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

Determining the degradation characteristics of proteins is difficult due tothe lack of appropriate methodologies, particularly in the case of leafproteins. Previous studies suggest that ribulose bisphosphate carboxylase(RuBP carboxylase; EC 4.1.1.39) proteolysis may be fundamentally differentin C3 and C4 plants. To test this hypothesis, the relative degradationrates of the total soluble protein, RuBP carboxylase and glycolate oxidase(EC 1.1.3.1) in the second leaves of intact C3 (Triticumaestivum L.) and C4 (Zea mays L) andSorghum bicolor L.)plants was measured. Themethodology utilized involved an efficient procedure to label the leafproteins, the use of a double-labelling method to measure proteindegradation and a single-step purification of the labelled proteins understudy. RuBP carboxylase is subjected to continuous degradation in allplants investigated. Its rate of degradation is higher for Z.mays, intermediate for T. aestivum andlower for S. bicolor. When the rate of RuBPcarboxylase degradation was compared with that of the total soluble proteina differential pattern was obtained for the plant species examined: whereasmaize presents a faster rate of RuBP carboxylase degradation than of thetotal soluble protein, wheat and sorghum show similar rates. However, therate of RuBP carboxylase proteolysis in the three plant species studied ismuch lower than the rate of glycolate oxidase degradation. The resultsobtained indicate that, under the conditions of study, the degradationcharacteristics of plant RuBP carboxylase, as those of glycolate oxidase,are species specific, in a way suggesting that they do not depend on thetype of photosynthetic metabolism of the species considered (C3 orC4).