Reutilization of Ribulose Bisphosphate Carboxylase

Abstract

Ribulose bisphosphate (RuBP) carboxylase is truly a multifunctional protein. Not only does it exhibit the well-known carboxylase and oxygenase activities, but also its high concentration and turnover characteristics in the leaf fit the classification of a storage protein. RuBP carboxylase varies in concentration by species but can be up to 65% of the total soluble protein of grass or alfalfa leaves. It is assembled and sequestered in a discrete organelle, the chloroplast, wherein it is protected from the proteolytic enzymes in the cytoplasm. In fact, it appears that carboxylase degradation is a cytoplasmically driven process. After its synthesis and assembly, little turnover is detected until plants require its remobilization. RuBP carboxylase can then be mobilized during senescence or when the plant requires its reserves because of deficits of either nitrogen or carbohydrates. As such, the RuBP carboxylase concentration in the leaf is very responsive to environmental stresses.