Oligomeric state-dependent functional switching of Arabidopsis universal stress protein

Universal stress proteins (USPs) are conserved stress-responsive proteins, yet the functional relevance of their oligomeric states remains poorly understood. Here, we show that recombinant Arabidopsis thaliana universal stress protein (AtUSP) exists as two stable oligomeric forms: a high-molecular-weight (HMW) assembly and a low-molecular-weight (LMW) species. These two fractions exhibited clearly distinct and non-overlapping biological activities. The HMW complexes displayed robust holdase chaperone activity, whereas the LMW form lacked detectable chaperone function. In contrast, antifungal activity was exclusively associated with the LMW form. LMW-AtUSP penetrated fungal cells and accumulated in the cytosol, where it induced both intracellular and mitochondrial reactive oxygen species (ROS) production and activated apoptosis-associated responses, including caspase-3/7 activation and phosphatidylserine externalization. The HMW form showed none of these antifungal-associated activities. Together, these results establish that the biological function of AtUSP is determined by its oligomeric state and identify the LMW species as the active antifungal form responsible for ROS-mediated apoptotic responses in fungal pathogens.