Ribosomal P3 protein AtP3B of Arabidopsis acts as both protein and RNA chaperone to increase tolerance of heat and cold stresses

The P3 proteins are plant‐specific ribosomal P‐proteins; however, their molecular functions have not been characterized. In a screen for components of heat‐stable high‐molecular weight (HMW) complexes, we isolated the P3 protein AtP3B from heat‐treated Arabidopsis suspension cultures. By size‐exclusion chromatography (SEC), SDS‐PAGE and native PAGE followed by immunoblotting with anti‐AtP3B antibody, we showed that AtP3B was stably retained in HMW complexes following heat shock. The level of AtP3B mRNA increased in response to both high‐ and low‐temperature stresses. Bacterially expressed recombinant AtP3B protein exhibited both protein and RNA chaperone activities. Knockdown of AtP3B by RNAi made plants sensitive to both high‐ and low‐temperature stresses, whereas overexpression of AtP3B increased tolerance of both conditions. Together, our results suggest that AtP3B protects cells against both high‐ and low‐temperature stresses. These findings provide novel insight into the molecular functions and in vivo roles of acidic ribosomal P‐proteins, thereby expanding our knowledge of the protein production machinery.