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Hu, Ping-Ping; Hou, Jian-Yun; Xu, Ya-Li; Niu, Nan-Nan; Zhao, Cheng; Lu, Lu; Zhou, Ming; Scheer, Hugo and Zhao, Kai-Hong (2020): The role of lyases, NblA and NblB proteins and bilin chromophore transfer in restructuring the cyanobacterial light-harvesting complex(double dagger). In: Plant Journal, Vol. 102, No. 3: pp. 529-540

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Phycobilisomes are large light-harvesting complexes attached to the stromal side of thylakoids in cyanobacteria and red algae. They can be remodeled or degraded in response to changing light and nutritional status. Both the core and the peripheral rods of phycobilisomes contain biliproteins. During biliprotein biosynthesis, open-chain tetrapyrrole chromophores are attached covalently to the apoproteins by dedicated lyases. Another set of non-bleaching (Nb) proteins has been implicated in phycobilisome degradation, among them NblA and NblB. We report in vitro experiments with lyases, biliproteins and NblA/B which imply that the situation is more complex than currently discussed: lyases can also detach the chromophores and NblA and NblB can modulate lyase-catalyzed binding and detachment of chromophores in a complex fashion. We show: (i) NblA and NblB can interfere with chromophorylation as well as chromophore detachment of phycobiliprotein, they are generally inhibitors but in some cases enhance the reaction;(ii) NblA and NblB promote dissociation of whole phycobilisomes, cores and, in particular, allophycocyanin trimers;(iii) while NblA and NblB do not interact with each other, both interact with lyases, apo- and holo-biliproteins;(iv) they promote synergistically the lyase-catalyzed chromophorylation of the beta-subunit of the major rod component, CPC;and (v) they modulate lyase-catalyzed and lyase-independent chromophore transfers among biliproteins, with the core protein, ApcF, the rod protein, CpcA, and sensory biliproteins (phytochromes, cyanobacteriochromes) acting as potential traps. The results indicate that NblA/B can cooperate with lyases in remodeling the phycobilisomes to balance the metabolic requirements of acclimating their light-harvesting capacity without straining the overall metabolic economy of the cell.

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