Chlorophyll synthase (ChlG) catalyses a terminal reaction in the chlorophyll biosynthesis pathway, attachment of phytol or geranylgeraniol to the C17 propionate of chlorophyllide. Cyanobacterial ChlG forms a stable complex with high light-inducible protein D (HliD), a small single-helix protein homologous to the third transmembrane helix of plant light-harvesting complexes (LHCs). The ChlG-HliD assembly binds chlorophyll, β-carotene, zeaxanthin and myxoxanthophyll and associates with the YidC insertase, most likely to facilitate incorporation of chlorophyll into translated photosystem apoproteins. HliD independently coordinates chlorophyll and β-carotene but the role of the xanthophylls, which appear to be exclusive to the core ChlG-HliD assembly, is unclear. Here we generated mutants of Synechocystis sp. PCC 6803 lacking specific combinations of carotenoids or HliD in a background with FLAG- or His-tagged ChlG. Immunoprecipitation experiments and analysis of isolated membranes demonstrate that the absence of zeaxanthin and myxoxanthophyll significantly weakens the interaction between HliD and ChlG. ChlG alone does not bind carotenoids and accumulation of the chlorophyllide substrate in the absence of xanthophylls indicates that activity/stability of the 'naked' enzyme is perturbed. In contrast, the interaction of HliD with a second partner, the photosystem II assembly factor Ycf39, is preserved in the absence of xanthophylls. We propose that xanthophylls are required for the stable association of ChlG and HliD, acting as a 'molecular glue' at the lateral transmembrane interface between these proteins; roles for zeaxanthin and myxoxanthophyll in ChlG-HliD complexation are discussed, as well as the possible presence of similar complexes between LHC-like proteins and chlorophyll biosynthesis enzymes in plants.

Department of Chemical and Biological Engineering University of Sheffield Sheffield S1 3JD U K

Department of Molecular Biology and Biotechnology University of Sheffield Sheffield S10 2TN U K

Faculty of Science University of South Bohemia 37005 České Budějovice Czech Republic

Institute of Microbiology of the Czech Academy of Sciences Centre Algatech 37981 Třeboň Czech Republic

Institute of Systems Molecular and Integrative Biology University of Liverpool Liverpool L69 7ZB U K

Biochem J. 2021 Jan 15;478(1):61-62. doi: 10.1042/BCJ20200803. PubMed

References provided by Crossref.org

High-light-inducible proteins control associations between chlorophyll synthase and the Photosystem II biogenesis factor Ycf39

High-light-inducible proteins HliA and HliB: pigment binding and protein-protein interactions

Plant LHC-like proteins show robust folding and static non-photochemical quenching