The biogenesis of Photosystem II is a complicated process requiring numerous auxiliary factors to assist in all steps of its assembly. The cyanobacterial protein Ycf39 forms a stress-induced complex with 2 small chlorophyll-binding, High-light-inducible proteins C and D (HliC and HliD), and has been reported to participate in the insertion of chlorophyll molecules into the central D1 subunit of Photosystem II. However, how this process is organized remains unknown. Here, we show that Ycf39 and both HliC and HliD can form distinct complexes with chlorophyll synthase (ChlG) in the model cyanobacterium Synechocystis sp. PCC 6803. We isolated and characterized ChlG complexes from various strains grown under different conditions and provide a mechanistic view of the docking of Ycf39 to ChlG via HliD and the structural role of HliC. In the absence of stress, chlorophyll is produced by the ChlG-HliD2-ChlG complex, which is stabilized by chlorophyll and zeaxanthin molecules bound to the HliD homodimer. The switch to high light leads to stress pressure and greatly elevated synthesis of HliC, resulting in the replacement of HliD homodimers with HliC-HliD heterodimers. Unlike HliD, HliC cannot interact directly with ChlG or Ycf39. Therefore, the original ChlG-HliD2-ChlG complex is converted into a ChlG-HliD-HliC hetero-trimer that presumably binds transiently to Ycf39 and the nascent D1 polypeptide. We speculate that this molecular machinery promotes the delivery of chlorophyll to D1 upon high-light-induced chlorophyll deficiency. The HliD homodimers formed under standard, nonstress growth conditions and attached to ChlG could serve as an emergency chlorophyll reserve.

• MeSH
• Bacterial Proteins * metabolism   genetics   MeSH
• Chlorophyll metabolism   MeSH
• Photosystem II Protein Complex * metabolism   MeSH
• Carbon-Oxygen Ligases * metabolism   genetics   MeSH
• Light * MeSH
• Synechocystis * metabolism   radiation effects   genetics   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Bacterial Proteins * MeSH
• Chlorophyll MeSH
• chlorophyll synthetase MeSH Browser
• Photosystem II Protein Complex * MeSH
• high light-inducible protein, cyanobacteria MeSH Browser
• Carbon-Oxygen Ligases * MeSH
• Light-Harvesting Protein Complexes MeSH

High-light-inducible proteins (Hlips) are single-helix transmembrane proteins that are essential for the survival of cyanobacteria under stress conditions. The model cyanobacterium Synechocystis sp. PCC 6803 contains four Hlip isoforms (HliA-D) that associate with Photosystem II (PSII) during its assembly. HliC and HliD are known to form pigmented (hetero)dimers that associate with the newly synthesized PSII reaction center protein D1 in a configuration that allows thermal dissipation of excitation energy. Thus, it is expected that they photoprotect the early steps of PSII biogenesis. HliA and HliB, on the other hand, bind the PSII inner antenna protein CP47, but the mode of interaction and pigment binding have not been resolved. Here, we isolated His-tagged HliA and HliB from Synechocystis and show that these two very similar Hlips do not interact with each other as anticipated, rather they form HliAC and HliBC heterodimers. Both dimers bind Chl and β-carotene in a quenching conformation and associate with the CP47 assembly module as well as later PSII assembly intermediates containing CP47. In the absence of HliC, the cellular levels of HliA and HliB were reduced, and both bound atypically to HliD. We postulate a model in which HliAC-, HliBC-, and HliDC-dimers are the functional Hlip units in Synechocystis. The smallest Hlip, HliC, acts as a 'generalist' that prevents unspecific dimerization of PSII assembly intermediates, while the N-termini of 'specialists' (HliA, B or D) dictate interactions with proteins other than Hlips.

• Keywords
• CP47, Chlorophyll, High-light-inducible proteins, Photosystem II, Synechocystis,
• MeSH
• Bacterial Proteins metabolism   MeSH
• Photosystem II Protein Complex metabolism   MeSH
• Tumor Necrosis Factor Ligand Superfamily Member 14 metabolism   MeSH
• Light-Harvesting Protein Complexes * metabolism   MeSH
• Synechocystis * metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Bacterial Proteins MeSH
• Photosystem II Protein Complex MeSH
• Tumor Necrosis Factor Ligand Superfamily Member 14 MeSH
• Light-Harvesting Protein Complexes * MeSH