Decomposition of cyanobacterial light harvesting complexes: NblA‐dependent role of the bilin lyase homolog NblB

Abstract

Phycobilisomes, the macromolecular light harvesting complexes of cyanobacteria are degraded under nutrient-limiting conditions. This crucial response is required to adjust light excitation to the metabolic status and avoid damage by excess excitation. Phycobilisomes are comprised of phycobiliproteins, apo-proteins that covalently bind bilin chromophores. In the cyanobacterium Synechococcus elongatus, the phycobiliproteins allophycocyanin and phycocyanin comprise the core and the rods of the phycobilisome, respectively. Previously, NblB was identified as an essential component required for phycocyanin degradation under nutrient starvation. This protein is homologous to bilin-lyases, enzymes that catalyze the covalent attachment of bilins to apo-proteins. However, the nblB-inactivated strain is not impaired in phycobiliprotein synthesis, but rather is characterized by aberrant phycocyanin degradation. Here, using a phycocyanin-deficient strain, we demonstrate that NblB is required for degradation of the core pigment, allophycocyanin. Furthermore, we show that the protein NblB is expressed under nutrient sufficient conditions, but during nitrogen starvation its level decreases about two-fold. This finding is in contrast to an additional component essential for degradation, NblA, the expression of which is highly induced under starvation. We further identified NblB residues required for phycocyanin degradation in vivo. Finally, we demonstrate phycocyanin degradation in a cell-free system, thereby providing support for the suggestion that NblB directly mediates pigment degradation by chromophore detachment. The dependence of NblB function on NblA revealed using this system, together with the results indicating presence of NblB under nutrient sufficient conditions, suggests a rapid mechanism for induction of pigment degradation, which requires only the expression of NblA.

Nitrogen chlorosis in unicellular cyanobacteria – a developmental program for surviving nitrogen deprivation

“…A genetic approach identified NblB as an essential component of the PBS degradation process (Dolganov and Grossman, 1999). Interestingly, chromophore detachment was not observed unless NblA was also included in these assays, indicating an NblA-dependent function of NblB (Levi et al, 2018). Together, these results support the hypothesis that NblB serves to detach bilin chromophores to allow the degradation of PBS pigments and the homology to bilin-lyases represents its ability to interact with the chromophore (Dolganov and Grossman, 1999).…”

“…A recent study, which established an in vitro system for PC degradation, provided experimental support for the role for NblB in chromophore detachment. Interestingly, chromophore detachment was not observed unless NblA was also included in these assays, indicating an NblA-dependent function of NblB (Levi et al, 2018).…”

“…Genetic studies using a PC-less mutant indicated the requirement for both NblA and NblB in the degradation of allophycocyanin (APC), the pigment comprising the core of the PBS complex (Sendersky et al, 2015;Levi et al, 2018). Furthermore, FLIM analyses using the PC-less mutants indicated interaction of NblA with core complexes attached to the thylakoid membranes (Sendersky et al, 2015).…”

“…In contrast to the nblA gene, which is highly induced upon nitrogen starvation, NblB is constitutively expressed under nutrient replete conditions (Levi et al, 2018); thus, induction of NblA expression is sufficient to drive PBS degradation. In vitro studies using a recombinant Clp protease (ClpC-ClpP1ClpR) demonstrated the degradation of NblA1 and NblA2 of Synechocystis (Baier et al, 2014).…”

“…To explain the observed dependence of NblB function on NblA (Levi et al, 2018), it was suggested that NblB interacts with PC-NblA complexes and detaches the chromophore (Fig. 2B) and, consequently, NblA introduces the apo-protein to the Clp protease ( Fig.…”