Unique Unfoldase/Aggregase Activity of a Molecular Chaperone Hsp33 in its Holding-Inactive State

The various chaperone activities of heat shock proteins contribute to ensuring cellular proteostasis. Here, we demonstrate the non-canonical unfoldase activity as an inherent functionality of the prokaryotic molecular chaperone, Hsp33. Hsp33 was originally identified as a holding chaperone that is post-translationally activated by oxidation. However, in this study, we verified that the holding-inactive reduced form of Hsp33 (^RHsp33) strongly bound to the translational elongation factor, EF-Tu. This interaction was critically mediated by the redox-switch domain of ^RHsp33 and the guanine nucleotide-binding domain of EF-Tu. The bound ^RHsp33, without undergoing any conformational change, catalyzed the EF-Tu aggregation by evoking the aberrant folding of EF-Tu to expose hydrophobic surfaces. Consequently, the oligomers/aggregates of EF-Tu, but not its functional monomeric form, were highly susceptible to proteolytic degradation by Lon protease. These findings present a unique example of an ATP-independent molecular chaperone with distinctive dual functions-as an unfoldase/aggregase and as a holding chaperone-depending on the redox status. It is also suggested that the unusual unfoldase/aggregase activity of ^RHsp33 can contribute to cellular proteostasis by dysregulating EF-Tu under heat-stressed conditions.