end of the protease domain. In this study we examined the role of each PDZ domain in the protease and chaperone activities of this protein. Specifically, DegP mutants with either one or both PDZ domains deleted were generated and tested to determine their protease and chaperone activities, as well as their abilities to sequester unfolded substrates. We found that the PDZ domains in DegP have different roles; the PDZ1 domain is essential for protease activity and is responsible for recognizing and sequestering unfolded substrates through C-terminal tags, whereas the PDZ2 domain is mostly involved in maintaining the hexameric cage of DegP. Interestingly, neither of the PDZ domains was required for the chaperone activity of DegP. In addition, we found that the loops connecting the protease domain to PDZ1 and connecting PDZ1 to PDZ2 are also essential for the protease activity of the hexameric DegP protein. New insights into the roles of the PDZ domains in the structure and function of DegP are provided. These results imply that DegP recognizes substrate molecules targeted for degradation and substrate molecules targeted for refolding in different manners and suggest that the substrate recognition mechanisms may play a role in the protease-chaperone switch, dictating whether the substrate is degraded or refolded.