Abstract

 Ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) degradation is a complex process involving a multitude of proteolytic pathways, and its mechanism is still unclear now. We previously confirmed that a new senescence-associated protease in the stroma of chloroplasts from senescing leaves was suggested to be involved in the appearance of a 51 kDa fragment of Rubisco. In this study, the 51-kDa fragment was also detected when the crude extracts of mature leaves were incubated in Tris-HCl buffer (pH 7.5) with 1 mM ATP or the chloroplast lysates of mature leaves were incubated in the same buffer with 1 mM ATP and 1 μM Zn²⁺ for 1.5 h. However, the special degradation of large subunits (LSU) in the crude extracts of senescing leaves seemed to not to be affected by ATP and Zn²⁺ presence. Furthermore, the 51-kDa fragment could be detected when chloroplast lysates of senescing leaves were incubated in pH 7.5 Tris-HCl buffer containing 1 mM ATP and 1 μM Zn²⁺ only for 0.5 h, but it would be at least for 1 h if the buffer did not contain 1 mM ATP and 1 μM Zn²⁺. The results from this study implied that there would be two types of protease, which could specially degrade LSU into 51-kDa fragment, in cytoplasm or vacuole and in chloroplast of mature leaves respectively before leaf senescence.

Key words: ATP, Zn²⁺, Rubisco, protein degradation, wheat.