Abstract
When cyanobacteria acclimate to nitrogen deficiency, they degrade their large (3-5-MDa), light-harvesting complexes, the phycobilisomes. This massive, yet specific, intracellular degradation of the pigmented phycobiliproteins causes a color change of cyanobacterial cultures from blue-green to yellow-green, a process referred to as chlorosis or bleaching. Phycobilisome degradation is induced by expression of the nblA gene, which encodes a protein of ~7 kDa. NblA most likely acts as an adaptor protein that guides a Clp protease to the phycobiliproteins, thereby initiating the degradation process. Most cyanobacteria and red algae possess just one nblA-homologous gene. As an exception, the widely used "model organism" Synechocystis sp. PCC6803 expresses two such genes, nblA16803 and nblA26803, both of whose products are required for phycobilisome degradation. Here, we demonstrate that the two NblA proteins heterodimerize in vitro and in vivo using pull-down assays and a Förster energy-transfer approach, respectively. We further show that the NblA proteins form a ternary complex with ClpC (the HSP100 chaperone partner of Clp proteases) and phycobiliproteins in vitro. This complex is susceptible to ATP-dependent degradation by a Clp protease, a finding that supports a proposed mechanism of the degradation process. Expression of the single nblA gene encoded by the genome of the N2-fixing, filamentous cyanobacterium Nostoc sp. PCC7120 in the nblA1/nblA2 mutant of Synechocystis sp. PCC6803 induced phycobilisome degradation, suggesting that the function of the NblA heterodimer of Synechocystis sp. PCC6803 is combined in the homodimeric protein of Nostoc sp. PCC7120.
Highlights
In cyanobacteria, starvation-induced phycobilisome degradation is caused by NblA
Homodimeric NblA of Nostoc 7120 Complements the Nonbleaching Phenotype of the nblA1/nblA2 Double Mutant of Synechocystis 6803—Genomes of most cyanobacteria, such as Synechococcus 7942 and Nostoc 7120, encode just one nblA gene, whose expression suffices to initiate the proteolytic degradation of the PBS
Both fragments were ligated into the self-replicating plasmid pVZ321 containing the predicted promoter and terminator regions of Synechocystis 6803 nblA16803 and nblA26803, which are cotranscribed from a common promoter [15]
Summary
Starvation-induced phycobilisome degradation is caused by NblA. Results: Synechocystis expresses two NblA proteins that form a heterodimer. We further show that the NblA proteins form a ternary complex with ClpC (the HSP100 chaperone partner of Clp proteases) and phycobiliproteins in vitro. In pull-down experiments, NblA was found to bind to ClpC, an HSP100 chaperone partner of a Clp protease, in an ATP-dependent manner [20] This result led to a proposed model of PBS degradation in which NblA acts as a so-called adaptor protein of a Clp protease [20]. We present evidence that NblA16803 and NblA26803 act as a heterodimer in which NblA16803 mediates the binding to ClpC6803 and phycobiliproteins Both NblA proteins are degraded by a Clp protease in vitro, a finding that strongly supports the proposed model of the role of NblA in PBS degradation. We further show that expression of the single nblA gene from Nostoc 7120 complements the non-bleaching phenotype of the nblA1/nblA2 double mutant of Synechocystis 6803
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