Xanthophyll carotenoids stabilise the association of cyanobacterial chlorophyll synthase with the LHC-like protein HliD.

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 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.