The role of lyases, NblA and NblB proteins and bilin chromophore transfer in restructuring the cyanobacterial light‐harvesting complex^‡

Abstract

SummaryPhycobilisomes 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 β‐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.

The phycobilisome core‐membrane linkers from Synechocystis sp. PCC 6803 and red‐algae assemble in the same topology

“…To further characterize the APC core of the Δ( rep3 ) mutant, we used Synechocystis Δ( cpcA/B ) mutant lacking CPC rods (Hu et al. , 2020) to prepare Δ( cpcA/B/rep3 ), the CPC‐free analog of Δ( rep3 ). Sucrose gradient ultracentrifugation revealed that most of the isolated PBSs concentrated in the H fraction of Δ( cpcA/B ) and the I fraction of Δ( cpcA/B/rep3 ) (Figure 4a,d).…”

Discovery of a small protein factor involved in the coordinated degradation of phycobilisomes in cyanobacteria

“…NblA targets CpcB as protease adaptor recruiting Clp protease. Our data show that NblD interacts with CpcB as well but has another function, likely in opening the tight phycobilisome structure to disrupt PC hexamers, freeing them up for tagging and degradation by the NblA-Clp system and removal of the bilin chromophores by enzymes involved in this process such as NblB(68,71).…”

“…PNAS | 7 of 12 Discovery of a small protein factor involved in the coordinated degradation of phycobilisomes in cyanobacteria https://doi.org/10.1073/pnas.2012277118 PLANT BIOLOGY 6803 lyases, biliproteins, and NblA/B implied complex interactions between these factors and processes during phycobilisome degradation (68). NblD, a small protein, is strongly up-regulated under nitrogen starvation in Synechocystis 6803 (50) and, as shown in this study, participates in phycobilisome disassembly.…”

Small but Smart: On the Diverse Role of Small Proteins in the Regulation of Cyanobacterial Metabolism

“…In contrast to NblA, NblB is expressed similarly in both cells facing nutrient limitation and nutrient-sufficient conditions [91], indicating another, yet unknown, layer of regulation for this protein. Interestingly, recent results show that NblA and NblB not only facilitate degradation of phycobilisomes, but also binding and rearrangement of chromophores to phycobilisomes [92]. Thus, NblA and NblB might play an important role for short-term adjustments of the photocomplexes to optimize light harvesting under changing light conditions in natural environments.…”