DETACHMENT OF COVALENTLY BOUND CHROMOPHORES FROM PHYCOBILISOME PIGMENTS

The cyanobacterial macromolecular pigment complex, the phycobilisome, is subjected to regulated proteolysis during nutrient starvation. This pivotal acclimation response allows `recycling of nutrients` and prevents excessive light excitation and its consequent oxidative damage. An essential component of the degradation process is NblB, which exhibits similarity to bilin lyase enzymes that are involved in pigment biogenesis. These enzymes catalyze the covalent attachment of bilin chromophores to the relevant apo-proteins. Inactivation of nblB, however, does not affect pigment synthesis; rather, it impairs pigment degradation. Therefore, it was proposed that the homology of NblB to the pigment synthesizing enzymes reflects its ability to interact with bilin chromophores and that this recognition is crucial for chromophore detachment and further pigment degradation. The aim of this research is to shed light on the role of NblB in degradation of phycocyanin, the major phycobilisome pigment of the cyanobacterium
Synechococcus elongatus. Using a mutational approach, we demonstrated that amino acids highly conserved in bilin lyases of the E/F subfamily, are crucial for NblB function. Additionally, co-expression of α-phycocyanin and NblB in Escherichia coli resulted in decreased pigment level compared to expression of α-phycocyanin by its own, providing support for the requirement of NblB for the degradation process. Based on these data as well as our previous studies demonstrating interaction of the proteolysis-adaptor, NblA, with phycobilisomes, we propose a sequential process for degradation of the light harvesting complex involving the following steps: Dissociation of small pigment assemblies from the phycobilisome by NblA, chromophore detachment by NblB and finally, introduction of the apo-protein to the degradation machinery by NblA.