Monitoring Editor: Jonathan S. Weissman Ynm3 is the only budding yeast protein posssessing a combination of serine protease and PDZ domains, a defining feature of the HtrA (high temperature requirement A) protein family. The bacterial HtrA/DegP is involved in protective stress response to aid survival at higher temperatures. The role of mammalian mitochondrial HtrA2/Omi in protein quality control is unclear although loss of its protease activity results in susceptibility toward Parkinson's disease, where mitochondrial dysfunction and impairment of protein folding and degradation are key pathogenetic features. We studied the role of the budding yeast HtrA, Ynm3, with respect to unfolding stresses. Similar to E. coli DegP, we find that Ynm3 is a dual chaperone-protease. Its proteolytic activity is crucial for cell survival at higher temperature. Ynm3 also exhibits strong general chaperone activity, a novel finding for a eukaryotic HtrA member. We propose that the chaperone activity of Ynm3 may be important to improve the efficiency of proteolysis of aberrant proteins by averting the formation of nonproductive toxic aggregates and presenting them in a soluble state to its protease domain. Suppression studies with Deltaynm3 led to the discovery of chaperone activity in a nucleolar PPIase, Fpr3, which could partly relieve the heat sensitivity of Deltaynm3.
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