Protein degradation in C3 and C4 plants with particular reference to ribulose bisphosphate carboxylase and glycolate oxidase

Abstract

Determining the degradation characteristics of proteins is difficult due to the lack of appropriate methodologies, particularly in the case of leaf proteins. Previous studies suggest that ribulose bisphosphate carboxylase (RuBP carboxylase; EC 4.1.1.39) proteolysis may be fundamentally different in C3 and C4 plants. To test this hypothesis, the relative degradation rates of the total soluble protein, RuBP carboxylase and glycolate oxidase (EC 1.1.3.1) in the second leaves of intact C3 (Triticum aestivum L.) and C4 (Zea mays L. and Sorghum bicolor L.) plants was measured. The methodology utilized involved an efficient procedure to label the leaf proteins, the use of a double-labelling method to measure protein degradation and a singlestep purification of the labelled proteins under study. RuBP carboxylase is subjected to continuous degradation in all plants investigated. Its rate of degradation is higher for Z. mays, intermediate for T. aestivum and lower for S. bicolor. When the rate of RuBP carboxylase degradation was compared with that of the total soluble protein a differential pattern was obtained for the plant species examined: whereas maize presents a faster rate of RuBP carboxylase degradation than of the total soluble protein, wheat and sorghum show similar rates. However, the rate of RuBP carboxylase proteolysis in the three plant species studied is much lower than the rate of glycolate oxidase degradation. The results obtained indicate that, under the conditions of study, the degradation characteristics of plant RuBP carboxylase, as those of glycolate oxidase, are species specific, in a way suggesting that they do not depend on the type of photosynthetic metabolism of the species considered (C3 or C4).