Substitution of Tyrosine Residues at the Aromatic Cluster around the βA−βB Loop of Rubisco Small Subunit Affects the Structural Stability of the Enzyme and the in Vivo Degradation under Stress Conditions

Abstract

Ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) plays a central metabolic role in photosynthetic eukaryotes, and its catabolism is a crucial process for the nutrient economy of higher plants. The rubisco holoenzyme is assembled from eight chloroplast-encoded large subunits and eight nuclear-encoded small subunits. We have identified a cluster of conserved tyrosines at the interface between subunits (comprising Y67, Y68, and Y72 from the betaA-betaB loop of the small subunit and Y226 from the large subunit) that may contribute to holoenzyme stability. To investigate the role of these tyrosines in rubisco structure and in vivo degradation, we have examined site-directed mutants of these residues (Y67A, Y68A, Y72A, and Y226L) in Chlamydomonas reinhardtii. Even if all mutant strains were able to grow photoautotrophically, they exhibited a reduction in rubisco activity and/or the level of expression, especially the Y67A and Y72A mutants. Besides, all mutant rubiscos were inactivated at a lower temperature than the wild type. The kinetics of proteolysis of the mutant enzymes with subtilisin revealed structural alterations, leading to facilitated disassembly (in the cases of Y67A and Y72A) or aggregation propensity (for Y68A and Y226L). When subjected to oxidative stress in vivo through exposure of liquid cultures to hydrogen peroxide, all mutant strains degraded rubisco at a faster rate than the wild type. These results demonstrate that the tyrosine cluster around the betaA-betaB loop of rubisco small subunit plays a stabilizing role by affecting the catalytic activity and the degradation rate of the enzyme in stressed cells.